



Internet Engineering Task Force (IETF)                      H. Brockhaus
Request for Comments: 9810                                 D. von Oheimb
Obsoletes: 4210, 9480                                            Siemens
Updates: 5912                                               M. Ounsworth
Category: Standards Track                                        J. Gray
ISSN: 2070-1721                                                  Entrust
                                                               July 2025


   Internet X.509 Public Key Infrastructure -- Certificate Management
                             Protocol (CMP)

Abstract

   This document describes the Internet X.509 Public Key Infrastructure
   (PKI) Certificate Management Protocol (CMP).  Protocol messages are
   defined for X.509v3 certificate creation and management.  CMP
   provides interactions between client systems and PKI components such
   as a Registration Authority (RA) and a Certification Authority (CA).

   This document adds support for management of certificates containing
   a Key Encapsulation Mechanism (KEM) public key and uses EnvelopedData
   instead of EncryptedValue.  This document also includes the updates
   specified in Section 2 and Appendix A.2 of RFC 9480.

   This document obsoletes RFC 4210, and together with RFC 9811, it also
   obsoletes RFC 9480.  Appendix F of this document updates Section 9 of
   RFC 5912.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc9810.

Copyright Notice

   Copyright (c) 2025 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Revised BSD License text as described in Section 4.e of the
   Trust Legal Provisions and are provided without warranty as described
   in the Revised BSD License.

Table of Contents

   1.  Introduction
     1.1.  Changes Made by RFC 4210
     1.2.  Updates Made by RFC 9480
     1.3.  Changes Made by This Document
   2.  Terminology and Abbreviations
   3.  PKI Management Overview
     3.1.  PKI Management Model
       3.1.1.  Definitions of PKI Entities
         3.1.1.1.  Subjects and End Entities
         3.1.1.2.  Certification Authority
         3.1.1.3.  Registration Authority
         3.1.1.4.  Key Generation Authority
       3.1.2.  PKI Management Requirements
       3.1.3.  PKI Management Operations
   4.  Assumptions and Restrictions
     4.1.  End Entity Initialization
     4.2.  Initial Registration/Certification
       4.2.1.  Criteria Used
         4.2.1.1.  Initiation of Registration/Certification
         4.2.1.2.  End Entity Message Origin Authentication
         4.2.1.3.  Location of Key Generation
         4.2.1.4.  Confirmation of Successful Certification
       4.2.2.  Initial Registration/Certification Schemes
         4.2.2.1.  Centralized Scheme
         4.2.2.2.  Basic Authenticated Scheme
     4.3.  POP of Private Key
       4.3.1.  Signature Keys
       4.3.2.  Encryption Keys
       4.3.3.  Key Agreement Keys
       4.3.4.  KEM Keys
     4.4.  Root CA Key Update
       4.4.1.  CA Operator Actions
       4.4.2.  Verifying Certificates
         4.4.2.1.  Verification in Cases 1 and 4
         4.4.2.2.  Verification in Case 2
         4.4.2.3.  Verification in Case 3
       4.4.3.  Revocation - Change of the CA Key
     4.5.  EKU for PKI Entities
   5.  Data Structures
     5.1.  Overall PKI Message
       5.1.1.  PKI Message Header
         5.1.1.1.  ImplicitConfirm
         5.1.1.2.  ConfirmWaitTime
         5.1.1.3.  OrigPKIMessage
         5.1.1.4.  CertProfile
         5.1.1.5.  KemCiphertextInfo
       5.1.2.  PKI Message Body
       5.1.3.  PKI Message Protection
         5.1.3.1.  Shared Secret Information
         5.1.3.2.  DH Key Pairs
         5.1.3.3.  Signature
         5.1.3.4.  Key Encapsulation
         5.1.3.5.  Multiple Protection
     5.2.  Common Data Structures
       5.2.1.  Requested Certificate Contents
       5.2.2.  Encrypted Values
       5.2.3.  Status Codes and Failure Information for PKI Messages
       5.2.4.  Certificate Identification
       5.2.5.  Out-of-Band Root CA Public Key
       5.2.6.  Archive Options
       5.2.7.  Publication Information
       5.2.8.  POP Structures
         5.2.8.1.  raVerified
         5.2.8.2.  POPOSigningKey Structure
         5.2.8.3.  POPOPrivKey Structure
         5.2.8.4.  Summary of POP Options
       5.2.9.  GeneralizedTime
     5.3.  Operation-Specific Data Structures
       5.3.1.  Initialization Request
       5.3.2.  Initialization Response
       5.3.3.  Certification Request
       5.3.4.  Certification Response
       5.3.5.  Key Update Request Content
       5.3.6.  Key Update Response Content
       5.3.7.  Key Recovery Request Content
       5.3.8.  Key Recovery Response Content
       5.3.9.  Revocation Request Content
       5.3.10. Revocation Response Content
       5.3.11. Cross-Certification Request Content
       5.3.12. Cross-Certification Response Content
       5.3.13. CA Key Update Announcement Content
       5.3.14. Certificate Announcement
       5.3.15. Revocation Announcement
       5.3.16. CRL Announcement
       5.3.17. PKI Confirmation Content
       5.3.18. Certificate Confirmation Content
       5.3.19. PKI General Message Content
         5.3.19.1.  CA Protocol Encryption Certificate
         5.3.19.2.  Signing Key Pair Types
         5.3.19.3.  Encryption / Key Agreement Key Pair Types
         5.3.19.4.  Preferred Symmetric Algorithm
         5.3.19.5.  Updated CA Key Pair
         5.3.19.6.  CRL
         5.3.19.7.  Unsupported Object Identifiers
         5.3.19.8.  Key Pair Parameters
         5.3.19.9.  Revocation Passphrase
         5.3.19.10. ImplicitConfirm
         5.3.19.11. ConfirmWaitTime
         5.3.19.12. Original PKIMessage
         5.3.19.13. Supported Language Tags
         5.3.19.14. CA Certificates
         5.3.19.15. Root CA Update
         5.3.19.16. Certificate Request Template
         5.3.19.17. CRL Update Retrieval
         5.3.19.18. KEM Ciphertext
       5.3.20. PKI General Response Content
       5.3.21. Error Message Content
       5.3.22. Polling Request and Response
   6.  Mandatory PKI Management Functions
     6.1.  Root CA Initialization
     6.2.  Root CA Key Update
     6.3.  Subordinate CA Initialization
     6.4.  CRL Production
     6.5.  PKI Information Request
     6.6.  Cross-Certification
       6.6.1.  One-Way Request-Response Scheme
     6.7.  End Entity Initialization
       6.7.1.  Acquisition of PKI Information
       6.7.2.  Out-of-Band Verification of the Root CA Key
     6.8.  Certificate Request
     6.9.  Key Update
   7.  Version Negotiation
     7.1.  Supporting RFC 2510 Implementations
       7.1.1.  Clients Talking to RFC 2510 Servers
       7.1.2.  Servers Receiving Version cmp1999 PKIMessages
   8.  Security Considerations
     8.1.  On the Necessity of POP
     8.2.  POP with a Decryption Key
     8.3.  POP by Exposing the Private Key
     8.4.  Attack Against DH Key Exchange
     8.5.  Perfect Forward Secrecy
     8.6.  Private Keys for Certificate Signing and CMP Message
            Protection
     8.7.  Entropy of Random Numbers, Key Pairs, and Shared Secret
            Information
     8.8.  Recurring Usage of KEM Keys for Message Protection
     8.9.  Trust Anchor Provisioning Using CMP Messages
     8.10. Authorizing Requests for Certificates with Specific EKUs
     8.11. Usage of CT Logs
   9.  IANA Considerations
   10. References
     10.1.  Normative References
     10.2.  Informative References
   Appendix A.  Reasons for the Presence of RAs
   Appendix B.  The Use of Revocation Passphrase
   Appendix C.  PKI Management Message Profiles (REQUIRED)
     C.1.  General Rules for Interpretation of These Profiles
     C.2.  Algorithm Use Profile
     C.3.  POP Profile
     C.4.  Initial Registration/Certification (Basic Authenticated
           Scheme)
     C.5.  Certificate Request
     C.6.  Key Update Request
   Appendix D.  PKI Management Message Profiles (OPTIONAL)
     D.1.  General Rules for Interpretation of These Profiles
     D.2.  Algorithm Use Profile
     D.3.  Self-Signed Certificates
     D.4.  Root CA Key Update
     D.5.  PKI Information Request/Response
     D.6.  Cross-Certification Request/Response (1-way)
     D.7.  In-Band Initialization Using External Identity Certificate
   Appendix E.  Variants of Using KEM Keys for PKI Message Protection
   Appendix F.  Compilable ASN.1 Definitions
   Acknowledgements
   Authors' Addresses

1.  Introduction

   This document describes the Internet X.509 PKI CMP.  Protocol
   messages are defined for certificate creation and management.  The
   term "certificate" in this document refers to an X.509v3 certificate
   as defined in [RFC5280].

1.1.  Changes Made by RFC 4210

   [RFC4210] differs from [RFC2510] in the following areas:

   *  The PKI management message profile section is split to two
      appendices: the required profile and the optional profile.  Some
      of the formerly mandatory functionality is moved to the optional
      profile.

   *  The message confirmation mechanism has changed substantially.

   *  A new polling mechanism is introduced, deprecating the old polling
      method at the CMP transport level.

   *  The CMP transport protocol issues are handled in a separate
      document [RFC6712], thus the "Transports" section is removed.

   *  A new implicit confirmation method is introduced to reduce the
      number of protocol messages exchanged in a transaction.

   *  The new specification contains some less prominent protocol
      enhancements and improved explanatory text on several issues.

1.2.  Updates Made by RFC 9480

   CMP Updates [RFC9480] and CMP Algorithms [RFC9481] updated [RFC4210],
   supporting the PKI management operations specified in the Lightweight
   CMP Profile [RFC9483], in the following areas:

   *  Added new extended key usages (EKUs) for various CMP server types,
      e.g., RA and CA, to express the authorization of the certificate
      holder that acts as the indicated type of PKI management entity.

   *  Extended the description of multiple protection to cover
      additional use cases, e.g., batch processing of messages.

   *  Used the Cryptographic Message Syntax (CMS) [RFC5652] type
      EnvelopedData as the preferred choice instead of EncryptedValue to
      better support crypto agility in CMP.

      For reasons of completeness and consistency, the type
      EncryptedValue has been exchanged in all occurrences.  This
      includes the protection of centrally generated private keys,
      encryption of certificates, Proof-of-Possession (POP) methods, and
      protection of revocation passphrases.  To properly differentiate
      the support of EnvelopedData instead of EncryptedValue, CMP
      version 3 is introduced in case a transaction is supposed to use
      EnvelopedData.

      Note: According to point 9 in Section 2.1 of [RFC4211], the use of
      the EncryptedValue structure has been deprecated in favor of the
      EnvelopedData structure.  [RFC4211] offers the EncryptedKey
      structure a choice of EncryptedValue and EnvelopedData for
      migration to EnvelopedData.

   *  Offered an optional hashAlg field in CertStatus supporting cases
      when a certificate needs to be confirmed, but the certificate was
      signed using a signature algorithm that does not indicate a
      specific hash algorithm to use for computing the certHash.  This
      is also in preparation for upcoming post-quantum algorithms.

   *  Added new general message types to request CA certificates, a root
      CA update, a certificate request template, or Certificate
      Revocation List (CRL) updates.

   *  Extended the use of polling to p10cr, certConf, rr, genm, and
      error messages.

   *  Deleted the mandatory algorithm profile in Appendix C.2 and
      instead referred to Section 7 of [RFC9481].

   *  Added Sections 8.6, 8.7, 8.9, and 8.10 to the security
      considerations.

1.3.  Changes Made by This Document

   This document obsoletes [RFC4210] and [RFC9480].

   Backward compatibility with CMP version 2 is maintained wherever
   possible.  Updates to CMP version 2 improve crypto agility, extend
   the polling mechanism, add new general message types, and add EKUs to
   identify special CMP server authorizations.  CMP version 3 is
   introduced for changes to the ASN.1 syntax, which support
   EnvelopedData, certConf with hashAlg, POPOPrivKey with agreeMAC, and
   RootCaKeyUpdateContent in ckuann messages.

   The updates made in this document include the changes specified by
   Section 2 and Appendix A.2 of [RFC9480] as described in Section 1.2.
   Additionally, this document updates the content of [RFC4210] in the
   following areas:

   *  Added Section 3.1.1.4 introducing the Key Generation Authority
      (KGA).

   *  Extended Section 3.1.2 regarding use of Certificate Transparency
      (CT) logs.

   *  Updated Section 4.4 introducing RootCaKeyUpdateContent as an
      alternative to using a repository to acquire new root CA
      certificates.

   *  Added Section 5.1.1.3 containing a description of origPKIMessage
      content, moved here from Section 5.1.3.4.

   *  Added support for KEM keys for POP to Sections 4.3 and 5.2.8, for
      message protection to Sections 5.1.1 and 5.1.3.4 and Appendix E,
      and for usage with CMS EnvelopedData to Section 5.2.2.

   *  Deprecated CAKeyUpdAnnContent in favor of RootCaKeyUpdateContent.

   *  Incorporated the request message behavioral clarifications from
      Appendix C of [RFC4210] to Section 5.  The definition of
      altCertTemplate was incorporated into Section 5.2.1, and the
      clarification on POPOSigningKey and on POPOPrivKey was
      incorporated into Section 5.2.8.

   *  Added support for CMS EnvelopedData to different POP methods for
      transferring encrypted private keys, certificates, and challenges
      to Section 5.2.8.

   *  Added Sections 8.1, 8.5, 8.8, and 8.11 to the security
      considerations.

2.  Terminology and Abbreviations

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

   This document relies on the terminology defined in [RFC5280].  The
   most important abbreviations are listed below:

   CA:  Certification Authority

   CMP:  Certificate Management Protocol

   CMS:  Cryptographic Message Syntax

   CRL:  Certificate Revocation List

   CRMF:  Certificate Request Message Format

   KEM:  Key Encapsulation Mechanism

   KGA:  Key Generation Authority

   LRA:  Local Registration Authority

   MAC:  Message Authentication Code

   PKI:  Public Key Infrastructure

   POP:  Proof-of-Possession

   RA:  Registration Authority

   TEE:  Trusted Execution Environment

3.  PKI Management Overview

   The PKI must be structured to be consistent with the types of
   individuals who must administer it.  Providing such administrators
   with unbounded choices not only complicates the software required but
   also increases the chances that a subtle mistake by an administrator
   or software developer will result in broader compromise.  Similarly,
   restricting administrators with cumbersome mechanisms will cause them
   not to use the PKI.

   Management protocols are REQUIRED to support online interactions
   between PKI components.  For example, a management protocol might be
   used between a CA and a client system with which a key pair is
   associated or between two CAs that issue cross-certificates for each
   other.

3.1.  PKI Management Model

   Before specifying particular message formats and procedures, we first
   define the entities involved in PKI management and their interactions
   (in terms of the PKI management functions required).  We then group
   these functions in order to accommodate different identifiable types
   of end entities.

3.1.1.  Definitions of PKI Entities

   The entities involved in PKI management include the end entity (i.e.,
   the entity to whom the certificate is issued) and the CA (i.e., the
   entity that issues the certificate).  An RA might also be involved in
   PKI management.

3.1.1.1.  Subjects and End Entities

   The term "subject" is used here to refer to the entity to whom the
   certificate is issued, typically named in the subject or
   subjectAltName field of a certificate.  When we wish to distinguish
   the tools and/or software used by the subject (e.g., a local
   certificate management module), we will use the term "subject
   equipment".  In general, the term "end entity", rather than
   "subject", is preferred in order to avoid confusion with the field
   name.  It is important to note that the end entities here will
   include not only human users of applications but also applications
   themselves (e.g., for Internet Key Exchange Protocol (IKE) / IPsec)
   or devices (e.g., routers or industrial control systems).  This
   factor influences the protocols that the PKI management operations
   use; for example, application software is far more likely to know
   exactly which certificate extensions are required than are human
   users.  PKI management entities are also end entities in the sense
   that they are sometimes named in the subject or subjectAltName field
   of a certificate or cross-certificate.  Where appropriate, the term
   "end entity" will be used to refer to end entities who are not PKI
   management entities.

   All end entities require secure local access to some information --
   at a minimum, their own name and private key, the name of a CA that
   is directly trusted by this entity, and that CA's public key (or a
   fingerprint of the public key where a self-certified version is
   available elsewhere).  Implementations MAY use secure local storage
   for more than this minimum (e.g., the end entity's own certificates
   or application-specific information).  The form of storage will also
   vary -- from files to tamper-resistant cryptographic tokens.  The
   information stored in such local, trusted storage is referred to here
   as the end entity's TEE, also known as Personal Security Environment
   (PSE).

   Though TEE formats are beyond the scope of this document (they are
   very dependent on equipment, et cetera), a generic interchange format
   for TEEs is defined here: a certification response message (see
   Section 5.3.4) MAY be used.

3.1.1.2.  Certification Authority

   The CA may or may not actually be a real "third party" from the end
   entity's point of view.  Quite often, the CA will actually belong to
   the same organization as the end entities it supports.

   Again, we use the term "CA" to refer to the entity named in the
   issuer field of a certificate.  When it is necessary to distinguish
   the software or hardware tools used by the CA, we use the term "CA
   equipment".

   The CA equipment will often include both an "offline" component and
   an "online" component, with the CA private key only available to the
   "offline" component.  This is, however, a matter for implementers
   (though it is also relevant as a policy issue).

   We use the term "root CA" to indicate a CA that is directly trusted
   by an end entity; that is, securely acquiring the value of a root CA
   public key requires some out-of-band step(s).  This term is not meant
   to imply that a root CA is necessarily at the top of any hierarchy,
   simply that the CA in question is trusted directly.  The "root CA"
   may provide its trust anchor information with or without using a
   certificate.  In some circumstances, such a certificate may be self-
   signed, but in other circumstances, it may be cross-signed, signed by
   a peer, signed by a superior CA, or unsigned.

   Note that other documents like [X509.2019] and [RFC5280] use the term
   "trusted CA" or "trust anchor" instead of "root CA".  This document
   continues using "root CA" based on the above definition because it is
   also present in the ASN.1 syntax that cannot be changed easily.

   A "subordinate CA" is one that is not a root CA for the end entity in
   question.  Often, a subordinate CA will not be a root CA for any
   entity, but this is not mandatory.

3.1.1.3.  Registration Authority

   In addition to end entities and CAs, many environments call for the
   existence of an RA separate from the CA.  The functions that the RA
   may carry out will vary from case to case but MAY include identity
   checking, token distribution, checking certificate requests and
   authentication of their origin, revocation reporting, name
   assignment, archival of key pairs, et cetera.

   This document views the RA as an OPTIONAL component: When it is not
   present, the CA is assumed to be able to carry out the RA's functions
   so that the PKI management protocols are the same from the end
   entity's point of view.

   Again, we distinguish, where necessary, between the RA and the tools
   used (the "RA equipment").

   Note that an RA is itself an end entity.  We further assume that all
   RAs are in fact certified end entities and that RAs have private keys
   that are usable for signing.  How a particular CA equipment
   identifies some end entities as RAs is an implementation issue (i.e.,
   this document specifies no special RA certification operation).  We
   do not mandate that the RA is certified by the CA with which it is
   interacting at the moment (so one RA may work with more than one CA
   whilst only being certified once).

   In some circumstances, end entities will communicate directly with a
   CA even where an RA is present.  For example, for initial
   registration and/or certification, the end entity may use its RA but
   communicate directly with the CA in order to refresh its certificate.

3.1.1.4.  Key Generation Authority

   A KGA is a PKI management entity generating key pairs on behalf of an
   end entity.  As the KGA generates the key pair, it knows the public
   and the private part.

   This document views the KGA as an OPTIONAL component.  When it is not
   present and central key generation is needed, the CA is assumed to be
   able to carry out the KGA's functions so that the PKI management
   protocol messages are the same from the end entity's point of view.
   If certain tasks of a CA are delegated to other components, this
   delegation needs authorization, which can be indicated by EKUs (see
   Section 4.5).

   Note: When doing central generation of key pairs, implementers should
   consider the implications of server-side retention on the overall
   security of the system; in some cases, retention is good, for
   example, for escrow reasons, but in other cases, the server should
   clear its copy after delivery to the end entity.

   Note: If the CA delegates key generation to a KGA, the KGA can be
   collocated with the RA.

3.1.2.  PKI Management Requirements

   The protocols given here meet the following requirements on PKI
   management

   1.   PKI management must conform to the ISO/IEC 9594-8/ITU-T X.509
        standards, in particular [X509.2019].

   2.   It must be possible to regularly update any key pair without
        affecting any other key pair.

   3.   The use of confidentiality in PKI management protocols must be
        kept to a minimum in order to ease acceptance in environments
        where strong confidentiality might cause regulatory problems.

   4.   PKI management protocols must allow the use of different
        industry-standard cryptographic algorithms (see CMP Algorithms
        [RFC9481]).  This means that any given CA, RA, or end entity
        may, in principle, use whichever algorithms suit it for its own
        key pair(s).

   5.   PKI management protocols must not preclude the generation of key
        pairs by the end entity concerned, by a KGA, or by a CA.  Key
        generation may also occur elsewhere, but for the purposes of PKI
        management, we can regard key generation as occurring wherever
        the key is first present at an end entity, KGA, or CA.

   6.   PKI management protocols must support the publication of
        certificates by the end entity concerned, by an RA, or by a CA.
        Different implementations and different environments may choose
        any of the above approaches.

   7.   PKI management protocols must support the production of
        Certificate Revocation Lists (CRLs) by allowing certified end
        entities to make requests for the revocation of certificates.
        This must be done in such a way that the denial-of-service
        attacks, which are possible, are not made simpler.

   8.   PKI management protocols must be usable over a variety of
        "transport" mechanisms, specifically including email, Hypertext
        Transfer Protocol (HTTP), Message Queuing Telemetry Transport
        (MQTT), Constrained Application Protocol (CoAP), and various
        offline and non-networked file transfer methods.

   9.   Final authority for certification creation rests with the CA.
        No RA or end entity equipment can assume that any certificate
        issued by a CA will contain what was requested; a CA may alter
        certificate field values or may add, delete, or alter extensions
        according to its operating policy.  In other words, all PKI
        entities (end entities, RAs, KGAs, and CAs) must be capable of
        handling responses to requests for certificates in which the
        actual certificate issued is different from that requested (for
        example, a CA may shorten the validity period requested).  Note
        that policy may dictate that the CA must not publish or
        otherwise distribute the certificate until the requesting entity
        has reviewed and accepted the newly created certificate or the
        POP is completed.  In case of publication of the certificate
        (when using indirect POP, see Section 8.11) or a precertificate
        in a CT log [RFC9162], the certificate must be revoked if it was
        not accepted by the end entity or the POP could not be
        completed.

   10.  A graceful, scheduled changeover from one non-compromised CA key
        pair to the next (CA key update) must be supported (note that if
        the CA key is compromised, re-initialization must be performed
        for all entities in the domain of that CA).  An end entity whose
        TEE contains the new CA public key (following a CA key update)
        may also need to be able to verify certificates verifiable using
        the old public key.  End entities who directly trust the old CA
        key pair may also need to be able to verify certificates signed
        using the new CA private key (required for situations where the
        old CA public key is "hardwired" into the end entity's
        cryptographic equipment).

   11.  The functions of an RA may, in some implementations or
        environments, be carried out by the CA itself.  The protocols
        must be designed so that end entities will use the same protocol
        regardless of whether the communication is with an RA or CA.
        Naturally, the end entity must use the correct RA or CA public
        key to verify the protection of the communication.

   12.  Where an end entity requests a certificate containing a given
        public key value, the end entity must be ready to demonstrate
        possession of the corresponding private key value.  This may be
        accomplished in various ways, depending on the type of
        certification request.  See Section 4.3 for details of the in-
        band methods defined for the PKIX-CMP (i.e., CMP) messages.

3.1.3.  PKI Management Operations

   The following diagram shows the relationship between the entities
   defined above in terms of the PKI management operations.  The letters
   in the diagram indicate "protocols" in the sense that a defined set
   of PKI management messages can be sent along each of the lettered
   lines.

     +---+     cert. publish        +------------+      j
     |   |  <---------------------  | End Entity | <-------
     | C |             g            +------------+      "out-of-band"
     | e |                            | ^                loading
     | r |                            | |      initial
     | t |                          a | | b     registration/
     |   |                            | |       certification
     | / |                            | |      key pair recovery
     |   |                            | |      key pair update
     | C |                            | |      certificate update
     | R |  PKI "USERS"               V |      revocation request
     | L | -------------------+-+-----+-+------+-+-------------------
     |   |  PKI MANAGEMENT    | ^              | ^
     |   |    ENTITIES      a | | b          a | | b
     | R |                    V |              | |
     | e |             g   +------+    d       | |
     | p |   <------------ | RA   | <-----+    | |
     | o |      cert.      |      | ----+ |    | |
     | s |       publish   +------+   c | |    | |
     | i |                              | |    | |
     | t |                              V |    V |
     | o |          g                 +------------+   i
     | r |   <------------------------|     CA     |------->
     | y |          h                 +------------+  "out-of-band"
     |   |      cert. publish              | ^         publication
     |   |      CRL publish                | |
     +---+                                 | |    cross-certification
                                         e | | f  cross-certificate
                                           | |       update
                                           | |
                                           V |
                                         +------+
                                         | CA-2 |
                                         +------+

                           Figure 1: PKI Entities

   At a high level, the set of operations for which management messages
   are defined can be grouped as follows.

   1.  CA establishment: When establishing a new CA, certain steps are
       required (e.g., production of initial CRLs and export of CA
       public key).

   2.  End entity initialization: This includes importing a root CA
       public key and requesting information about the options supported
       by a PKI management entity.

   3.  Certification: Various operations result in the creation of new
       certificates:

       a.  initial registration/certification: This is the process
           whereby an end entity first makes itself known to a CA or RA,
           prior to the CA issuing a certificate or certificates for
           that end entity.  The end result of this process (when it is
           successful) is that a CA issues a certificate for an end
           entity's public key and returns that certificate to the end
           entity and/or posts that certificate in a repository.  This
           process may, and typically will, involve multiple "steps",
           possibly including an initialization of the end entity's
           equipment.  For example, the end entity's equipment must be
           securely initialized with the public key of a CA, e.g., using
           zero-touch methods like Bootstrapping Remote Secure Key
           Infrastructure (BRSKI) [RFC8995] or Secure Zero Touch
           Provisioning (SZTP) [RFC8572], to be used in validating
           certificate paths.  Furthermore, an end entity typically
           needs to be initialized with its own key pair(s).

       b.  key pair update: Every key pair needs to be updated regularly
           (i.e., replaced with a new key pair), and a new certificate
           needs to be issued.

       c.  certificate update: As certificates expire, they may be
           "refreshed" if nothing relevant in the environment has
           changed.

       d.  CA key pair update: As with end entities, CA key pairs need
           to be updated regularly; however, different mechanisms are
           required.

       e.  cross-certification request: One CA requests issuance of a
           cross-certificate from another CA.  For the purposes of this
           standard, the following terms are defined.  A "cross-
           certificate" is a certificate in which the subject CA and the
           issuer CA are distinct and SubjectPublicKeyInfo contains a
           verification key (i.e., the certificate has been issued for
           the subject CA's signing key pair).  When it is necessary to
           distinguish more finely, the following terms may be used: A
           cross-certificate is called an "inter-domain cross-
           certificate" if the subject and issuer CAs belong to
           different administrative domains; it is called an "intra-
           domain cross-certificate" otherwise.

           Note 1:  The above definition of "cross-certificate" aligns
                    with the defined term "CA-certificate" in X.509.
                    Note that this term is not to be confused with the
                    X.500 "cACertificate" attribute type, which is
                    unrelated.

           Note 2:  In many environments, the term "cross-certificate",
                    unless further qualified, will be understood to be
                    synonymous with "inter-domain cross-certificate" as
                    defined above.

           Note 3:  Issuance of cross-certificates may be, but is not
                    necessarily, mutual; that is, two CAs may issue
                    cross-certificates for each other.

       f.  cross-certificate update: Similar to a normal certificate
           update but involving a cross-certificate.

   4.  Certificate/CRL discovery operations: Some PKI management
       operations result in the publication of certificates or CRLs:

       a.  certificate publication: Having gone to the trouble of
           producing a certificate, some means for publishing may be
           needed.  The "means" defined in PKIX MAY involve the messages
           specified in Sections 5.3.13 to 5.3.16 or MAY involve other
           methods (for example, Lightweight Directory Access Protocol
           (LDAP)) as described in [RFC4511] or [RFC2585] (the
           "Operational Protocols" documents of the PKIX series of
           specifications).

       b.  CRL publication: As for certificate publication.

   5.  Recovery operations: Some PKI management operations are used when
       an end entity has "lost" its TEE:

       a.  key pair recovery: As an option, user client key materials
           (e.g., a user's private key used for decryption purposes) MAY
           be backed up by a CA, an RA, or a key backup system
           associated with a CA or RA.  If an entity needs to recover
           these backed up key materials (e.g., as a result of a
           forgotten password or a lost key chain file), a protocol
           exchange may be needed to support such recovery.

   6.  Revocation operations: Some PKI management operations result in
       the creation of new CRL entries and/or new CRLs:

       a.  revocation request: An authorized person advises a CA of an
           abnormal situation requiring certificate revocation.

   7.  TEE operations: Whilst the definition of TEE operations (e.g.,
       moving a TEE, changing a PIN, etc.) are beyond the scope of this
       specification, we do define a PKIMessage (CertRepMessage) that
       can form the basis of such operations.

   Note that online protocols are not the only way of implementing the
   above operations.  For all operations, there are offline methods of
   achieving the same result, and this specification does not mandate
   use of online protocols.  For example, when hardware tokens are used,
   many of the operations MAY be achieved as part of the physical token
   delivery.

   Later sections define a set of standard messages supporting the above
   operations.  Transfer protocols for conveying these exchanges in
   various environments (e.g., offline: file-based; online: email, HTTP
   [RFC9811], MQTT, and CoAP [RFC9482]) are beyond the scope of this
   document and must be specified separately.  Appropriate transfer
   protocols MUST be capable of delivering the CMP messages reliably.

   CMP provides inbuilt integrity protection and authentication.  The
   information communicated unencrypted in CMP messages does not contain
   sensitive information endangering the security of the PKI when
   intercepted.  However, it might be possible for an eavesdropper to
   utilize the available information to gather confidential technical or
   business-critical information.  Therefore, users should consider
   protection of confidentiality on lower levels of the protocol stack,
   e.g., by using TLS [RFC8446], DTLS [RFC9147], or IPsec
   [RFC7296][RFC4303].

4.  Assumptions and Restrictions

4.1.  End Entity Initialization

   The first step for an end entity in dealing with PKI management
   entities is to request information about the PKI functions supported
   and to securely acquire a copy of the relevant root CA public key(s).

4.2.  Initial Registration/Certification

   There are many schemes that can be used to achieve initial
   registration and certification of end entities.  No one method is
   suitable for all situations due to the range of policies that a CA
   may implement and the variation in the types of end entity that can
   occur.

   However, we can classify the initial registration/certification
   schemes that are supported by this specification.  Note that the word
   "initial", above, is crucial: We are dealing with the situation where
   the end entity in question has had no previous contact with the PKI,
   except having received the root CA certificate of that PKI by some
   zero-touch method like BRSKI [RFC8995] [RFC9733] or SZTP [RFC8572].
   In case the end entity already possesses certified keys, then some
   simplifications/alternatives are possible.

   Having classified the schemes that are supported by this
   specification, we can then specify some as mandatory and some as
   optional.  The goal is that the mandatory schemes cover a sufficient
   number of the cases that will arise in real use, whilst the optional
   schemes are available for special cases that arise less frequently.
   In this way, we achieve a balance between flexibility and ease of
   implementation.

   Further classification of mandatory and optional schemes addressing
   different environments is available, e.g., in Appendices C and D of
   this specification on managing human user certificates as well as in
   the Lightweight CMP Profile [RFC9483] on fully automating certificate
   management in a machine-to-machine and Internet of Things (IoT)
   environment.  Industry standards such as [ETSI-3GPP.33.310] for
   mobile networks and [UNISIG.Subset-137] for railroad automation have
   adopted CMP and defined a series of mandatory schemes for their use
   cases.

   We will now describe the classification of initial registration/
   certification schemes.

4.2.1.  Criteria Used

4.2.1.1.  Initiation of Registration/Certification

   In terms of the PKI messages that are produced, we can regard the
   initiation of the initial registration/certification exchanges as
   occurring wherever the first PKI message relating to the end entity
   is produced.  Note that the real-world initiation of the
   registration/certification procedure may occur elsewhere (e.g., a
   personnel department may telephone an RA operator or use zero-touch
   methods like BRSKI [RFC8995] or SZTP [RFC8572]).

   The possible locations are at the end entity, an RA, or a CA.

4.2.1.2.  End Entity Message Origin Authentication

   The online messages produced by the end entity that requires a
   certificate may be authenticated or not.  The requirement here is to
   authenticate the origin of any messages from the end entity to the
   PKI (CA/RA).

   In this specification, such authentication is achieved by two
   different means:

   *  symmetric: The PKI (CA/RA) issuing the end entity with a secret
      value (initial authentication key) and reference value (used to
      identify the secret value) via some out-of-band means.  The
      initial authentication key can then be used to protect relevant
      PKI messages.

   *  asymmetric: Using a private key and certificate issued by another
      PKI trusted for initial authentication, e.g., an Initial Device
      Identifier (IDevID) IEEE 802.1AR [IEEE.802.1AR-2018].  The trust
      establishment in this external PKI is out of scope of this
      document.

   Thus, we can classify the initial registration/certification scheme
   according to whether or not the online 'end entity -> PKI management
   entity' messages are authenticated or not.

   Note 1:  We do not discuss the authentication of the 'PKI management
            entity -> end entity' messages here, as this is always
            REQUIRED.  In any case, it can be achieved simply once the
            root-CA public key has been installed at the end entity's
            equipment or it can be based on the initial authentication
            key.

   Note 2:  An initial registration/certification procedure can be
            secure where the messages from the end entity are
            authenticated via some out-of-band means (e.g., a subsequent
            visit).

4.2.1.3.  Location of Key Generation

   In this specification, "key generation" is regarded as occurring
   wherever either the public or private component of a key pair first
   occurs in a PKIMessage.  Note that this does not preclude a
   centralized key generation service by a KGA; the actual key pair MAY
   have been generated elsewhere and transported to the end entity, RA,
   or CA using a (proprietary or standardized) key generation request/
   response protocol (outside the scope of this specification).

   Thus, there are three possibilities for the location of "key
   generation": the end entity, a KGA, or a CA.

4.2.1.4.  Confirmation of Successful Certification

   Following the creation of a certificate for an end entity, additional
   assurance can be gained by having the end entity explicitly confirm
   successful receipt of the message containing (or indicating the
   creation of) the certificate.  Naturally, this confirmation message
   must be protected (based on the initial symmetric or asymmetric
   authentication key or other means).

   This gives two further possibilities: confirmed or not.

4.2.2.  Initial Registration/Certification Schemes

   The criteria above allow for a large number of initial registration/
   certification schemes.  Examples of possible initial registration/
   certification schemes can be found in the following subsections.  An
   entity may support other schemes specified in profiles of PKIX-CMP,
   such as Appendices C and D or [RFC9483].

4.2.2.1.  Centralized Scheme

   In terms of the classification above, this scheme is, in some ways,
   the simplest possible, where:

   *  initiation occurs at the certifying CA;

   *  no online message authentication is required;

   *  "key generation" occurs at the certifying CA (see
      Section 4.2.1.3); and

   *  no confirmation message is required.

   In terms of message flow, this scheme means that the only message
   required is sent from the CA to the end entity.  The message must
   contain the entire TEE for the end entity.  Some out-of-band means
   must be provided to allow the end entity to authenticate the message
   received and to decrypt any encrypted values.

4.2.2.2.  Basic Authenticated Scheme

   In terms of the classification above, this scheme is where:

   *  initiation occurs at the end entity;

   *  message authentication is required;

   *  "key generation" occurs at the end entity (see Section 4.2.1.3);
      and

   *  a confirmation message is recommended.

   Note: An Initial Authentication Key (IAK) can be either a symmetric
   key or an asymmetric private key with a certificate issued by another
   PKI trusted for this purpose.  The establishment of such trust is out
   of scope of this document.

   In terms of message flow, the basic authenticated scheme is as
   follows:

     End Entity                                              RA/CA
     ==========                                      =============
          out-of-band distribution of Initial Authentication
          Key (IAK) and reference value (RA/CA -> end entity)
     Key generation
     Creation of certification request
     Protect request with IAK
                   -----> certification request ----->
                                                    verify request
                                                    process request
                                                    create response
                   <----- certification response <-----
     handle response
     create confirmation
                   -----> cert conf message      ----->
                                                    verify confirmation
                                                    create response
                   <----- conf ack (optional)    <-----
     handle response

   Note: Where verification of the cert confirmation message fails, the
   RA/CA MUST revoke the newly issued certificate if it has been
   published or otherwise made available.

4.3.  POP of Private Key

   POP is where a PKI management entity (CA/RA) verifies if an end
   entity has access to the private key corresponding to a given public
   key.  The question of whether, and in what circumstances, POPs add
   value to a PKI is a debate as old as PKI itself!  See Section 8.1 for
   a further discussion on the necessity of POP in PKI.

   The PKI management operations specified here make it possible for an
   end entity to prove to a CA/RA that it has possession of (i.e., is
   able to use) the private key corresponding to the public key for
   which a certificate is requested (see Section 5.2.8 for different POP
   methods).  A given CA/RA is free to choose how to enforce POP (e.g.,
   out-of-band procedural means versus PKIX-CMP in-band messages) in its
   certification exchanges (i.e., this may be a policy issue).  However,
   it is REQUIRED that CAs/RAs MUST enforce POP by some means because
   there are currently many non-PKIX operational protocols in use
   (various electronic mail protocols are one example) that do not
   explicitly check the binding between the end entity and the private
   key.  Until operational protocols that do verify the binding (for
   signature, encryption, key agreement, and KEM key pairs) exist, and
   are ubiquitous, this binding can only be assumed to have been
   verified by the CA/RA.  Therefore, if the binding is not verified by
   the CA/RA, certificates in the Internet PKI end up being somewhat
   less meaningful.

   POP is accomplished in different ways depending upon the type of key
   for which a certificate is requested.  If a key can be used for
   multiple purposes (e.g., an RSA key), then any appropriate method MAY
   be used (e.g., a key that may be used for signing, as well as other
   purposes, MUST NOT be sent to the CA/RA in order to prove possession
   unless archival of the private key is explicitly desired).

   This specification explicitly allows for cases where an end entity
   supplies the relevant proof to an RA and the RA subsequently attests
   to the CA that the required proof has been received (and validated!).
   For example, an end entity wishing to have a signing key certified
   could send the appropriate signature to the RA, which then simply
   notifies the relevant CA that the end entity has supplied the
   required proof.  Of course, such a situation may be disallowed by
   some policies (e.g., CAs may be the only entities permitted to verify
   POP during certification).

4.3.1.  Signature Keys

   For signature keys, the end entity can sign a value to prove
   possession of the private key; see Section 5.2.8.2.

4.3.2.  Encryption Keys

   For encryption keys, the end entity can provide the private key to
   the CA/RA (e.g., for archiving), see Section 5.2.8.3.1, or can be
   required to decrypt a value in order to prove possession of the
   private key.  Decrypting a value can be achieved either directly (see
   Section 5.2.8.3.3) or indirectly (see Section 5.2.8.3.2).

   The direct method is for the RA/CA to issue a random challenge to
   which an immediate response by the end entity is required.

   The indirect method is to issue a certificate that is encrypted for
   the end entity (and have the end entity demonstrate its ability to
   decrypt this certificate in the confirmation message).  This allows a
   CA to issue a certificate in a form that can only be used by the
   intended end entity.

   This specification encourages use of the indirect method because it
   requires no extra messages to be sent (i.e., the proof can be
   demonstrated using the {request, response, confirmation} triple of
   messages).

4.3.3.  Key Agreement Keys

   For key agreement keys, the end entity and the PKI management entity
   (i.e., CA or RA) must establish a shared secret key in order to prove
   that the end entity has possession of the private key.

   Note that this need not impose any restrictions on the keys that can
   be certified by a given CA.  In particular, for Diffie-Hellman (DH)
   keys, the end entity may freely choose its algorithm parameters
   provided that the CA can generate a short-term (or one-time) key pair
   with the appropriate parameters when necessary.

4.3.4.  KEM Keys

   For KEM keys, the end entity can provide the private key to the CA/RA
   (e.g., for archiving), see Section 5.2.8.3.1, or can be required to
   decrypt a value in order to prove possession of the private key.
   Decrypting a value can be achieved either directly (see
   Section 5.2.8.3.3) or indirectly (see Section 5.2.8.3.2).

   Note: A definition of KEMs can be found in Section 1 of [RFC9629].

   The direct method is for the RA/CA to issue a random challenge to
   which an immediate response by the end entity is required.

   The indirect method is to issue a certificate that is encrypted for
   the end entity using a shared secret key derived from a key
   encapsulated using the public key (and have the end entity
   demonstrate its ability to use its private key for decapsulation of
   the KEM ciphertext, derive the shared secret key, decrypt this
   certificate, and provide a hash of the certificate in the
   confirmation message).  This allows a CA to issue a certificate in a
   form that can only be used by the intended end entity.

   This specification encourages use of the indirect method because it
   requires no extra messages to be sent (i.e., the proof can be
   demonstrated using the {request, response, confirmation} triple of
   messages).

   A certification request message for a KEM certificate SHALL use
   POPOPrivKey by using the keyEncipherment choice of ProofOfPossession
   (see Section 5.2.8) in the popo field of CertReqMsg as long as no
   KEM-specific choice is available.

4.4.  Root CA Key Update

   This discussion only applies to CAs that are directly trusted by some
   end entities.  Recognizing whether a self-signed or non-self-signed
   CA is supposed to be directly trusted for some end entities is a
   matter of CA policy and end entity configuration.  Thus, this is
   beyond the scope of this document.

   The basis of the procedure described here is that the CA protects its
   new public key using its previous private key and vice versa.  Thus,
   when a CA updates its key pair, it may generate two link
   certificates: "old with new" and "new with old".

   Note: The usage of link certificates has been shown to be very
   specific for each use case, and no assumptions are done on this
   aspect.  RootCaKeyUpdateContent is updated to specify these link
   certificates as optional.

   Note: When an LDAP directory is used to publish root CA updates, the
   old and new root CA certificates together with the two link
   certificates are stored as cACertificate attribute values.

   When a CA changes its key pair, those entities who have acquired the
   old CA public key via "out-of-band" means are most affected.  These
   end entities need to acquire the new CA public key in a trusted way.
   This may be achieved "out-of-band" by using a repository or by using
   online messages also containing the link certificates "new with old".
   Once the end entity acquired and properly verified the new CA public
   key, it must load the new trust anchor information into its trusted
   store.

   The data structure used to protect the new and old CA public keys is
   typically a standard X.509v3 certificate (which may also contain
   extensions).  There are no new data structures required.

   Note: Sometimes self-signed root CA certificates do not make use of
   X.509v3 extensions and may be X.509v1 certificates.  Therefore, a
   root CA key update must be able to work for version 1 certificates.
   The use of the X.509v3 KeyIdentifier extension is recommended for
   easier path building.

   Note: While the scheme could be generalized to cover cases where the
   CA updates its key pair more than once during the validity period of
   one of its end entities' certificates, this generalization seems of
   dubious value.  Not having this generalization simply means that the
   validity periods of certificates issued with the old CA key pair
   cannot exceed the end of the "old with new" certificate validity
   period.

   Note: This scheme offers a mechanism to ensures that end entities
   will acquire the new CA public key, at the latest by the expiry of
   the last certificate they owned that was signed with the old CA
   private key.  Certificate and/or key update operations occurring at
   other times do not necessarily require this (depending on the end
   entity's equipment).

   Note: In practice, a new root CA may have a slightly different
   subject Distinguished Name (DN), e.g., indicating a generation
   identifier like the year of issuance or a version number, for
   instance, in an Organizational Unit (OU) element.  How to bridge
   trust to the new root CA certificate in a CA DN change or a cross-
   certificate scenario is out of scope for this document.

4.4.1.  CA Operator Actions

   To change the key of the CA, the CA operator does the following:

   1.  Generate a new key pair.

   2.  Create a certificate containing the new CA public key signed with
       the new private key or by the private key of some other CA (the
       "new with new" certificate).

   3.  Optionally: Create a link certificate containing the new CA
       public key signed with the old private key (the "new with old"
       certificate).

   4.  Optionally: Create a link certificate containing the old CA
       public key signed with the new private key (the "old with new"
       certificate).

   5.  Publish these new certificates so that end entities may acquire
       it, e.g., using a repository or RootCaKeyUpdateContent.

   The old CA private key is then no longer required when the validity
   of the "old with old" certificate ended.  However, the old CA public
   key will remain in use for validating the "new with old" link
   certificate until the new CA public key is loaded into the trusted
   store.  The old CA public key is no longer required (other than for
   non-repudiation) when all end entities of this CA have securely
   acquired and stored the new CA public key.

   The "new with new" certificate must have a validity period with a
   notBefore time that is before the notAfter time of the "old with old"
   certificate and a notAfter time that is after the notBefore time of
   the next update of this certificate.

   The "new with old" certificate must have a validity period with the
   same notBefore time as the "new with new" certificate and a notAfter
   time by which all end entities of this CA will securely possess the
   new CA public key (at the latest, at the notAfter time of the "old
   with old" certificate).

   The "old with new" certificate must have a validity period with the
   same notBefore and notAfter time as the "old with old" certificate.

   Note: Further operational considerations on transition from one root
   CA self-signed certificate to the next is available in Section 5 of
   [RFC8649].

4.4.2.  Verifying Certificates

   Normally when verifying a signature, the verifier verifies (among
   other things) the certificate containing the public key of the
   signer.  However, once a CA is allowed to update its key, there are a
   range of new possibilities.  These are shown in the table below.

   +======================+======================+=====================+
   |                      | Verifier's TEE       | Verifier's TEE      |
   |                      | contains NEW public  | contains OLD        |
   |                      | key                  | public key          |
   +======================+======================+=====================+
   | Signer's certificate | Case 1: The verifier | Case 2: The         |
   | is protected using   | can directly verify  | verifier is         |
   | NEW key pair         | the certificate.     | missing the NEW     |
   |                      |                      | public key.         |
   +======================+----------------------+---------------------+
   | Signer's certificate | Case 3: The verifier | Case 4: The         |
   | is protected using   | is missing the OLD   | verifier can        |
   | OLD key pair         | public key.          | directly verify     |
   |                      |                      | the certificate.    |
   +======================+----------------------+---------------------+

                                  Table 1

4.4.2.1.  Verification in Cases 1 and 4

   In these cases, the verifier has a local copy of the CA public key
   that can be used to verify the certificate directly.  This is the
   same as the situation where no key change has occurred.

4.4.2.2.  Verification in Case 2

   In case 2, the verifier must get access to the new public key of the
   CA.  Case 2 will arise when the CA operator has issued the verifier's
   certificate, then changed the CA's key, and then issued the signer's
   certificate; so it is quite a typical case.

   The verifier does the following:

   1.  Get the "new with new" and "new with old" certificates.  The
       location of where to retrieve these certificates may be available
       in the authority information access extension of the "old with
       old" certificate (see the access method for caIssuers in
       Section 4.2.2.1 of [RFC5280]), or it may be locally configured.

       a.  If a repository is available, look up the certificates in the
           caCertificate attribute.

       b.  If an HTTP or FTP server is available, pick the certificates
           from the "certs-only" CMS message.

       c.  If a CMP server is available, request the certificates using
           the root CA update the general message (see
           Section 5.3.19.15).

       d.  Otherwise, get the certificates "out-of-band" using any
           trustworthy mechanism.

   2.  If the certificates are received, check that the validity periods
       and the subject and issuer fields match.  Verify the signatures
       using the old root CA key (which the verifier has locally).

   3.  If all checks are successful, securely store the new trust anchor
       information and validate the signer's certificate.

4.4.2.3.  Verification in Case 3

   In case 3, the verifier must get access to the old public key of the
   CA.  Case 3 will arise when the CA operator has issued the signer's
   certificate, then changed the key, and then issued the verifier's
   certificate.

   The verifier does the following:

   1.  Get the "old with new" certificate.  The location of where to
       retrieve these certificates may be available in the authority
       information access extension of the "new with new" certificate
       (see caIssuers access method in Section 4.2.2.1 of [RFC5280]), or
       it may be locally configured.

       a.  If a repository is available, look up the certificate in the
           caCertificate attribute.

       b.  If an HTTP or FTP server is available, pick the certificate
           from the "certs-only" CMS message.

       c.  If a CMP server and an untrusted copy of the old root CA
           certificate are available (e.g., the signer provided it in-
           band in the CMP extraCerts filed), request the certificate
           using the root CA update the general message (see
           Section 5.3.19.15).

       d.  Otherwise, get the certificate "out-of-band" using any
           trustworthy mechanism.

   2.  If the certificate is received, check that the validity periods
       and the subject and issuer fields match.  Verify the signatures
       using the new root CA key (which the verifier has locally).

   3.  If all checks were successful, securely store the old trust
       anchor information and validate the signer's certificate.

4.4.3.  Revocation - Change of the CA Key

   As we saw above, the verification of a certificate becomes more
   complex once the CA is allowed to change its key.  This is also true
   for revocation checks, as the CA may have signed the CRL using a
   newer private key than the one within the user's TEE.

   The analysis of the alternatives is the same as for certificate
   verification.

4.5.  EKU for PKI Entities

   The EKU extension indicates the purposes for which the certified key
   pair may be used.  Therefore, it restricts the use of a certificate
   to specific applications.

   A CA may want to delegate parts of its duties to other PKI management
   entities.  This section provides a mechanism to both prove this
   delegation and enable automated means for checking the authorization
   of this delegation.  Such delegation may also be expressed by other
   means, e.g., explicit configuration.

   To offer automatic validation for the delegation of a role by a CA to
   another entity, the certificates used for CMP message protection or
   signed data for central key generation MUST be issued by the
   delegating CA and MUST contain the respective EKUs.  This proves that
   the delegating CA authorized this entity to act in the given role, as
   described below.

   The OIDs to be used for these EKUs are:

     id-kp-cmcCA OBJECT IDENTIFIER ::= {
        iso(1) identified-organization(3) dod(6) internet(1)
        security(5) mechanisms(5) pkix(7) kp(3) 27 }

     id-kp-cmcRA OBJECT IDENTIFIER ::= {
        iso(1) identified-organization(3) dod(6) internet(1)
        security(5) mechanisms(5) pkix(7) kp(3) 28 }

     id-kp-cmKGA OBJECT IDENTIFIER ::= {
        iso(1) identified-organization(3) dod(6) internet(1)
        security(5) mechanisms(5) pkix(7) kp(3) 32 }

   Note: Section 2.10 of [RFC6402] specifies OIDs for a Certificate
   Management over CMS (CMC) CA and a CMC RA.  As the functionality of a
   CA and RA is not specific to any protocol used for managing
   certificates (such as CMC or CMP), these EKUs are reused by CMP.

   The meaning of the id-kp-cmKGA EKU is as follows:

   CMP KGA:  CMP KGAs are CAs or are identified by the id-kp-cmKGA EKU.
      The CMP KGA knows the private key it generated on behalf of the
      end entity.  This is a very sensitive service and needs specific
      authorization, which by default is with the CA certificate itself.
      The CA may delegate its authorization by placing the id-kp-cmKGA
      EKU in the certificate used to authenticate the origin of the
      generated private key.  The authorization may also be determined
      through local configuration of the end entity.

5.  Data Structures

   This section contains descriptions of the data structures required
   for PKI management messages.  Section 6 describes constraints on
   their values and the sequence of events for each of the various PKI
   management operations.

5.1.  Overall PKI Message

   All of the messages used in this specification for the purposes of
   PKI management use the following structure:

     PKIMessage ::= SEQUENCE {
        header           PKIHeader,
        body             PKIBody,
        protection   [0] PKIProtection OPTIONAL,
        extraCerts   [1] SEQUENCE SIZE (1..MAX) OF CMPCertificate
                          OPTIONAL
     }

     PKIMessages ::= SEQUENCE SIZE (1..MAX) OF PKIMessage

   The PKIHeader contains information that is common to many PKI
   messages.

   The PKIBody contains message-specific information.

   The PKIProtection, when used, contains bits that protect the PKI
   message.

   The extraCerts field can contain certificates that may be useful to
   the recipient.  For example, this can be used by a CA or RA to
   present an end entity with certificates that it needs to verify its
   own new certificate (for example, if the CA that issued the end
   entity's certificate is not a root CA for the end entity).  Note that
   this field does not necessarily contain a certification path; the
   recipient may have to sort, select from, or otherwise process the
   extra certificates in order to use them.

5.1.1.  PKI Message Header

   All PKI messages require some header information for addressing and
   transaction identification.  Some of this information will also be
   present in a transport-specific envelope.  However, if the PKI
   message is protected, then this information is also protected (i.e.,
   we make no assumption about secure transport).

   The following data structure is used to contain this information:

     PKIHeader ::= SEQUENCE {
        pvno                INTEGER     { cmp1999(1), cmp2000(2),
                                          cmp2021(3) },
        sender              GeneralName,
        recipient           GeneralName,
        messageTime     [0] GeneralizedTime         OPTIONAL,
        protectionAlg   [1] AlgorithmIdentifier{ALGORITHM, {...}}
                            OPTIONAL,
        senderKID       [2] KeyIdentifier           OPTIONAL,
        recipKID        [3] KeyIdentifier           OPTIONAL,
        transactionID   [4] OCTET STRING            OPTIONAL,
        senderNonce     [5] OCTET STRING            OPTIONAL,
        recipNonce      [6] OCTET STRING            OPTIONAL,
        freeText        [7] PKIFreeText             OPTIONAL,
        generalInfo     [8] SEQUENCE SIZE (1..MAX) OF
                            InfoTypeAndValue     OPTIONAL
     }

     PKIFreeText ::= SEQUENCE SIZE (1..MAX) OF UTF8String

   The usage of the protocol version number (pvno) is described in
   Section 7.

   The sender field contains the name of the sender of the PKIMessage.
   This name (in conjunction with senderKID, if supplied) should be
   sufficient to indicate the key to use to verify the protection on the
   message.  If nothing about the sender is known to the sending entity
   (e.g., in the initial request message, where the end entity may not
   know its own DN, email name, IP address, etc.), then the "sender"
   field MUST contain a "NULL-DN" value in the directoryName choice.  A
   "NULL-DN" is a SEQUENCE OF relative DNs of zero length and is encoded
   as 0x3000.  In such a case, the senderKID field MUST hold an
   identifier (i.e., a reference number) that indicates to the receiver
   the appropriate shared secret information to use to verify the
   message.

   The recipient field contains the name of the recipient of the
   PKIMessage.  This name (in conjunction with recipKID, if supplied)
   should be usable to verify the protection on the message.

   The protectionAlg field specifies the algorithm used to protect the
   message.  If no protection bits are supplied (note that PKIProtection
   is OPTIONAL), then this field MUST be omitted; if protection bits are
   supplied, then this field MUST be supplied.

   senderKID and recipKID are usable to indicate which keys have been
   used to protect the message (recipKID will normally only be required
   where protection of the message uses DH or Elliptic Curve Diffie-
   Hellman (ECDH) keys).  These fields MUST be used if required to
   uniquely identify a key (e.g., if more than one key is associated
   with a given sender name).  The senderKID SHOULD be used in any case.

   Note: The recommendation of using senderKID has changed since
   [RFC4210], where it was recommended to be omitted if not needed to
   identify the protection key.

   The transactionID field within the message header is to be used to
   allow the recipient of a message to correlate this with an ongoing
   transaction.  This is needed for all transactions that consist of
   more than just a single request/response pair.  For transactions that
   consist of a single request/response pair, the rules are as follows.
   A client MUST populate the transactionID field if the message
   contains an infoValue of type KemCiphertextInfo (see
   Section 5.1.3.4).  In all other cases, the client MAY populate the
   transactionID field of the request.  If a server receives such a
   request that has the transactionID field set, then it MUST set the
   transactionID field of the response to the same value.  If a server
   receives such request with a missing transactionID field, then it
   MUST populate the transactionID field if the message contains a
   KemCiphertextInfo field.  In all other cases, the server MAY set the
   transactionID field of the response.

   For transactions that consist of more than just a single request/
   response pair, the rules are as follows.  If the message contains an
   infoValue of type KemCiphertextInfo, the client MUST generate a
   transactionID; otherwise, the client SHOULD generate a transactionID
   for the first request.  If a server receives such a request that has
   the transactionID field set, then it MUST set the transactionID field
   of the response to the same value.  If a server receives such request
   with a missing transactionID field, then it MUST populate the
   transactionID field of the response with a server-generated ID.
   Subsequent requests and responses MUST all set the transactionID
   field to the thus established value.  In all cases where a
   transactionID is being used, a given client MUST NOT have more than
   one transaction with the same transactionID in progress at any time
   (to a given server).  Servers are free to require uniqueness of the
   transactionID or not, as long as they are able to correctly associate
   messages with the corresponding transaction.  Typically, this means
   that a server will require the {client, transactionID} tuple to be
   unique, or even the transactionID alone to be unique, if it cannot
   distinguish clients based on any transport-level information.  A
   server receiving the first message of a transaction (which requires
   more than a single request/response pair) that contains a
   transactionID that does not allow it to meet the above constraints
   (typically because the transactionID is already in use) MUST send
   back an ErrorMsgContent with a PKIFailureInfo of transactionIdInUse.
   It is RECOMMENDED that the clients fill the transactionID field with
   128 bits of (pseudo-)random data for the start of a transaction to
   reduce the probability of having the transactionID in use at the
   server.

   The senderNonce and recipNonce fields protect the PKIMessage against
   replay attacks.  The senderNonce will typically be 128 bits of
   (pseudo-)random data generated by the sender, whereas the recipNonce
   is copied from the senderNonce field of the previous message in the
   transaction.

   The messageTime field contains the time at which the sender created
   the message.  This may be useful to allow end entities to correct/
   check their local time for consistency with the time on a central
   system.

   The freeText field may be used to send a human-readable message to
   the recipient (in any number of languages).  Each UTF8String MAY
   include a language tag [RFC5646] to indicate the language of the
   contained text.  The first language used in this sequence indicates
   the desired language for replies.

   The generalInfo field may be used to send machine-processable
   additional data to the recipient.  The following generalInfo
   extensions are defined and MAY be supported.

5.1.1.1.  ImplicitConfirm

   This is used by the end entity to inform the CA or RA that it does
   not wish to send a certificate confirmation for issued certificates.

     id-it-implicitConfirm OBJECT IDENTIFIER ::= {id-it 13}
     ImplicitConfirmValue ::= NULL

   If the CA grants the request to the end entity, it MUST put the same
   extension in the PKIHeader of the response.  If the end entity does
   not find the extension in the response, it MUST send the certificate
   confirmation.

5.1.1.2.  ConfirmWaitTime

   This is used by the CA or RA to inform the end entity how long it
   intends to wait for the certificate confirmation before revoking the
   certificate and deleting the transaction.

     id-it-confirmWaitTime OBJECT IDENTIFIER ::= {id-it 14}
     ConfirmWaitTimeValue ::= GeneralizedTime

5.1.1.3.  OrigPKIMessage

   An RA MAY include the original PKIMessage from the end entity in the
   generalInfo field of the PKIHeader of a PKIMessage.  This is used by
   the RA to inform the CA of the original PKIMessage that it received
   from the end entity and modified in some way (e.g., added or modified
   particular field values or added new extensions) before forwarding
   the new PKIMessage.  This accommodates, for example, cases in which
   the CA wishes to check the message origin, the POP, or other
   information on the original end entity message.

   Note: If the changes made by the RA to the original PKIMessage break
   the POP of a certificate request, the RA can set the popo field of
   the new PKIMessage to raVerified (see Section 5.2.8.4).

   Unless the OrigPKIMessage infoValue is in the header of a nested
   message, it MUST contain exactly one PKIMessage.  The contents of
   OrigPKIMessage infoValue in the header of a nested message MAY
   contain multiple PKIMessage structures, which MUST be in the same
   order as the PKIMessage structures in PKIBody.

     id-it-origPKIMessage OBJECT IDENTIFIER ::= {id-it 15}
     OrigPKIMessageValue ::= PKIMessages

5.1.1.4.  CertProfile

   This is used by the end entity to indicate specific certificate
   profiles, e.g., when requesting a new certificate or a certificate
   request template (see Section 5.3.19.16).

     id-it-certProfile OBJECT IDENTIFIER ::= {id-it 21}
     CertProfileValue ::= SEQUENCE SIZE (1..MAX) OF UTF8String

   When used in a p10cr message, the CertProfileValue sequence MUST NOT
   contain multiple certificate profile names.  When used in an
   ir/cr/kur/genm message, the CertProfileValue sequence MUST NOT
   contain more certificate profile names than the number of CertReqMsg
   or GenMsgContent InfoTypeAndValue elements contained in the message
   body.

   The certificate profile names in the CertProfileValue sequence relate
   to the CertReqMsg or GenMsgContent InfoTypeAndValue elements in the
   given order.  An empty string means no certificate profile name is
   associated with the respective CertReqMsg or GenMsgContent
   InfoTypeAndValue element.  If the CertProfileValue sequence contains
   less certificate profile entries than CertReqMsg or GenMsgContent
   InfoTypeAndValue elements, the remaining CertReqMsg or GenMsgContent
   InfoTypeAndValue elements have no profile name associated with them.

5.1.1.5.  KemCiphertextInfo

   A PKI entity MAY provide the KEM ciphertext for MAC-based message
   protection using KEM (see Section 5.1.3.4) in the generalInfo field
   of a request message to a PKI management entity if it knows that the
   PKI management entity uses a KEM key pair and has its public key.

     id-it-KemCiphertextInfo OBJECT IDENTIFIER ::= { id-it 24 }
     KemCiphertextInfoValue ::= KemCiphertextInfo

   For more details of KEM-based message protection, see
   Section 5.1.3.4.  See Section 5.3.19.18 for the definition of {id-it
   24}.

5.1.2.  PKI Message Body

     PKIBody ::= CHOICE {
        ir       [0]  CertReqMessages,        --Initialization Req
        ip       [1]  CertRepMessage,         --Initialization Resp
        cr       [2]  CertReqMessages,        --Certification Req
        cp       [3]  CertRepMessage,         --Certification Resp
        p10cr    [4]  CertificationRequest,   --PKCS #10 Cert.  Req.
        popdecc  [5]  POPODecKeyChallContent, --pop Challenge
        popdecr  [6]  POPODecKeyRespContent,  --pop Response
        kur      [7]  CertReqMessages,        --Key Update Request
        kup      [8]  CertRepMessage,         --Key Update Response
        krr      [9]  CertReqMessages,        --Key Recovery Req
        krp      [10] KeyRecRepContent,       --Key Recovery Resp
        rr       [11] RevReqContent,          --Revocation Request
        rp       [12] RevRepContent,          --Revocation Response
        ccr      [13] CertReqMessages,        --Cross-Cert.  Request
        ccp      [14] CertRepMessage,         --Cross-Cert.  Resp
        ckuann   [15] CAKeyUpdContent,        --CA Key Update Ann.
        cann     [16] CertAnnContent,         --Certificate Ann.
        rann     [17] RevAnnContent,          --Revocation Ann.
        crlann   [18] CRLAnnContent,          --CRL Announcement
        pkiconf  [19] PKIConfirmContent,      --Confirmation
        nested   [20] NestedMessageContent,   --Nested Message
        genm     [21] GenMsgContent,          --General Message
        genp     [22] GenRepContent,          --General Response
        error    [23] ErrorMsgContent,        --Error Message
        certConf [24] CertConfirmContent,     --Certificate Confirm
        pollReq  [25] PollReqContent,         --Polling Request
        pollRep  [26] PollRepContent          --Polling Response
     }

   The specific types are described in Section 5.3 below.

5.1.3.  PKI Message Protection

   Some PKI messages will be protected for integrity.

   Note: If an asymmetric algorithm is used to protect a message and the
   relevant public component has been certified already, then the origin
   of the message can also be authenticated.  On the other hand, if the
   public component is uncertified, then the message origin cannot be
   automatically authenticated but may be authenticated via out-of-band
   means.

   When protection is applied, the following structure is used:

     PKIProtection ::= BIT STRING

   The input to the calculation of PKIProtection is the DER encoding of
   the following data structure:

     ProtectedPart ::= SEQUENCE {
        header    PKIHeader,
        body      PKIBody
     }

   There MAY be cases in which the PKIProtection BIT STRING is
   deliberately not used to protect a message (i.e., this OPTIONAL field
   is omitted) because other protection, external to PKIX, will be
   applied instead.  Such a choice is explicitly allowed in this
   specification.  Examples of such external protection include CMS
   [RFC5652] and Security Multiparts [RFC1847] encapsulation of the
   PKIMessage (or simply the PKIBody (omitting the CHOICE tag), if the
   relevant PKIHeader information is securely carried in the external
   mechanism).  It is noted, however, that many such external mechanisms
   require that the end entity already possesses a public-key
   certificate, a unique DN, and/or other such infrastructure-related
   information.  Thus, they may not be appropriate for initial
   registration, key-recovery, or any other process with "bootstrapping"
   characteristics.  For those cases, it may be necessary that the
   PKIProtection parameter be used.  In the future, if/when external
   mechanisms are modified to accommodate bootstrapping scenarios, the
   use of PKIProtection may become rare or non-existent.

   Depending on the circumstances, the PKIProtection bits may contain a
   MAC or signature.  Only the following cases can occur:

5.1.3.1.  Shared Secret Information

   In this case, the sender and recipient share secret information with
   sufficient entropy (established via out-of-band means).
   PKIProtection will contain a MAC value, and the protectionAlg MAY be
   one of the options described in Section 6.1 of CMP Algorithms
   [RFC9481].

   The algorithm identifier id-PasswordBasedMac is defined in
   Section 4.4 of [RFC4211] and updated by [RFC9045].  It is mentioned
   in Section 6.1.1 of [RFC9481] for backward compatibility.  More
   modern alternatives are listed in Section 6.1 of [RFC9481].

     id-PasswordBasedMac OBJECT IDENTIFIER ::= {1 2 840 113533 7 66 13}
     PBMParameter ::= SEQUENCE {
        salt                OCTET STRING,
        owf                 AlgorithmIdentifier,
        iterationCount      INTEGER,
        mac                 AlgorithmIdentifier
     }

   The following text gives a method of key expansion to be used when
   the MAC algorithm requires an input length that is larger than the
   size of the one-way function (OWF).

   Note: Section 4.4 of [RFC4211] and [RFC9045] do not mention this key
   expansion method or give an example using HMAC algorithms where key
   expansion is not needed.  It is recognized that this omission in
   [RFC4211] can lead to confusion and possible incompatibility if key
   expansion [RFC4210] is not used when needed.  Therefore, when key
   expansion is required (when K > H), the key expansion defined in the
   following text MUST be used.

   In the above protectionAlg, the salt value is appended to the shared
   secret input.  The OWF is then applied iterationCount times, where
   the salted secret is the input to the first iteration and, for each
   successive iteration, the input is set to be the output of the
   previous iteration.  The output of the final iteration (called
   "BASEKEY" for ease of reference, with a size of "H") is what is used
   to form the symmetric key.  If the MAC algorithm requires a K-bit key
   and K <= H, then the most significant K bits of BASEKEY are used.  If
   K > H, then all of BASEKEY is used for the most significant H bits of
   the key, OWF("1" || BASEKEY) is used for the next most significant H
   bits of the key, OWF("2" || BASEKEY) is used for the next most
   significant H bits of the key, and so on, until all K bits have been
   derived.  [Here "N" is the ASCII byte encoding the number N and "||"
   represents concatenation.]

   Note: It is RECOMMENDED that the fields of PBMParameter remain
   constant throughout the messages of a single transaction (e.g.,
   ir/ip/certConf/pkiConf) to reduce the overhead associated with
   PasswordBasedMac computation.

5.1.3.2.  DH Key Pairs

   Where the sender and receiver possess finite-field or elliptic-curve-
   based DH certificates with compatible DH parameters in order to
   protect the message, the end entity must generate a symmetric key
   based on its private DH key value and the DH public key of the
   recipient of the PKI message.  PKIProtection will contain a MAC value
   keyed with this derived symmetric key, and the protectionAlg will be
   the following:

     id-DHBasedMac OBJECT IDENTIFIER ::= {1 2 840 113533 7 66 30}

     DHBMParameter ::= SEQUENCE {
        owf                 AlgorithmIdentifier,
        -- AlgId for an OWF
        mac                 AlgorithmIdentifier
        -- the MAC AlgId
     }

   In the above protectionAlg, OWF is applied to the result of the DH
   computation.  The OWF output (called "BASEKEY" for ease of reference,
   with a size of "H") is what is used to form the symmetric key.  If
   the MAC algorithm requires a K-bit key and K <= H, then the most
   significant K bits of BASEKEY are used.  If K > H, then all of
   BASEKEY is used for the most significant H bits of the key,
   OWF("1" || BASEKEY) is used for the next most significant H bits of
   the key, OWF("2" || BASEKEY) is used for the next most significant H
   bits of the key, and so on, until all K bits have been derived.
   [Here "N" is the ASCII byte encoding the number N and "||" represents
   concatenation.]

   Note: Hash algorithms that can be used as OWFs are listed in
   Section 2 of CMP Algorithms [RFC9481].

5.1.3.3.  Signature

   In this case, the sender possesses a signature key pair and simply
   signs the PKI message.  PKIProtection will contain the signature
   value and the protectionAlg will be an AlgorithmIdentifier for a
   digital signature, which MAY be one of the options described in
   Section 3 of CMP Algorithms [RFC9481].

5.1.3.4.  Key Encapsulation

   In case the sender of a message has a KEM key pair, it can be used to
   establish a shared secret key for MAC-based message protection.  This
   can be used for message authentication.

   This approach uses the definition of KEM algorithm functions in
   Section 1 of [RFC9629] as follows:

   A KEM algorithm provides three functions:

   1.  KeyGen() -> (pk, sk): Generate a public key (pk) and a private
       (secret) key (sk).

   2.  Encapsulate(pk) -> (ct, ss): Given the public key (pk), produce a
       ciphertext (ct) and a shared secret (ss).

   3.  Decapsulate(sk, ct) -> (ss): Given the private key (sk) and the
       ciphertext (ct), produce the shared secret (ss).

   To support a particular KEM algorithm, the PKI entity that possesses
   a KEM key pair and wishes to use it for MAC-based message protection
   MUST support the KEM Decapsulate() function.  The PKI entity that
   wishes to verify the MAC-based message protection MUST support the
   KEM Encapsulate() function.  The respective public KEM key is usually
   carried in a certificate [ML-KEM].

   Note: Both PKI entities send and receive messages in a PKI management
   operation.  Both PKI entities may independently wish to protect
   messages using their KEM key pairs.  For ease of explanation, we use
   the terms "Alice" to denote the PKI entity possessing the KEM key
   pair and who wishes to provide MAC-based message protection and "Bob"
   to denote the PKI entity having Alice's authentic public KEM key and
   who needs to verify the MAC-based protection provided by Alice.

   Assuming Bob has Alice's KEM public key, he generates the ciphertext
   using KEM encapsulation and transfers it to Alice in an
   InfoTypeAndValue structure.  Alice then retrieves the KEM shared
   secret from the ciphertext using KEM decapsulation and the associated
   KEM private key.  Using a key derivation function (KDF), Alice
   derives a shared secret key from the KEM shared secret and other data
   sent by Bob. PKIProtection will contain a MAC value calculated using
   that shared secret key, and the protectionAlg will be the following:

     id-KemBasedMac OBJECT IDENTIFIER ::= {1 2 840 113533 7 66 16}

     KemBMParameter ::= SEQUENCE {
       kdf              AlgorithmIdentifier{KEY-DERIVATION, {...}},
       kemContext   [0] OCTET STRING OPTIONAL,
       len              INTEGER (1..MAX),
       mac              AlgorithmIdentifier{MAC-ALGORITHM, {...}}
     }

   Note: The OID for id-KemBasedMac was assigned on the private-use arc
   { iso(1) member-body(2) us(840) nortelnetworks(113533) entrust(7) }
   and not assigned on an IANA-owned arc because the authors wished to
   place it on the same branch as the existing OIDs for id-
   PasswordBasedMac and id-DHBasedMac.

   kdf is the algorithm identifier of the chosen KDF, and any associated
   parameters, used to derive the shared secret key.

   kemContext MAY be used to transfer additional algorithm-specific
   context information (see also the definition of ukm in Section 3 of
   [RFC9629]).

   len is the output length of the KDF and MUST be the desired size of
   the key to be used for MAC-based message protection.

   mac is the algorithm identifier of the chosen MAC algorithm, and any
   associated parameters, used to calculate the MAC value.

   The KDF and MAC algorithms MAY be chosen from the options in CMP
   Algorithms [RFC9481].

   The InfoTypeAndValue transferring the KEM ciphertext uses OID id-it-
   KemCiphertextInfo.  It contains a KemCiphertextInfo structure, as
   defined in Section 5.3.19.18.

   Note: This InfoTypeAndValue can be carried in a genm/genp message
   body, as specified in Section 5.3.19.18, or in the generalInfo field
   of PKIHeader in messages of other types (see Section 5.1.1.5).

   In the following, a generic message flow for MAC-based protection
   using KEM is specified in more detail.  It is assumed that Bob
   possesses Alice's public KEM key.  Alice can be the initiator of a
   PKI management operation or the responder.  For more detailed
   figures, see Appendix E.

   Generic Message Flow:

   Step# Alice                                Bob
   ---------------------------------------------------------------------
     1                                        perform KEM Encapsulate
                         <-- KEM Ciphertext <--
     2   perform KEM Decapsulate,
           perform key derivation,
           format message with
           MAC-based protection
                         -->    message     -->
     3                                        perform key derivation,
                                                verify MAC-based
                                                protection
   -------------------  Alice authenticated by Bob  --------------------

        Figure 2: Generic Message Flow When Alice Has a KEM Key Pair

   1.  Bob needs to possess Alice's authentic public KEM key (pk), for
       instance, contained in a KEM certificate that was received and
       successfully validated by Bob beforehand.

       Bob generates a shared secret (ss) and the associated ciphertext
       (ct) using the KEM Encapsulate function with Alice's public KEM
       key (pk).  Bob MUST NOT reuse the ss and ct for other PKI
       management operations.  From this data, Bob produces a
       KemCiphertextInfo structure, including the KEM algorithm
       identifier and the ciphertext (ct) and sends it to Alice in an
       InfoTypeAndValue structure, as defined in Section 5.3.19.18.

         Encapsulate(pk) -> (ct, ss)

   2.  Alice decapsulates the shared secret (ss) from the ciphertext
       (ct) using the KEM Decapsulate function and its private KEM key
       (sk).

         Decapsulate(ct, sk) -> (ss)

       If the decapsulation operation outputs an error, any failInfo
       field in an error response message SHALL contain the value
       badMessageCheck and the PKI management operation SHALL be
       terminated.

       Alice derives the shared secret key (ssk) using a KDF.  The
       shared secret (ss) is used as input key material for the KDF, and
       the value len is the desired output length of the KDF as required
       by the MAC algorithm to be used for message protection.  KDF,
       len, and MAC will be transferred to Bob in the protectionAlg
       KemBMParameter.  The DER-encoded KemOtherInfo structure, as
       defined below, is used as context for the KDF.

         KDF(ss, len, context)->(ssk)

       The shared secret key (ssk) is used for MAC-based protection by
       Alice.

   3.  Bob derives the same shared secret key (ssk) using the KDF.  Also
       here, the shared secret (ss) is used as input key material for
       the KDF, the value len is the desired output length for the KDF,
       and the DER-encoded KemOtherInfo structure constructed in the
       same way as on Alice's side is used as context for the KDF.

         KDF(ss, len, context)->(ssk)

       Bob uses the shared secret key (ssk) for verifying the MAC-based
       protection of the message received and in this way authenticates
       Alice.

   This shared secret key (ssk) can be reused by Alice for MAC-based
   protection of further messages sent to Bob within the current PKI
   management operation.

   This approach employs the notation of KDF(IKM, L, info) as described
   in Section 5 of [RFC9629] with the following changes:

   *  IKM is the input key material.  It is the symmetric secret called
      "ss" resulting from the KEM.

   *  L is dependent of the MAC algorithm that is used with the shared
      secret key for CMP message protection and is called "len" in this
      document.

   *  info is an additional input to the KDF, is called "context" in
      this document, and contains the DER-encoded KemOtherInfo structure
      defined as:

        KemOtherInfo ::= SEQUENCE {
          staticString      PKIFreeText,
          transactionID     OCTET STRING,
          kemContext    [0] OCTET STRING OPTIONAL
        }

      staticString MUST be "CMP-KEM".

      transactionID MUST be the value from the message containing the
      ciphertext (ct) in KemCiphertextInfo.

      Note: The transactionID is used to ensure domain separation of the
      derived shared secret key between different PKI management
      operations.  For all PKI management operations with more than one
      exchange, the transactionID MUST be set anyway (see
      Section 5.1.1).  In case Bob provided an infoValue of type
      KemCiphertextInfo to Alice in the initial request message (see
      Figure 4 of Appendix E), the transactionID MUST be set by Bob.

      kemContext MAY contain additional algorithm-specific context
      information.

   *  OKM is the output keying material of the KDF used for MAC-based
      message protection of length len and is called "ssk" in this
      document.

   There are various ways that Alice can request and Bob can provide the
   KEM ciphertext (see Appendix E for details).  The KemCiphertextInfo
   can be requested using PKI general messages, as described in
   Section 5.3.19.18.  Alternatively, the generalInfo field of the
   PKIHeader can be used to convey the same request and response
   InfoTypeAndValue structures, as described in Section 5.1.1.5.  The
   procedure also works without Alice explicitly requesting the KEM
   ciphertext in case Bob knows one of Alice's KEM keys beforehand and
   can expect that she is ready to use it.

   If both the initiator and responder in a PKI management operation
   have KEM key pairs, this procedure can be applied by both entities
   independently, establishing and using different shared secret keys
   for either direction.

5.1.3.5.  Multiple Protection

   When receiving a protected PKI message, a PKI management entity, such
   as an RA, MAY forward that message adding its own protection.
   Additionally, multiple PKI messages MAY be aggregated.  There are
   several use cases for such messages.

   *  The RA confirms having validated and authorized a message and
      forwards the original message unchanged.

   *  A PKI management entity collects several messages that are to be
      forwarded in the same direction and forwards them in a batch.
      Request messages can be transferred as a batch upstream (towards
      the CA); response or announce messages can be transferred as a
      batch downstream (towards an RA but not to the end entity).  For
      instance, this can be used when bridging an offline connection
      between two PKI management entities.

   These use cases are accomplished by nesting the messages within a new
   PKI message.  The structure used is as follows:

     NestedMessageContent ::= PKIMessages

   In case an RA needs to modify a request message, it MAY include the
   original PKIMessage in the generalInfo field of the modified message,
   as described in Section 5.1.1.3.

5.2.  Common Data Structures

   Before specifying the specific types that may be placed in a PKIBody,
   we define some data structures that are used in more than one case.

5.2.1.  Requested Certificate Contents

   Various PKI management messages require that the originator of the
   message indicate some of the fields that are required to be present
   in a certificate.  The CertTemplate structure allows entities
   requesting a certificate to specify the data fields that they want to
   be included.  Typically, they are required to provide at least the
   publicKey field.  A CertTemplate structure is identical to a
   TBSCertificate structure (see [RFC5280]) but with all fields
   optional/situational.

   Note: Even if the originator completely specifies the contents of a
   certificate it requires, a CA is free to modify fields within the
   certificate actually issued.  If the modified certificate is
   unacceptable to the requester, the requester MUST send back a
   certConf message that either does not include this certificate (via a
   CertHash) or does include this certificate (via a CertHash) along
   with a status of "rejected".  See Section 5.3.18 for the definition
   and use of CertHash and the certConf message.

   Note: Before requesting a new certificate, an end entity can request
   a certTemplate structure as a kind of certificate request blueprint
   in order to learn which data the CA expects to be present in the
   certificate request (see Section 5.3.19.16).

   See CRMF [RFC4211] for CertTemplate syntax.

   If certTemplate is an empty SEQUENCE (i.e., all fields omitted), then
   the controls field in the CertRequest structure MAY contain the id-
   regCtrl-altCertTemplate control, specifying a template for a
   certificate other than an X.509v3 public-key certificate.
   Conversely, if certTemplate is not empty (i.e., at least one field is
   present), then controls MUST NOT contain id-regCtrl-altCertTemplate.
   The new control is defined as follows:

     id-regCtrl-altCertTemplate OBJECT IDENTIFIER ::= { iso(1)
        identified-organization(3) dod(6) internet(1) security(5)
        mechanisms(5) pkix(7) pkip(5) regCtrl(1) 7}

     AltCertTemplate ::= AttributeTypeAndValue

   Also see [RFC4212] for more details on how to manage certificates in
   alternative formats using CRMF [RFC4211] syntax.

5.2.2.  Encrypted Values

   When encrypted data like a private key, certificate, POP challenge,
   or revocation passphrase is sent in PKI messages, it is RECOMMENDED
   to use the EnvelopedData structure.  In some cases, this is
   accomplished by using the EncryptedKey data structure instead of
   EncryptedValue.

     EncryptedKey ::= CHOICE {
        encryptedValue       EncryptedValue, -- deprecated
        envelopedData    [0] EnvelopedData }

   See Certificate Request Message Format (CRMF) [RFC4211] for
   EncryptedKey and EncryptedValue syntax and Cryptographic Message
   Syntax (CMS) [RFC5652] for EnvelopedData syntax.  Using the
   EncryptedKey data structure offers the choice to either use
   EncryptedValue (for backward compatibility only) or EnvelopedData.
   The use of the EncryptedValue structure has been deprecated in favor
   of the EnvelopedData structure.  Therefore, it is RECOMMENDED to use
   EnvelopedData.

   Note: The EncryptedKey structure defined in CRMF [RFC4211] is used
   here, which makes the update backward compatible.  Using the new
   syntax with the untagged default choice EncryptedValue is bits-on-
   the-wire compatible with the old syntax.

   To indicate support for EnvelopedData, the pvno cmp2021 has been
   introduced.  Details on the usage of the protocol version number are
   described in Section 7.

   The EnvelopedData structure is RECOMMENDED to be used in CMP to
   transport a private key, certificate, POP challenge, or revocation
   passphrase in encrypted form as follows:

   *  It contains only one RecipientInfo structure because the content
      is encrypted only for one recipient.

   *  It may contain a private key in the AsymmetricKeyPackage structure
      (which is placed in the encryptedContentInfo field), as defined in
      [RFC5958], that is wrapped in a SignedData structure, as specified
      in Section 5 of [RFC5652] and [RFC8933], signed by the KGA or CA.

   *  It may contain a certificate, POP challenge, or revocation
      passphrase directly in the encryptedContent field.

   The content of the EnvelopedData structure, as specified in Section 6
   of [RFC5652], MUST be encrypted using a newly generated symmetric
   content-encryption key.  This content-encryption key MUST be securely
   provided to the recipient using one of four key management
   techniques.

   The choice of the key management technique to be used by the sender
   depends on the credential available at the recipient:

   *  recipient's certificate with an algorithm identifier and a public
      key that supports key transport and where any given key usage
      extension allows keyEncipherment: The content-encryption key will
      be protected using the key transport key management technique, as
      specified in Section 6.2.1 of [RFC5652].

   *  recipient's certificate with an algorithm identifier and a public
      key that supports key agreement and where any given key usage
      extension allows keyAgreement: The content-encryption key will be
      protected using the key agreement key management technique, as
      specified in Section 6.2.2 of [RFC5652].

   *  a password or shared secret: The content-encryption key will be
      protected using the password-based key management technique, as
      specified in Section 6.2.4 of [RFC5652].

   *  recipient's certificate with an algorithm identifier and a public
      key that supports KEM and where any given key usage extension
      allows keyEncipherment: The content-encryption key will be
      protected using the key management technique for KEM keys, as
      specified in [RFC9629].

   Note: There are cases where the algorithm identifier, the type of the
   public key, and the key usage extension will not be sufficient to
   decide on the key management technique to use, e.g., when
   rsaEncryption is the algorithm identifier.  In such cases, it is a
   matter of local policy to decide.

5.2.3.  Status Codes and Failure Information for PKI Messages

   All response messages will include some status information.  The
   following values are defined.

     PKIStatus ::= INTEGER {
        accepted               (0),
        grantedWithMods        (1),
        rejection              (2),
        waiting                (3),
        revocationWarning      (4),
        revocationNotification (5),
        keyUpdateWarning       (6)
     }

   Responders may use the following syntax to provide more information
   about failure cases.

     PKIFailureInfo ::= BIT STRING {
        badAlg                 (0),
        badMessageCheck        (1),
        badRequest             (2),
        badTime                (3),
        badCertId              (4),
        badDataFormat          (5),
        wrongAuthority         (6),
        incorrectData          (7),
        missingTimeStamp       (8),
        badPOP                 (9),
        certRevoked            (10),
        certConfirmed          (11),
        wrongIntegrity         (12),
        badRecipientNonce      (13),
        timeNotAvailable       (14),
        unacceptedPolicy       (15),
        unacceptedExtension    (16),
        addInfoNotAvailable    (17),
        badSenderNonce         (18),
        badCertTemplate        (19),
        signerNotTrusted       (20),
        transactionIdInUse     (21),
        unsupportedVersion     (22),
        notAuthorized          (23),
        systemUnavail          (24),
        systemFailure          (25),
        duplicateCertReq       (26)
     }

     PKIStatusInfo ::= SEQUENCE {
        status        PKIStatus,
        statusString  PKIFreeText     OPTIONAL,
        failInfo      PKIFailureInfo  OPTIONAL
     }

5.2.4.  Certificate Identification

   In order to identify particular certificates, the CertId data
   structure is used.

   See [RFC4211] for CertId syntax.

5.2.5.  Out-of-Band Root CA Public Key

   Each root CA that provides a self-signed certificate must be able to
   publish its current public key via some "out-of-band" means or
   together with the respective link certificate using an online
   mechanism.  While such mechanisms are beyond the scope of this
   document, we define data structures that can support such mechanisms.

   There are generally two methods available: Either the CA directly
   publishes its self-signed certificate, or this information is
   available via the directory (or equivalent) and the CA publishes a
   hash of this value to allow verification of its integrity before use.

   Note: As an alternative to out-of-band distribution of root CA public
   keys, the CA can provide the self-signed certificate together with
   link certificates, e.g., using RootCaKeyUpdateContent
   (Section 5.3.19.15).

     OOBCert ::= Certificate

   The fields within this certificate are restricted as follows:

   *  The certificate MUST be self-signed (i.e., the signature must be
      verifiable using the SubjectPublicKeyInfo field);

   *  The subject and issuer fields MUST be identical;

   *  If the subject field contains a "NULL-DN", then both
      subjectAltNames and issuerAltNames extensions MUST be present and
      have exactly the same value; and

   *  The values of all other extensions must be suitable for a self-
      signed certificate (e.g., key identifiers for the subject and
      issuer must be the same).

     OOBCertHash ::= SEQUENCE {
        hashAlg     [0] AlgorithmIdentifier     OPTIONAL,
        certId      [1] CertId                  OPTIONAL,
        hashVal         BIT STRING
     }

   The intention of the hash value is that anyone who has securely
   received the hash value (via the out-of-band means) can verify a
   self-signed certificate for that CA.

5.2.6.  Archive Options

   Requesters may indicate that they wish the PKI to archive a private
   key value using the PKIArchiveOptions structure.

   See [RFC4211] for PKIArchiveOptions syntax.

5.2.7.  Publication Information

   Requesters may indicate that they wish the PKI to publish a
   certificate using the PKIPublicationInfo structure.

   See [RFC4211] for PKIPublicationInfo syntax.

5.2.8.  POP Structures

   The POP structure used is indicated in the popo field of type
   ProofOfPossession in the CertReqMsg sequence (see Section 4 of
   [RFC4211]).

      ProofOfPossession ::= CHOICE {
         raVerified      [0] NULL,
         signature       [1] POPOSigningKey,
         keyEncipherment [2] POPOPrivKey,
         keyAgreement    [3] POPOPrivKey
      }

5.2.8.1.  raVerified

   An end entity MUST NOT use raVerified.  If an RA performs changes to
   a certification request breaking the provided POP, or if the RA
   requests a certificate on behalf of an end entity and cannot provide
   the POP itself, the RA MUST use raVerified.  Otherwise, it SHOULD NOT
   use raVerified.

   When introducing raVerified, the RA MUST check the existing POP, or
   it MUST ensure by other means that the end entity is the holder of
   the private key.  The RA MAY provide the original message containing
   the POP in the generalInfo field using the id-it-origPKIMessage (see
   Section 5.1.1.3) enabling the CA to verify it.

5.2.8.2.  POPOSigningKey Structure

   If the certification request is for a key pair that supports signing
   (i.e., a request for a verification certificate), then the POP of the
   private key is demonstrated through use of the POPOSigningKey
   structure; for details, see Section 4.1 of [RFC4211].

      POPOSigningKey ::= SEQUENCE {
         poposkInput [0] POPOSigningKeyInput OPTIONAL,
         algorithmIdentifier AlgorithmIdentifier,
         signature BIT STRING
      }

      POPOSigningKeyInput ::= SEQUENCE {
         authInfo CHOICE {
            sender [0] GeneralName,
            publicKeyMAC PKMACValue
         },
         publicKey SubjectPublicKeyInfo
      }

      PKMACValue ::= SEQUENCE {
         algId AlgorithmIdentifier,
         value BIT STRING
      }

   Note: For the purposes of this specification, the ASN.1 comment given
   in Appendix C of [RFC4211] pertains not only to certTemplate but also
   to the altCertTemplate control, as defined in Section 5.2.1.

   If certTemplate (or the altCertTemplate control) contains the subject
   and publicKey values, then poposkInput MUST be omitted and the
   signature MUST be computed on the DER-encoded value of the certReq
   field of the CertReqMsg (or the DER-encoded value of
   AltCertTemplate).  If certTemplate/altCertTemplate does not contain
   both the subject and public key values (i.e., if it contains only one
   of these or neither), then poposkInput MUST be present and the
   signature MUST be computed on the DER-encoded value of poposkInput
   (i.e., the "value" OCTETs of the POPOSigningKeyInput DER).

   In the special case that the CA/RA has a DH certificate that is known
   to the end entity and the certification request is for a key
   agreement key pair, the end entity can also use the POPOSigningKey
   structure (where the algorithmIdentifier field is DHBasedMAC and the
   signature field is the MAC) for demonstrating POP.

5.2.8.3.  POPOPrivKey Structure

   If the certification request is for a key pair that does not support
   signing (i.e., a request for an encryption or key agreement
   certificate), then the POP of the private key is demonstrated through
   use of the POPOPrivKey structure in one of the following three ways;
   for details see Sections 4.2 and 4.3 in [RFC4211].

      POPOPrivKey ::= CHOICE {
         thisMessage [0] BIT STRING, -- deprecated
         subsequentMessage [1] SubsequentMessage,
         dhMAC [2] BIT STRING, -- deprecated
         agreeMAC [3] PKMACValue,
         encryptedKey [4] EnvelopedData
      }

      SubsequentMessage ::= INTEGER {
         encrCert (0),
         challengeResp (1)
      }

   When using agreeMAC or encryptedKey choices, the pvno cmp2021(3) MUST
   be used.  Details on the usage of the protocol version number are
   described in Section 7.

5.2.8.3.1.  Inclusion of the Private Key

   This method mentioned previously in Section 4.3 demonstrates POP of
   the private key by including the encrypted private key in the
   CertRequest in the POPOPrivKey structure or in the PKIArchiveOptions
   control structure.  This method SHALL only be used if archival of the
   private key is desired.

   For a certification request message indicating cmp2021(3) in the pvno
   field of the PKIHeader, the encrypted private key MUST be transferred
   in the encryptedKey choice of POPOPrivKey (or within the
   PKIArchiveOptions control) in a CMS EnvelopedData structure, as
   defined in Section 5.2.2.

   Note: The thisMessage choice has been deprecated in favor of
   encryptedKey.  When using cmp2000(2) in the certification request
   message header for backward compatibility, the thisMessage choice of
   POPOPrivKey is used containing the encrypted private key in an
   EncryptedValue structure wrapped in a BIT STRING.  This allows the
   necessary conveyance and protection of the private key while
   maintaining bits-on-the-wire compatibility with [RFC4211].

5.2.8.3.2.  Indirect Method - Encrypted Certificate

   The indirect method mentioned previously in Section 4.3 demonstrates
   POP of the private key by having the CA return the requested
   certificate in encrypted form (see Section 5.2.2).  This method is
   indicated in the CertRequest by requesting the encrCert option in the
   subsequentMessage choice of POPOPrivKey.

           end entity                         RA/CA
                    ----       req        ---->
                    <---  rep (enc cert)  -----
                    ---- conf (cert hash) ---->
                    <---       ack        -----

   The end entity proves knowledge of the private key to the CA by
   providing the correct CertHash for this certificate in the certConf
   message.  This demonstrates POP because the end entity can only
   compute the correct CertHash if it is able to recover the encrypted
   certificate, and it can only recover the certificate if it is able to
   obtain the symmetric key using the required private key.  Clearly,
   for this to work, the CA MUST NOT publish the certificate until the
   certConf message arrives (when certHash is to be used to demonstrate
   POP).  See Section 5.3.18 for further details, and see Section 8.11
   for security considerations regarding use of CT logs.

   The recipient SHOULD maintain a context of the PKI management
   operation, e.g., using transactionID and certReqId, to identify the
   private key to use when decrypting the EnvelopedData containing the
   newly issued certificate.  The recipient may be unable to use the
   RecipientInfo structure as it refers to the certificate that is still
   encrypted.  The sender MUST populate the rid field as specified by
   CMS, and the client MAY ignore it.

5.2.8.3.3.  Direct Method - Challenge-Response Protocol

   The direct method mentioned previously in Section 4.3 demonstrates
   POP of the private key by having the end entity engage in a
   challenge-response protocol (using the messages popdecc of type
   POPODecKeyChall and popdecr of type POPODecKeyResp; see below)
   between CertReqMessages and CertRepMessage.  This method is indicated
   in the CertRequest by requesting the challengeResp option in the
   subsequentMessage choice of POPOPrivKey.

   Note: This method would typically be used in an environment in which
   an RA verifies POP and then makes a certification request to the CA
   on behalf of the end entity.  In such a scenario, the CA trusts the
   RA to have done POP correctly before the RA requests a certificate
   for the end entity.

   The complete protocol then looks as follows (note that req' does not
   necessarily encapsulate req as a nested message):

           end entity            RA            CA
                    ---- req ---->
                    <--- chall ---
                    ---- resp --->
                                  ---- req' --->
                                  <--- rep -----
                                  ---- conf --->
                                  <--- ack -----
                    <--- rep -----
                    ---- conf --->
                    <--- ack -----

   This protocol is obviously much longer than the exchange given in
   Section 5.2.8.3.2 above but allows a Local Registration Authority
   (LRA) to be involved and has the property that the certificate itself
   is not actually created until the POP is complete.  In some
   environments, a different order of the above messages may be
   required, such as the following (this may be determined by policy):

           end entity            RA            CA
                    ---- req ---->
                    <--- chall ---
                    ---- resp --->
                                  ---- req' --->
                                  <--- rep -----
                    <--- rep -----
                    ---- conf --->
                                  ---- conf --->
                                  <--- ack -----
                    <--- ack -----

   The challenge-response messages for POP of a private key are
   specified as follows (for decryption keys, see [MvOV97], p.404 for
   details).  This challenge-response exchange is associated with the
   preceding certification request message (and subsequent certification
   response and confirmation messages) by the transactionID used in the
   PKIHeader and by the protection applied to the PKIMessage.

      POPODecKeyChallContent ::= SEQUENCE OF Challenge

      Challenge ::= SEQUENCE {
         owf AlgorithmIdentifier OPTIONAL,
         witness OCTET STRING,
         challenge OCTET STRING, -- deprecated
         encryptedRand [0] EnvelopedData OPTIONAL
      }

      Rand ::= SEQUENCE {
         int INTEGER,
         sender GeneralName
      }

   More details on the fields in this syntax are available in
   Appendix F.

   For a popdecc message indicating cmp2021(3) in the pvno field of the
   PKIHeader, the encryption of Rand MUST be transferred in the
   encryptedRand field in a CMS EnvelopedData structure as defined in
   Section 5.2.2.  The challenge field MUST contain an empty OCTET
   STRING.

   The recipient SHOULD maintain a context of the PKI management
   operation, e.g., using transactionID and certReqId, to identify the
   private key to use when decrypting encryptedRand.  The sender MUST
   populate the rid field in the EnvelopedData sequence using the
   issuerAndSerialNumber choice containing a NULL-DN as issuer and the
   certReqId as serialNumber.  The client MAY ignore the rid field.

   Note: The challenge field has been deprecated in favor of
   encryptedRand.  When using cmp2000(2) in the popdecc message header
   for backward compatibility, the challenge field MUST contain the
   encryption (involving the public key for which the certification
   request is being made) of Rand and encryptedRand MUST be omitted.
   Using challenge (omitting the optional encryptedRand field) is bit-
   compatible with [RFC4210].  Note that the size of Rand, when used
   with challenge, needs to be appropriate for encryption, involving the
   public key of the requester.  If, in some environment, names are so
   long that they cannot fit (e.g., very long DNs), then whatever
   portion will fit should be used (as long as it includes at least the
   common name, and as long as the receiver is able to deal meaningfully
   with the abbreviation).

      POPODecKeyRespContent ::= SEQUENCE OF INTEGER

   On receiving the popdecc message, the end entity decrypts all
   included challenges and responds with a popdecr message containing
   the decrypted integer values in the same order.

5.2.8.4.  Summary of POP Options

   The text in this section provides several options with respect to POP
   techniques.  Using "SK" for "signing key", "EK" for "encryption key",
   "KAK" for "key agreement key", and "KEMK" for "key encapsulation
   mechanism key", the techniques may be summarized as follows:

      RAVerified;
      SKPOP;
      EKPOPThisMessage; -- deprecated
      KAKPOPThisMessage; -- deprecated
      EKPOPEncryptedKey;
      KAKPOPEncryptedKey;
      KEMKPOPEncryptedKey;
      KAKPOPThisMessageDHMAC;
      EKPOPEncryptedCert;
      KAKPOPEncryptedCert;
      KEMKPOPEncryptedCert;
      EKPOPChallengeResp;
      KAKPOPChallengeResp; and
      KEMKPOPChallengeResp.

   Given this array of options, it is natural to ask how an end entity
   can know what is supported by the CA/RA (i.e., which options it may
   use when requesting certificates).  The following guidelines should
   clarify this situation for end entity implementers.

   *  RAVerified: This is not an end entity decision; the RA uses this
      if and only if it has verified POP before forwarding the request
      on to the CA, so it is not possible for the end entity to choose
      this technique.

   *  SKPOP: If the end entity has a signing key pair, this is the only
      POP method specified for use in the request for a corresponding
      certificate.

   *  EKPOPThisMessage (deprecated), KAKPOPThisMessage (deprecated),
      EKPOPEncryptedKey, KAKPOPEncryptedKey, KEMKPOPEncryptedKey:
      Whether or not to give up its private key to the CA/RA is an end
      entity decision.  If the end entity decides to reveal its key,
      then these are the only POP methods available in this
      specification to achieve this (and the key pair type and protocol
      version used will determine which of these methods to use).  The
      reason for deprecating EKPOPThisMessage and KAKPOPThisMessage
      options has been given in Section 5.2.8.3.1.

   *  KAKPOPThisMessageDHMAC: The end entity can only use this method if
      (1) the CA/RA has a DH certificate available for this purpose and
      (2) the end entity already has a copy of this certificate.  If
      both these conditions hold, then this technique is clearly
      supported and may be used by the end entity, if desired.

   *  EKPOPEncryptedCert, KAKPOPEncryptedCert, KEMKPOPEncryptedCert,
      EKPOPChallengeResp, KAKPOPChallengeResp, and KEMKPOPChallengeResp:
      The end entity picks one of these (in the subsequentMessage field)
      in the request message, depending upon preference and key pair
      type.  The end entity is not doing POP at this point; it is simply
      indicating which method it wants to use.  Therefore, if the CA/RA
      replies with a "badPOP" error, the end entity can re-request using
      the other POP method chosen in subsequentMessage.  Note, however,
      that this specification encourages the use of the EncryptedCert
      choice and, furthermore, says that the challenge-response would
      typically be used when an RA is involved and doing POP
      verification.  Thus, the end entity should be able to make an
      intelligent decision regarding which of these POP methods to
      choose in the request message.

5.2.9.  GeneralizedTime

   GeneralizedTime is a standard ASN.1 type and SHALL be used as
   specified in Section 4.1.2.5.2 of [RFC5280].

5.3.  Operation-Specific Data Structures

5.3.1.  Initialization Request

   An Initialization request message contains as the PKIBody a
   CertReqMessages data structure, which specifies the requested
   certificate(s).  Typically, SubjectPublicKeyInfo, KeyId, and Validity
   are the template fields that may be supplied for each certificate
   requested (see the profiles defined in Section 4.1.1 of [RFC9483] and
   Appendices C.4 and D.7 for further information).  This message is
   intended to be used for entities when first initializing into the
   PKI.

   See Section 5.2.1 and [RFC4211] for CertReqMessages syntax.

5.3.2.  Initialization Response

   An Initialization response message contains as the PKIBody a
   CertRepMessage data structure, which has for each certificate
   requested a PKIStatusInfo field, a subject certificate, and possibly
   a private key (normally encrypted using EnvelopedData; see
   Section 4.1.6 of [RFC9483] for further information).

   See Section 5.3.4 for CertRepMessage syntax.  Note that if the PKI
   message protection is "shared secret information" (see
   Section 5.1.3.1), then any certificate transported in the caPubs
   field may be directly trusted as a root CA certificate by the
   initiator.

5.3.3.  Certification Request

   A Certification request message contains as the PKIBody a
   CertReqMessages data structure, which specifies the requested
   certificates (see the profiles defined in Section 4.1.2 of [RFC9483]
   and Appendix C.2 for further information).  This message is intended
   to be used for existing PKI entities who wish to obtain additional
   certificates.

   See Section 5.2.1 and [RFC4211] for CertReqMessages syntax.

   Alternatively, the PKIBody MAY be a CertificationRequest (this
   structure is fully specified by the ASN.1 structure
   CertificationRequest given in [RFC2986]; see the profiles defined in
   Section 4.1.4 of [RFC9483] for further information).  This structure
   may be required for certificate requests for signing key pairs when
   interoperation with legacy systems is desired, but its use is
   strongly discouraged whenever not absolutely necessary.

5.3.4.  Certification Response

   A Certification response message contains as the PKIBody a
   CertRepMessage data structure, which has a status value for each
   certificate requested and optionally has a CA public key, failure
   information, a subject certificate, and an encrypted private key.

     CertRepMessage ::= SEQUENCE {
        caPubs          [1] SEQUENCE SIZE (1..MAX) OF CMPCertificate
                            OPTIONAL,
        response            SEQUENCE OF CertResponse
     }

     CertResponse ::= SEQUENCE {
        certReqId           INTEGER,
        status              PKIStatusInfo,
        certifiedKeyPair    CertifiedKeyPair     OPTIONAL,
        rspInfo             OCTET STRING         OPTIONAL
        -- analogous to the id-regInfo-utf8Pairs string defined
        -- for regInfo in CertReqMsg [RFC4211]
     }

     CertifiedKeyPair ::= SEQUENCE {
        certOrEncCert       CertOrEncCert,
        privateKey      [0] EncryptedKey         OPTIONAL,
        -- See [RFC4211] for comments on encoding.
        publicationInfo [1] PKIPublicationInfo   OPTIONAL
     }

     CertOrEncCert ::= CHOICE {
        certificate     [0] CMPCertificate,
        encryptedCert   [1] EncryptedKey
     }

   A p10cr message contains exactly one CertificationRequestInfo data
   structure, as specified in PKCS #10 [RFC2986], but no certReqId.
   Therefore, the certReqId in the corresponding Certification Response
   (cp) message MUST be set to -1.

   Only one of the failInfo (in PKIStatusInfo) and certificate (in
   CertifiedKeyPair) fields can be present in each CertResponse
   (depending on the status).  For some status values (e.g., waiting),
   neither of the optional fields will be present.

   Given an EncryptedCert and the relevant decryption key, the
   certificate may be obtained.  The purpose of this is to allow a CA to
   return the value of a certificate but with the constraint that only
   the intended recipient can obtain the actual certificate.  The
   benefit of this approach is that a CA may reply with a certificate
   even in the absence of proof that the requester is the end entity
   that can use the relevant private key (note that the proof is not
   obtained until the certConf message is received by the CA).  Thus,
   the CA will not have to revoke that certificate in the event that
   something goes wrong with the POP (but MAY do so anyway, depending
   upon policy).

   The use of EncryptedKey is described in Section 5.2.2.

   Note: To indicate support for EnvelopedData, the pvno cmp2021 has
   been introduced.  Details on the usage of different protocol version
   numbers are described in Section 7.

5.3.5.  Key Update Request Content

   For key update requests, the CertReqMessages syntax is used.
   Typically, SubjectPublicKeyInfo, KeyId, and Validity are the template
   fields that may be supplied for each key to be updated (see the
   profiles defined in Section 4.1.3 of [RFC9483] and Appendix C.6 for
   further information).  This message is intended to be used to request
   updates to existing (non-revoked and non-expired) certificates
   (therefore, it is sometimes referred to as a "Certificate Update"
   operation).  An update is a replacement certificate containing either
   a new subject public key or the current subject public key (although
   the latter practice may not be appropriate for some environments).

   See Section 5.2.1 and [RFC4211] for CertReqMessages syntax.

5.3.6.  Key Update Response Content

   For key update responses, the CertRepMessage syntax is used.  The
   response is identical to the initialization response.

   See Section 5.3.4 for CertRepMessage syntax.

5.3.7.  Key Recovery Request Content

   For key recovery requests, the syntax used is identical to the
   initialization request CertReqMessages.  Typically,
   SubjectPublicKeyInfo and KeyId are the template fields that may be
   used to supply a signature public key for which a certificate is
   required.

   See Section 5.2.1 and [RFC4211] for CertReqMessages syntax.  Note
   that if a key history is required, the requester must supply a
   protocol encryption key control in the request message.

5.3.8.  Key Recovery Response Content

   For key recovery responses, the following syntax is used.  For some
   status values (e.g., waiting), none of the optional fields will be
   present.

     KeyRecRepContent ::= SEQUENCE {
        status            PKIStatusInfo,
        newSigCert    [0] Certificate                 OPTIONAL,
        caCerts       [1] SEQUENCE SIZE (1..MAX) OF
                                     Certificate      OPTIONAL,
        keyPairHist   [2] SEQUENCE SIZE (1..MAX) OF
                                     CertifiedKeyPair OPTIONAL
     }

5.3.9.  Revocation Request Content

   When requesting revocation of a certificate (or several
   certificates), the following data structure is used (see the profiles
   defined in Section 4.2 of [RFC9483] for further information).  The
   name of the requester is present in the PKIHeader structure.

     RevReqContent ::= SEQUENCE OF RevDetails

     RevDetails ::= SEQUENCE {
        certDetails         CertTemplate,
        crlEntryDetails     Extensions       OPTIONAL
     }

5.3.10.  Revocation Response Content

   The revocation response is the response to the above message.  If
   produced, this is sent to the requester of the revocation.  (A
   separate revocation announcement message MAY be sent to the subject
   of the certificate for which revocation was requested.)

     RevRepContent ::= SEQUENCE {
        status        SEQUENCE SIZE (1..MAX) OF PKIStatusInfo,
        revCerts  [0] SEQUENCE SIZE (1..MAX) OF CertId OPTIONAL,
        crls      [1] SEQUENCE SIZE (1..MAX) OF CertificateList
                      OPTIONAL
     }

5.3.11.  Cross-Certification Request Content

   Cross-certification requests use the same syntax (CertReqMessages) as
   normal certification requests, with the restriction that the key pair
   MUST have been generated by the requesting CA and the private key
   MUST NOT be sent to the responding CA (see the profiles defined in
   Appendix D.6 for further information).  This request MAY also be used
   by subordinate CAs to get their certificates signed by the parent CA.

   See Section 5.2.1 and [RFC4211] for CertReqMessages syntax.

5.3.12.  Cross-Certification Response Content

   Cross-certification responses use the same syntax (CertRepMessage) as
   normal certification responses, with the restriction that no
   encrypted private key can be sent.

   See Section 5.3.4 for CertRepMessage syntax.

5.3.13.  CA Key Update Announcement Content

   When a CA updates its own key pair, the following data structure MAY
   be used to announce this event.

     RootCaKeyUpdateContent ::= SEQUENCE {
        newWithNew              CMPCertificate,
        newWithOld          [0] CMPCertificate OPTIONAL,
        oldWithNew          [1] CMPCertificate OPTIONAL
     }

   CAKeyUpdContent ::= CHOICE {
       cAKeyUpdAnnV2      CAKeyUpdAnnContent, -- deprecated
       cAKeyUpdAnnV3  [0] RootCaKeyUpdateContent
   }

   When using RootCaKeyUpdateContent in the ckuann message, the pvno
   cmp2021 MUST be used.  Details on the usage of the protocol version
   number are described in Section 7.

   In contrast to CAKeyUpdAnnContent as supported with cmp2000,
   RootCaKeyUpdateContent offers omitting newWithOld and oldWithNew,
   depending on the needs of the end entity.

5.3.14.  Certificate Announcement

   This structure MAY be used to announce the existence of certificates.

   Note that this message is intended to be used for those cases (if
   any) where there is no pre-existing method for publication of
   certificates; it is not intended to be used where, for example, X.500
   is the method for publication of certificates.

     CertAnnContent ::= Certificate

5.3.15.  Revocation Announcement

   When a CA has revoked, or is about to revoke, a particular
   certificate, it MAY issue an announcement of this (possibly upcoming)
   event.

     RevAnnContent ::= SEQUENCE {
        status              PKIStatus,
        certId              CertId,
        willBeRevokedAt     GeneralizedTime,
        badSinceDate        GeneralizedTime,
        crlDetails          Extensions  OPTIONAL
     }

   A CA MAY use such an announcement to warn (or notify) a subject that
   its certificate is about to be (or has been) revoked.  This would
   typically be used where the request for revocation did not come from
   the subject concerned.

   The willBeRevokedAt field contains the time at which a new entry will
   be added to the relevant CRLs.

5.3.16.  CRL Announcement

   When a CA issues a new CRL (or set of CRLs), the following data
   structure MAY be used to announce this event.

     CRLAnnContent ::= SEQUENCE OF CertificateList

5.3.17.  PKI Confirmation Content

   This data structure is used in the protocol exchange as the final
   PKIMessage.  Its content is the same in all cases -- actually, there
   is no content since the PKIHeader carries all the required
   information.

     PKIConfirmContent ::= NULL

   Use of this message for certificate confirmation is NOT RECOMMENDED;
   certConf SHOULD be used instead.  Upon receiving a pkiconf for a
   certificate response, the recipient MAY treat it as a certConf with
   all certificates being accepted.

5.3.18.  Certificate Confirmation Content

   This data structure is used by the client to send a confirmation to
   the CA/RA to accept or reject certificates.

     CertConfirmContent ::= SEQUENCE OF CertStatus

     CertStatus ::= SEQUENCE {
        certHash    OCTET STRING,
        certReqId   INTEGER,
        statusInfo  PKIStatusInfo OPTIONAL,
        hashAlg [0] AlgorithmIdentifier{DIGEST-ALGORITHM, {...}}
                    OPTIONAL
     }

   The hashAlg field SHOULD be used only in exceptional cases where the
   signatureAlgorithm of the certificate to be confirmed does not
   specify a hash algorithm in the OID or in the parameters or no hash
   algorithm is specified for hashing certificates signed using the
   signatureAlgorithm.  Note that for EdDSA, a hash algorithm is
   specified in Section 3.3 of [RFC9481], such that the hashAlg field is
   not needed for EdDSA.  Otherwise, the certHash value SHALL be
   computed using the same hash algorithm as used to create and verify
   the certificate signature or as specified for hashing certificates
   signed using the signatureAlgorithm.  If hashAlg is used, the CMP
   version indicated by the certConf message header must be cmp2021(3).

   For any particular CertStatus, omission of the statusInfo field
   indicates acceptance of the specified certificate.  Alternatively,
   explicit status details (with respect to acceptance or rejection) MAY
   be provided in the statusInfo field, perhaps for auditing purposes at
   the CA/RA.

   Within CertConfirmContent, omission of a CertStatus structure
   corresponding to a certificate supplied in the previous response
   message indicates rejection of the certificate.  Thus, an empty
   CertConfirmContent (a zero-length SEQUENCE) MAY be used to indicate
   rejection of all supplied certificates.  See Section 5.2.8.3.2 for a
   discussion of the certHash field with respect to POP.

5.3.19.  PKI General Message Content

     InfoTypeAndValue ::= SEQUENCE {
        infoType               OBJECT IDENTIFIER,
        infoValue              ANY DEFINED BY infoType  OPTIONAL
     }

     -- where {id-it} = {id-pkix 4} = {1 3 6 1 5 5 7 4}
     GenMsgContent ::= SEQUENCE OF InfoTypeAndValue

5.3.19.1.  CA Protocol Encryption Certificate

   This MAY be used by the end entity to get a certificate from the CA
   to use to protect sensitive information during the protocol.

     GenMsg:    {id-it 1}, < absent >
     GenRep:    {id-it 1}, Certificate | < absent >

   End entities MUST ensure that the correct certificate is used for
   this purpose.

5.3.19.2.  Signing Key Pair Types

   This MAY be used by the end entity to get the list of signature
   algorithms whose subject public key values the CA is willing to
   certify.

     GenMsg:    {id-it 2}, < absent >
     GenRep:    {id-it 2}, SEQUENCE SIZE (1..MAX) OF
                             AlgorithmIdentifier

   Note: For the purposes of this exchange, rsaEncryption and
   sha256WithRSAEncryption, for example, are considered to be
   equivalent; the question being asked is, "Is the CA willing to
   certify an RSA public key?"

   Note: In case several elliptic curves are supported, several id-
   ecPublicKey elements as defined in [RFC5480] need to be given, one
   per named curve.

5.3.19.3.  Encryption / Key Agreement Key Pair Types

   This MAY be used by the client to get the list of encryption / key
   agreement algorithms whose subject public key values the CA is
   willing to certify.

     GenMsg:    {id-it 3}, < absent >
     GenRep:    {id-it 3}, SEQUENCE SIZE (1..MAX) OF
                             AlgorithmIdentifier

   Note: In case several elliptic curves are supported, several id-
   ecPublicKey elements as defined in [RFC5480] need to be given, one
   per named curve.

5.3.19.4.  Preferred Symmetric Algorithm

   This MAY be used by the client to get the CA-preferred symmetric
   encryption algorithm for any confidential information that needs to
   be exchanged between the end entity and the CA (for example, if the
   end entity wants to send its private decryption key to the CA for
   archival purposes).

     GenMsg:    {id-it 4}, < absent >
     GenRep:    {id-it 4}, AlgorithmIdentifier

5.3.19.5.  Updated CA Key Pair

   This MAY be used by the CA to announce a CA key update event.

     GenMsg:    {id-it 18}, RootCaKeyUpdateValue

   See Section 5.3.13 for details of CA key update announcements.

5.3.19.6.  CRL

   This MAY be used by the client to get a copy of the latest CRL.

     GenMsg:    {id-it 6}, < absent >
     GenRep:    {id-it 6}, CertificateList

5.3.19.7.  Unsupported Object Identifiers

   This is used by the server to return a list of object identifiers
   that it does not recognize or support from the list submitted by the
   client.

     GenRep:    {id-it 7}, SEQUENCE SIZE (1..MAX) OF OBJECT IDENTIFIER

5.3.19.8.  Key Pair Parameters

   This MAY be used by the end entity to request the domain parameters
   to use for generating the key pair for certain public-key algorithms.
   It can be used, for example, to request the appropriate P, Q, and G
   to generate the DH/DSA key or to request a set of well-known elliptic
   curves.

     GenMsg:    {id-it 10}, OBJECT IDENTIFIER -- (Algorithm object-id)
     GenRep:    {id-it 11}, AlgorithmIdentifier | < absent >

   An absent infoValue in the GenRep indicates that the algorithm
   specified in GenMsg is not supported.

   End entities MUST ensure that the parameters are acceptable to it and
   that the GenRep message is authenticated (to avoid substitution
   attacks).

5.3.19.9.  Revocation Passphrase

   This MAY be used by the end entity to send a passphrase to a CA/RA
   for the purpose of authenticating a later revocation request (in the
   case that the appropriate signing private key is no longer available
   to authenticate the request).  See Appendix B for further details on
   the use of this mechanism.

     GenMsg:    {id-it 12}, EncryptedKey
     GenRep:    {id-it 12}, < absent >

   The use of EncryptedKey is described in Section 5.2.2.

5.3.19.10.  ImplicitConfirm

   See Section 5.1.1.1 for the definition and use of {id-it 13}.

5.3.19.11.  ConfirmWaitTime

   See Section 5.1.1.2 for the definition and use of {id-it 14}.

5.3.19.12.  Original PKIMessage

   See Section 5.1.1.3 for the definition and use of {id-it 15}.

5.3.19.13.  Supported Language Tags

   This MAY be used to determine the appropriate language tag [RFC5646]
   to use in subsequent messages.  The sender sends its list of
   supported languages (in order of most to least preferred); the
   receiver returns the one it wishes to use.  (Note: Each UTF8String
   MUST include a language tag.)  If none of the offered tags are
   supported, an error MUST be returned.

     GenMsg:    {id-it 16}, SEQUENCE SIZE (1..MAX) OF UTF8String
     GenRep:    {id-it 16}, SEQUENCE SIZE (1) OF UTF8String

5.3.19.14.  CA Certificates

   This MAY be used by the client to get CA certificates.

     GenMsg:    {id-it 17}, < absent >
     GenRep:    {id-it 17}, SEQUENCE SIZE (1..MAX) OF
                              CMPCertificate | < absent >

5.3.19.15.  Root CA Update

   This MAY be used by the client to get an update of a root CA
   certificate, which is provided in the body of the request message.
   In contrast to the ckuann message, this approach follows the request/
   response model.

   The end entity SHOULD reference its current trust anchor in
   RootCaCertValue in the request body, giving the root CA certificate
   if available.

     GenMsg:    {id-it 20}, RootCaCertValue | < absent >
     GenRep:    {id-it 18}, RootCaKeyUpdateValue | < absent >

     RootCaCertValue ::= CMPCertificate

     RootCaKeyUpdateValue ::= RootCaKeyUpdateContent

     RootCaKeyUpdateContent ::= SEQUENCE {
        newWithNew              CMPCertificate,
        newWithOld          [0] CMPCertificate OPTIONAL,
        oldWithNew          [1] CMPCertificate OPTIONAL
     }

   Note: In contrast to CAKeyUpdAnnContent (which was deprecated with
   pvno cmp2021), RootCaKeyUpdateContent offers omitting newWithOld and
   oldWithNew, depending on the needs of the end entity.

5.3.19.16.  Certificate Request Template

   This MAY be used by the client to get a template containing
   requirements for certificate request attributes and extensions.  The
   controls id-regCtrl-algId and id-regCtrl-rsaKeyLen MAY contain
   details on the types of subject public keys the CA is willing to
   certify.

   The id-regCtrl-algId control MAY be used to identify a cryptographic
   algorithm (see Section 4.1.2.7 of [RFC5280]) other than
   rsaEncryption.  The algorithm field SHALL identify a cryptographic
   algorithm.  The contents of the optional parameters field will vary
   according to the algorithm identified.  For example, when the
   algorithm is set to id-ecPublicKey, the parameters identify the
   elliptic curve to be used; see [RFC5480].

   Note: The client may specify a profile name in the certProfile field
   (see Section 5.1.1.4).

   The id-regCtrl-rsaKeyLen control SHALL be used for algorithm
   rsaEncryption and SHALL contain the intended modulus bit length of
   the RSA key.

     GenMsg:    {id-it 19}, < absent >
     GenRep:    {id-it 19}, CertReqTemplateContent | < absent >

     CertReqTemplateValue  ::= CertReqTemplateContent

     CertReqTemplateContent ::= SEQUENCE {
        certTemplate           CertTemplate,
        keySpec                Controls OPTIONAL }

     Controls  ::= SEQUENCE SIZE (1..MAX) OF AttributeTypeAndValue

     id-regCtrl-algId OBJECT IDENTIFIER ::= { iso(1)
        identified-organization(3) dod(6) internet(1) security(5)
        mechanisms(5) pkix(7) pkip(5) regCtrl(1) 11 }

     AlgIdCtrl ::= AlgorithmIdentifier{ALGORITHM, {...}}

     id-regCtrl-rsaKeyLen OBJECT IDENTIFIER ::= { iso(1)
        identified-organization(3) dod(6) internet(1) security(5)
        mechanisms(5) pkix(7) pkip(5) regCtrl(1) 12 }

     RsaKeyLenCtrl ::= INTEGER (1..MAX)

   The CertReqTemplateValue contains the prefilled certTemplate to be
   used for a future certificate request.  The publicKey field in the
   certTemplate MUST NOT be used.  In case the PKI management entity
   wishes to specify supported public-key algorithms, the keySpec field
   MUST be used.  One AttributeTypeAndValue per supported algorithm or
   RSA key length MUST be used.

   Note: The controls for an ASN.1 type are defined in Section 6 of CRMF
   [RFC4211].

5.3.19.17.  CRL Update Retrieval

   This MAY be used by the client to get new CRLs, specifying the source
   of the CRLs and the thisUpdate value of the latest CRL it already
   has, if available.  A CRL source is given either by a
   DistributionPointName or the GeneralNames of the issuing CA.  The
   DistributionPointName should be treated as an internal pointer to
   identify a CRL that the server already has and not as a way to ask
   the server to fetch CRLs from external locations.  The server SHALL
   only provide those CRLs that are more recent than the ones indicated
   by the client.

     GenMsg:    {id-it 22}, SEQUENCE SIZE (1..MAX) OF CRLStatus
     GenRep:    {id-it 23}, SEQUENCE SIZE (1..MAX) OF
                              CertificateList  |  < absent >

     CRLSource ::= CHOICE {
        dpn          [0] DistributionPointName,
        issuer       [1] GeneralNames }

     CRLStatus ::= SEQUENCE {
        source       CRLSource,
        thisUpdate   Time OPTIONAL }

5.3.19.18.  KEM Ciphertext

   This MAY be used by a PKI entity to get the KEM ciphertext for MAC-
   based message protection using KEM (see Section 5.1.3.4).

   The PKI entity that possesses a KEM key pair can request the
   ciphertext by sending an InfoTypeAndValue structure of type
   KemCiphertextInfo where the infoValue is absent.  The ciphertext can
   be provided in the following genp message with an InfoTypeAndValue
   structure of the same type.

     GenMsg:    {id-it 24}, < absent >
     GenRep:    {id-it 24}, KemCiphertextInfo

     KemCiphertextInfo ::= SEQUENCE {
       kem              AlgorithmIdentifier{KEM-ALGORITHM, {...}},
       ct               OCTET STRING
     }

   kem is the algorithm identifier of the KEM algorithm, and any
   associated parameters, used to generate the ciphertext (ct).

   ct is the ciphertext output from the KEM Encapsulate function.

   Note: These InfoTypeAndValue structures can also be transferred in
   the generalInfo field of the PKIHeader in messages of other types
   (see Section 5.1.1.5).

5.3.20.  PKI General Response Content

     GenRepContent ::= SEQUENCE OF InfoTypeAndValue

   Examples of GenReps that MAY be supported include those listed in the
   subsections of Section 5.3.19.

5.3.21.  Error Message Content

   This data structure MAY be used by an end entity, CA, or RA to convey
   error information and by a PKI management entity to initiate delayed
   delivery of responses.

     ErrorMsgContent ::= SEQUENCE {
        pKIStatusInfo          PKIStatusInfo,
        errorCode              INTEGER           OPTIONAL,
        errorDetails           PKIFreeText       OPTIONAL
     }

   This message MAY be generated at any time during a PKI transaction.
   If the client sends this request, the server MUST respond with a
   pkiconf response or another error message if any part of the header
   is not valid.

   In case a PKI management entity sends an error message to the end
   entity with the pKIStatusInfo field containing the status "waiting",
   the end entity SHOULD initiate polling as described in
   Section 5.3.22.  If the end entity does not initiate polling, both
   sides MUST treat this message as the end of the transaction (if a
   transaction is in progress).

   If protection is desired on the message, the client MUST protect it
   using the same technique (i.e., signature or MAC) as the starting
   message of the transaction.  The CA MUST always sign it with a
   signature key.

5.3.22.  Polling Request and Response

   This pair of messages is intended to handle scenarios in which the
   client needs to poll the server to determine the status of an
   outstanding response (i.e., when the "waiting" PKIStatus has been
   received).

     PollReqContent ::= SEQUENCE OF SEQUENCE {
        certReqId    INTEGER }

     PollRepContent ::= SEQUENCE OF SEQUENCE {
        certReqId    INTEGER,
        checkAfter   INTEGER,  -- time in seconds
        reason       PKIFreeText OPTIONAL }

   Unless implicit confirmation has been requested and granted, in
   response to an ir, cr, p10cr, kur, krr, or ccr request message,
   polling is initiated with an ip, cp, kup, krp, or ccp response
   message containing status "waiting".  For any type of request
   message, polling can be initiated with an error response message with
   status "waiting".  The following clauses describe how polling
   messages are used.  It is assumed that multiple certConf messages can
   be sent during transactions.  There will be one sent in response to
   each ip, cp, kup, krp, or ccp that contains a CertStatus for an
   issued certificate.

   1.  In response to an ip, cp, kup, krp, or ccp message, an end entity
       will send a certConf for all issued certificates and expect a
       pkiconf for each certConf.  An end entity will send a pollReq
       message in response to each CertResponse element of an ip, cp, or
       kup message with status "waiting" and in response to an error
       message with status "waiting".  Its certReqId MUST be either the
       index of a CertResponse data structure with status "waiting" or
       -1 referring to the complete response.

   2.  In response to a pollReq, a CA/RA will return an ip, cp, kup,
       krp, or ccp if one or more of the still pending requested
       certificates are ready or the final response to some other type
       of request is available; otherwise, it will return a pollRep.

   3.  If the end entity receives a pollRep, it will wait for at least
       the number of seconds given in the checkAfter field before
       sending another pollReq.

       Note that the checkAfter value heavily depends on the certificate
       management environment.  There are different possible reasons for
       a delayed delivery of response messages, e.g., high load on the
       server's backend, offline transfer of messages between two PKI
       management entities, or required RA operator approval.
       Therefore, the checkAfter time can vary greatly.  This should
       also be considered by the transfer protocol.

   4.  If the end entity receives an ip, cp, kup, krp, or ccp, then it
       will be treated in the same way as the initial response; if it
       receives any other response, then this will be treated as the
       final response to the original request.

   The following client-side state machine describes polling for
   individual CertResponse elements at the example of an ir request
   message.

                               START
                                 |
                                 v
                              Send ir
                                 | ip
                                 v
                            Check status
                            of returned <------------------------+
                               certs                             |
                                 |                               |
       +------------------------>|<------------------+           |
       |                         |                   |           |
       |        (issued)         v       (waiting)   |           |
     Add to <----------- Check CertResponse ------> Add to       |
    conf list           for each certificate      pending list   |
                                / \                              |
                               /   \  (empty conf list)          |
                  (conf list) /     \                            |
                             /       \              ip           |
                            /         \        +-----------------+
      (empty pending list) V           V       |    pollRep
        END <---- Send certConf        Send pollReq---------->Wait
                         |                 ^   ^               |
                         |                 |   |               |
                         +-----------------+   +---------------+
                            (pending list)

   In the following exchange, the end entity is enrolling for two
   certificates in one request.

   Step# End entity                       PKI
   ---------------------------------------------------------------------
     1   format ir
     2                   --> ir       -->
     3                                    handle ir
     4                                    manual intervention is
                                            required for both certs
     5                   <-- ip       <--
     6   process ip
     7   format pollReq
     8                   --> pollReq  -->
     9                                    check status of cert requests,
                                            certificates not ready
    10                                    format pollRep
    11                   <-- pollRep  <--
    12   wait
    13   format pollReq
    14                   --> pollReq  -->
    15                                    check status of cert requests,
                                            one certificate is ready
    16                                    format ip
    17                   <-- ip       <--
    18   handle ip
    19   format certConf
    20                   --> certConf -->
    21                                    handle certConf
    22                                    format ack
    23                   <-- pkiconf  <--
    24   format pollReq
    25                   --> pollReq  -->
    26                                    check status of certificate,
                                            certificate is ready
    27                                    format ip
    28                   <-- ip       <--
    29   handle ip
    30   format certConf
    31                   --> certConf -->
    32                                    handle certConf
    33                                    format ack
    34                   <-- pkiconf  <--

   The following client-side state machine describes polling for a
   complete response message.

                                   Start
                                     |
                                     | Send request
                                     v
                +----------- Receive response ------------+
                |                                         |
                | ip/cp/kup/krp/ccp/error with            | other
                | status "waiting"                        | response
                |                                         |
                v                                         |
    +------> Polling                                      |
    |           |                                         |
    |           | Send pollReq                            |
    |           | Receive response                        |
    |           |                                         |
    |           v                                         |
    +-----------+------------------->+<-------------------+
        pollRep   other response     |
                                     v
                               Handle response
                                     |
                                     v
                                    End

   In the following exchange, the end entity is sending a general
   message request, and the response is delayed by the server.

   Step# End entity                       PKI
   ---------------------------------------------------------------------
     1   format genm
     2                  --> genm    -->
     3                                 handle genm
     4                                 delay in response is necessary
     5                                 format error message "waiting"
                                         with certReqId set to -1
     6                  <-- error   <--
     7   process error
     8   format pollReq
     9                  --> pollReq -->
    10                                 check status of original request,
                                         general message response not
                                         ready
    11                                 format pollRep
    12                  <-- pollRep <--
    13   wait
    14   format pollReq
    15                  --> pollReq -->
    16                                 check status of original request,
                                         general message response is
                                         ready
    17                                 format genp
    18                  <-- genp    <--
    19   handle genp

6.  Mandatory PKI Management Functions

   Some of the PKI management functions outlined in Section 3.1 are
   described in this section.

   This section deals with functions that are "mandatory" in the sense
   that all end entity and CA/RA implementations MUST be able to provide
   the functionality described.  This part is effectively the profile of
   the PKI management functionality that MUST be supported.  Note,
   however, that the management functions described in this section do
   not need to be accomplished using the PKI messages defined in
   Section 5 if alternate means are suitable for a given environment.
   See Section 7 of [RFC9483] and Appendix C for profiles of the
   PKIMessage structures that MUST be supported for specific use cases.

6.1.  Root CA Initialization

   [See Section 3.1.1.2 for this document's definition of "root CA".]

   If a newly created root CA is at the top of a PKI hierarchy, it
   usually produces a "self-certificate", which is a certificate
   structure with the profile defined for the "newWithNew" certificate
   issued following a root CA key update.

   In order to make the CA's self-certificate useful to end entities
   that do not acquire the self-certificate via "out-of-band" means, the
   CA must also produce a fingerprint for its certificate.  End entities
   that acquire this fingerprint securely via some "out-of-band" means
   can then verify the CA's self-certificate and, hence, the other
   attributes contained therein.

   The data structure used to carry the fingerprint may be the
   OOBCertHash (see Section 5.2.5).

6.2.  Root CA Key Update

   CA keys (as all other keys) have a finite lifetime and will have to
   be updated on a periodic basis.  The certificates NewWithNew,
   NewWithOld, and OldWithNew (see Section 4.4.1) MAY be issued by the
   CA to aid existing end entities who hold the current root CA
   certificate (OldWithOld) to transition securely to the new root CA
   certificate (NewWithNew) and to aid new end entities who will hold
   NewWithNew to acquire OldWithOld securely for verification of
   existing data.

6.3.  Subordinate CA Initialization

   [See Section 3.1.1.2 for this document's definition of "subordinate
   CA".]

   From the perspective of PKI management protocols, the initialization
   of a subordinate CA is the same as the initialization of an end
   entity.  The only difference is that the subordinate CA must also
   produce an initial revocation list.

6.4.  CRL Production

   Before issuing any certificates, a newly established CA (which issues
   CRLs) must produce "empty" versions of each CRL, which are to be
   periodically produced.

6.5.  PKI Information Request

   When a PKI entity (CA, RA, or end entity) wishes to acquire
   information about the current status of a CA, it MAY send that CA a
   request for such information.

   The CA MUST respond to the request by providing (at least) all of the
   information requested by the requester.  If some of the information
   cannot be provided, then an error must be conveyed to the requester.

   If PKIMessages are used to request and supply this PKI information,
   then the request MUST be the GenMsg message, the response MUST be the
   GenRep message, and the error MUST be the Error message.  These
   messages are protected using a MAC based on shared secret information
   (e.g., password-based MAC; see Section 6.1 of "CMP Algorithms"
   [RFC9481]) or using any asymmetric authentication means such as a
   signature (if the end entity has an existing certificate).

6.6.  Cross-Certification

   The requester CA is the CA that will become the subject of the cross-
   certificate; the responder CA will become the issuer of the cross-
   certificate.

   The requester CA must be "up and running" before initiating the
   cross-certification operation.

6.6.1.  One-Way Request-Response Scheme

   The cross-certification scheme is essentially a one-way operation;
   that is, when successful, this operation results in the creation of
   one new cross-certificate.  If the requirement is that cross-
   certificates be created in "both directions", then each CA, in turn,
   must initiate a cross-certification operation (or use another
   scheme).

   This scheme is suitable where the two CAs in question can already
   verify each other's signatures (they have some common points of
   trust) or where there is an out-of-band verification of the origin of
   the certification request.

   Detailed Description:

      Cross-certification is initiated at one CA known as the responder.
      The CA administrator for the responder identifies the CA it wants
      to cross-certify and the responder CA equipment generates an
      authorization code.  The responder CA administrator passes this
      authorization code by out-of-band means to the requester CA
      administrator.  The requester CA administrator enters the
      authorization code at the requester CA in order to initiate the
      online exchange.

      The authorization code is used for authentication and integrity
      purposes.  This is done by generating a symmetric key based on the
      authorization code and using the symmetric key for generating MACs
      on all messages exchanged.  (Authentication may alternatively be
      done using signatures instead of MACs, if the CAs are able to
      retrieve and validate the required public keys by some means, such
      as an out-of-band hash comparison.)

      The requester CA initiates the exchange by generating a cross-
      certification request (ccr) with a fresh random number (requester
      random number).  The requester CA then sends the ccr message to
      the responder CA.  The fields in this message are protected from
      modification with a MAC based on the authorization code.

      Upon receipt of the ccr message, the responder CA validates the
      message and the MAC, saves the requester random number, and
      generates its own random number (responder random number).  It
      then generates (and archives, if desired) a new requester
      certificate that contains the requester CA public key and is
      signed with the responder CA signature private key.  The responder
      CA responds with the cross-certification response (ccp) message.
      The fields in this message are protected from modification with a
      MAC based on the authorization code.

      Upon receipt of the ccp message, the requester CA validates the
      message (including the received random numbers) and the MAC.  The
      requester CA responds with the certConf message.  The fields in
      this message are protected from modification with a MAC based on
      the authorization code.  The requester CA MAY write the requester
      certificate to the Repository as an aid to later certificate path
      construction.

      Upon receipt of the certConf message, the responder CA validates
      the message and the MAC and sends back an acknowledgement using
      the pkiconf message.  It MAY also publish the requester
      certificate as an aid to later path construction.

   Notes:

   1.  The ccr message must contain a "complete" certification request;
       that is, all fields except the serial number (including, e.g., a
       BasicConstraints extension) must be specified by the requester
       CA.

   2.  The ccp message SHOULD contain the verification certificate of
       the responder CA; if present, the requester CA must then verify
       this certificate (for example, via the "out-of-band" mechanism).

   (A simpler, non-interactive model of cross-certification may also be
   envisioned, in which the issuing CA acquires the subject CA's public
   key from some repository, verifies it via some out-of-band mechanism,
   and creates and publishes the cross-certificate without the subject
   CA's explicit involvement.  This model may be perfectly legitimate
   for many environments, but since it does not require any protocol
   message exchanges, its detailed description is outside the scope of
   this specification.)

6.7.  End Entity Initialization

   As with CAs, end entities must be initialized.  Initialization of end
   entities requires at least two steps:

   *  acquisition of PKI information

   *  out-of-band verification of one root-CA public key

   (Other possible steps include the retrieval of trust condition
   information and/or out-of-band verification of other CA public keys.)

6.7.1.  Acquisition of PKI Information

   The information REQUIRED is:

   *  the current root-CA public key

   *  (if the certifying CA is not a root-CA) the certification path
      from the root CA to the certifying CA together with appropriate
      revocation lists

   *  the algorithms and algorithm parameters that the certifying CA
      supports for each relevant usage

   Additional information could be required (e.g., supported extensions
   or CA policy information) in order to produce a certification request
   that will be successful.  However, for simplicity, we do not mandate
   that the end entity acquires this information via the PKI messages.
   The end result is simply that some certification requests may fail
   (e.g., if the end entity wants to generate its own encryption key,
   but the CA doesn't allow that).

   The required information MAY be acquired as described in Section 6.5.

6.7.2.  Out-of-Band Verification of the Root CA Key

   An end entity must securely possess the public key of its root CA.
   One method to achieve this is to provide the end entity with the CA's
   self-certificate fingerprint via some secure "out-of-band" means.
   The end entity can then securely use the CA's self-certificate.

   See Section 6.1 for further details.

6.8.  Certificate Request

   An initialized end entity MAY request an additional certificate at
   any time (for any purpose).  This request will be made using the
   certification request (cr) message.  If the end entity already
   possesses a signing key pair (with a corresponding verification
   certificate), then this cr message will typically be protected by the
   entity's digital signature.  The CA returns the new certificate (if
   the request is successful) in a CertRepMessage.

6.9.  Key Update

   When a key pair is due to expire, the relevant end entity MAY request
   a key update; that is, it MAY request that the CA issue a new
   certificate for a new key pair (or, in certain circumstances, a new
   certificate for the same key pair).  The request is made using a key
   update request (kur) message (referred to, in some environments, as a
   "Certificate Update" operation).  If the end entity already possesses
   a signing key pair (with a corresponding verification certificate),
   then this message will typically be protected by the entity's digital
   signature.  The CA returns the new certificate (if the request is
   successful) in a key update response (kup) message, which is
   syntactically identical to a CertRepMessage.

7.  Version Negotiation

   This section defines the version negotiation used to support older
   protocols between clients and servers.

   If a client knows the protocol version(s) supported by the server
   (e.g., from a previous PKIMessage exchange or via some out-of-band
   means), then it MUST send a PKIMessage with the highest version
   supported by both it and the server.  If a client does not know what
   version(s) the server supports, then it MUST send a PKIMessage using
   the highest version it supports with the following exception: Version
   cmp2021 SHOULD only be used if cmp2021 syntax is needed for the
   request being sent or for the expected response.

   Note: Using cmp2000 as the default pvno value is done to avoid extra
   message exchanges for version negotiation and to foster compatibility
   with cmp2000 implementations.  Version cmp2021 syntax is only needed
   if a message exchange uses EnvelopedData, hashAlg (in CertStatus),
   POPOPrivKey with agreeMAC, or ckuann with RootCaKeyUpdateContent.

   If a server receives a message with a version that it supports, then
   the version of the response message MUST be the same as the received
   version.  If a server receives a message with a version higher or
   lower than it supports, then it MUST send back an ErrorMsg with the
   unsupportedVersion bit set (in the failureInfo field of the
   pKIStatusInfo).  If the received version is higher than the highest
   supported version, then the version in the error message MUST be the
   highest version the server supports; if the received version is lower
   than the lowest supported version, then the version in the error
   message MUST be the lowest version the server supports.

   If a client gets back an ErrorMsgContent with the unsupportedVersion
   bit set and a version it supports, then it MAY retry the request with
   that version.

7.1.  Supporting RFC 2510 Implementations

   [RFC2510] did not specify the behavior of implementations receiving
   versions they did not understand since there was only one version in
   existence.  With the introduction of the revision in [RFC4210], the
   following versioning behavior is recommended.

7.1.1.  Clients Talking to RFC 2510 Servers

   If, after sending a message with a pvno value higher than cmp1999, a
   client receives an ErrorMsgContent with a version of cmp1999, then it
   MUST abort the current transaction.

   If a client receives a non-error PKIMessage with a version of
   cmp1999, then it MAY decide to continue the transaction (if the
   transaction hasn't finished) using the semantics described in
   [RFC2510].  If it does not choose to do so and the transaction is not
   finished, then it MUST abort the transaction and send an
   ErrorMsgContent with a version of cmp1999.

7.1.2.  Servers Receiving Version cmp1999 PKIMessages

   If a server receives a version cmp1999 message, it MAY revert to the
   behavior described in [RFC2510] and respond with version cmp1999
   messages.  If it does not choose to do so, then it MUST send back an
   ErrorMsgContent as described above in Section 7.

8.  Security Considerations

8.1.  On the Necessity of POP

   It is well established that the role of a CA is to verify that the
   name and public key belong to the end entity prior to issuing a
   certificate.  If an entity holding a private key obtains a
   certificate containing the corresponding public key issued for a
   different entity, it can authenticate as the entity named in the
   certificate.  This facilitates masquerading.  It is not entirely
   clear what security guarantees are lost if an end entity is able to
   obtain a certificate containing a public key that they do not possess
   the corresponding private key for.  There are some scenarios,
   described as "forwarding attacks" in Appendix A of [Gueneysu], in
   which this can lead to protocol attacks against a naively implemented
   sign-then-encrypt protocol, but in general, it merely results in the
   end entity obtaining a certificate that they cannot use.

8.2.  POP with a Decryption Key

   Some cryptographic considerations are worth explicitly spelling out.
   In the protocols specified above, when an end entity is required to
   prove possession of a decryption key, it is effectively challenged to
   decrypt something (its own certificate).  This scheme (and many
   others!) could be vulnerable to an attack if the possessor of the
   decryption key in question could be fooled into decrypting an
   arbitrary challenge and returning the cleartext to an attacker.
   Although in this specification a number of other failures in security
   are required in order for this attack to succeed, it is conceivable
   that some future services (e.g., notary, trusted time) could
   potentially be vulnerable to such attacks.  For this reason, we
   reiterate the general rule that implementations should be very
   careful about decrypting arbitrary "ciphertext" and revealing
   recovered "plaintext" since such a practice can lead to serious
   security vulnerabilities.

   The client MUST return the decrypted values only if they match the
   expected content type.  In an indirect method, the decrypted value
   MUST be a valid certificate, and in a direct method, the decrypted
   value MUST be a Rand as defined in Section 5.2.8.3.3.

8.3.  POP by Exposing the Private Key

   Note also that exposing a private key to the CA/RA as a POP technique
   can carry some security risks (depending upon whether or not the CA/
   RA can be trusted to handle such material appropriately).
   Implementers are advised to:

   *  Exercise caution in selecting and using this particular POP
      mechanism.

   *  Only use this POP mechanism if archival of the private key is
      desired.

   *  When appropriate, have the user of the application explicitly
      state that they are willing to trust the CA/RA to have a copy of
      their private key before proceeding to reveal the private key.

8.4.  Attack Against DH Key Exchange

   A small subgroup attack during a DH key exchange may be carried out
   as follows.  A malicious end entity may deliberately choose DH
   parameters that enable it to derive (a significant number of bits of)
   the DH private key of the CA during a key archival or key recovery
   operation.  Armed with this knowledge, the end entity would then be
   able to retrieve the decryption private key of another unsuspecting
   end entity, EE2, during EE2's legitimate key archival or key recovery
   operation with that CA.  In order to avoid the possibility of such an
   attack, two courses of action are available.  (1) The CA may generate
   a fresh DH key pair to be used as a protocol encryption key pair for
   each end entity with which it interacts.  (2) The CA may enter into a
   key validation protocol (not specified in this document) with each
   requesting end entity to ensure that the end entity's protocol
   encryption key pair will not facilitate this attack.  Option (1) is
   clearly simpler (requiring no extra protocol exchanges from either
   party) and is therefore RECOMMENDED.

8.5.  Perfect Forward Secrecy

   Long-term security typically requires perfect forward secrecy (pfs).
   When transferring encrypted long-term confidential values such as
   centrally generated private keys or revocation passphrases, pfs is
   likely important.  Yet, it is not needed for CMP message protection
   providing integrity and authenticity because transfer of PKI messages
   is usually completed in very limited time.  For the same reason, it
   is not typically required for the indirect method to provide a POP
   (Section 5.2.8.3.2) delivering the newly issued certificate in
   encrypted form.

   Encrypted values (Section 5.2.2) are transferred using CMS
   EnvelopedData [RFC5652], which does not offer pfs.  In cases where
   long-term security is needed, CMP messages SHOULD be transferred over
   a mechanism that provides pfs, such as TLS with appropriate cipher
   suites selected.

8.6.  Private Keys for Certificate Signing and CMP Message Protection

   A CA should not reuse its certificate signing key for other purposes,
   such as protecting CMP responses and TLS connections.  This way,
   exposure to other parts of the system and the number of uses of this
   particularly critical key are reduced to a minimum.

8.7.  Entropy of Random Numbers, Key Pairs, and Shared Secret
      Information

   Implementations must generate nonces and private keys from random
   input.  The use of inadequate pseudorandom number generators (PRNGs)
   to generate cryptographic keys can result in little or no security.
   An attacker may find it much easier to reproduce the PRNG environment
   that produced the keys and to search the resulting small set of
   possibilities than brute-force searching the whole key space.  As an
   example of predictable random numbers, see [CVE-2008-0166];
   consequences of low-entropy random numbers are discussed in Mining
   Your Ps and Qs [MiningPsQs].  The generation of quality random
   numbers is difficult.  ISO/IEC 20543:2019 [ISO.20543-2019], NIST SP
   800-90A Rev.1 [NIST.SP.800_90Ar1], BSI AIS 31 V2.0 [AIS31], and other
   specifications offer valuable guidance in this area.

   If shared secret information is generated by a cryptographically
   secure random number generator (CSRNG), it is safe to assume that the
   entropy of the shared secret information equals its bit length.  If
   no CSRNG is used, the entropy of shared secret information depends on
   the details of the generation process and cannot be measured securely
   after it has been generated.  If user-generated passwords are used as
   shared secret information, their entropy cannot be measured.
   Passwords generated from user-supplied entropy are typically
   insufficient for protected delivery of centrally generated keys or
   trust anchors.

   If the entropy of shared secret information protecting the delivery
   of a centrally generated key pair is known, it should not be less
   than the security strength of that key pair; if the shared secret
   information is reused for different key pairs, the security of the
   shared secret information should exceed the security strength of each
   individual key pair.

   For the case of a PKI management operation that delivers a new trust
   anchor (e.g., a root CA certificate), using caPubs or genp that is
   (a) not concluded in a timely manner or (b) where the shared secret
   information is reused for several key management operations, the
   entropy of the shared secret information, if known, should not be
   less than the security strength of the trust anchor being managed by
   the operation.  The shared secret information should have an entropy
   that at least matches the security strength of the key material being
   managed by the operation.  Certain use cases may require shared
   secret information that may be of a low security strength, e.g., a
   human-generated password.  It is RECOMMENDED that such secret
   information be limited to a single PKI management operation.

   Importantly for this section, further information about algorithm use
   profiles and their security strength is available in Section 7 of CMP
   Algorithms [RFC9481].

8.8.  Recurring Usage of KEM Keys for Message Protection

   For each PKI management operation using MAC-based message protection
   involving KEM (see Section 5.1.3.4), the KEM Encapsulate() function,
   providing a fresh KEM ciphertext (ct) and shared secret (ss), MUST be
   invoked.

   It is assumed that the overall data size of the CMP messages in a PKI
   management operation protected by a single shared secret key is small
   enough not to introduce extra security risks.

   To be appropriate for use with this specification, the KEM algorithm
   MUST explicitly be designed to be secure when the public key is used
   many times.  For example, a KEM algorithm with a single-use public
   key is not appropriate because the public key is expected to be
   carried in a long-lived certificate [RFC5280] and used over and over.
   Thus, KEM algorithms that offer indistinguishability under adaptive
   chosen ciphertext attack (IND-CCA2) security are appropriate.  A
   common design pattern for obtaining IND-CCA2 security with public key
   reuse is to apply the Fujisaki-Okamoto (FO) transform [Fujisaki] or a
   variant of the FO transform [Hofheinz].

   Therefore, given a long-term public key using an IND-CCA2-secure KEM
   algorithm, there is no limit to the number of CMP messages that can
   be authenticated using KEM keys for MAC-based message protection.

8.9.  Trust Anchor Provisioning Using CMP Messages

   A provider of trust anchors, which may be an RA involved in
   configuration management of its clients, MUST NOT include to-be-
   trusted CA certificates in a CMP message unless the specific
   deployment scenario can ensure that it is adequate that the receiving
   end entity trusts these certificates, e.g., by loading them into its
   trust store.

   Whenever an end entity receives in a CMP message a CA certificate to
   be used as a trust anchor (for example, in the caPubs field of a
   certificate response or in a general response), it MUST properly
   authenticate the message sender with existing trust anchors without
   requiring new trust anchor information included in the message.

   Additionally, the end entity MUST verify that the sender is an
   authorized source of trust anchors.  This authorization is governed
   by local policy and typically indicated using shared secret
   information or with a signature-based message protection using a
   certificate issued by a PKI that is explicitly authorized for this
   purpose.

8.10.  Authorizing Requests for Certificates with Specific EKUs

   When a CA issues a certificate containing EKU extensions as defined
   in Section 4.5, this expresses delegation of an authorization that
   originally is only with the CA certificate itself.  Such delegation
   is a very sensitive action in a PKI, and therefore, special care must
   be taken when approving such certificate requests to ensure that only
   legitimate entities receive a certificate containing such an EKU.

8.11.  Usage of CT Logs

   CAs that support indirect POP MUST NOT also publish final
   certificates to CT logs [RFC9162] before having received the certConf
   message containing the certHash of that certificate to complete the
   POP.  The risk is that a malicious actor could fetch the final
   certificate from the CT log and use that to spoof a response to the
   implicit POP challenge via a certConf response.  This risk does not
   apply to CT precertificates, so those are OK to publish.

   If a certificate or its precertificate was published in a CT log, it
   must be revoked if a required certConf message could not be verified,
   especially when the implicit POP was used.

9.  IANA Considerations

   This document updates the ASN.1 modules in Appendix A.2 of CMP
   Updates [RFC9480].  The OID 116 (id-mod-cmp2023-02) was registered in
   the "SMI Security for PKIX Module Identifier" registry to identify
   the updated ASN.1 module.

   IANA has added the following entry in the "SMI Security for PKIX CMP
   Information Types" registry within the SMI Numbers registry group
   (see <https://www.iana.org/assignments/smi-numbers>) [RFC7299]:

   Decimal:  24
   Description:  id-it-KemCiphertextInfo
   Reference:  RFC 9810

   Note that the new OID 1.2.840.113533.7.66.16 was registered by
   Entrust, and not by IANA, for id-KemBasedMac in the arc
   1.2.840.113533.7.66.  This was done to match the previous
   registrations for id-PasswordBasedMac and id-DHBasedMac, which are
   also on the Entrust private arc.

   All existing references to [RFC2510], [RFC4210], and [RFC9480] at
   <https://www.iana.org/assignments/smi-numbers> except those in the
   "SMI Security for PKIX Module Identifier" registry have been replaced
   with references to this document.

10.  References

10.1.  Normative References

   [MvOV97]   Menezes, A., van Oorschot, P., and S. Vanstone, "Handbook
              of Applied Cryptography", CRC Press ISBN 0-8493-8523-7,
              1996, <https://cacr.uwaterloo.ca/hac/>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC2985]  Nystrom, M. and B. Kaliski, "PKCS #9: Selected Object
              Classes and Attribute Types Version 2.0", RFC 2985,
              DOI 10.17487/RFC2985, November 2000,
              <https://www.rfc-editor.org/info/rfc2985>.

   [RFC2986]  Nystrom, M. and B. Kaliski, "PKCS #10: Certification
              Request Syntax Specification Version 1.7", RFC 2986,
              DOI 10.17487/RFC2986, November 2000,
              <https://www.rfc-editor.org/info/rfc2986>.

   [RFC3629]  Yergeau, F., "UTF-8, a transformation format of ISO
              10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November
              2003, <https://www.rfc-editor.org/info/rfc3629>.

   [RFC4211]  Schaad, J., "Internet X.509 Public Key Infrastructure
              Certificate Request Message Format (CRMF)", RFC 4211,
              DOI 10.17487/RFC4211, September 2005,
              <https://www.rfc-editor.org/info/rfc4211>.

   [RFC5280]  Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
              Housley, R., and W. Polk, "Internet X.509 Public Key
              Infrastructure Certificate and Certificate Revocation List
              (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
              <https://www.rfc-editor.org/info/rfc5280>.

   [RFC5480]  Turner, S., Brown, D., Yiu, K., Housley, R., and T. Polk,
              "Elliptic Curve Cryptography Subject Public Key
              Information", RFC 5480, DOI 10.17487/RFC5480, March 2009,
              <https://www.rfc-editor.org/info/rfc5480>.

   [RFC5646]  Phillips, A., Ed. and M. Davis, Ed., "Tags for Identifying
              Languages", BCP 47, RFC 5646, DOI 10.17487/RFC5646,
              September 2009, <https://www.rfc-editor.org/info/rfc5646>.

   [RFC5652]  Housley, R., "Cryptographic Message Syntax (CMS)", STD 70,
              RFC 5652, DOI 10.17487/RFC5652, September 2009,
              <https://www.rfc-editor.org/info/rfc5652>.

   [RFC5958]  Turner, S., "Asymmetric Key Packages", RFC 5958,
              DOI 10.17487/RFC5958, August 2010,
              <https://www.rfc-editor.org/info/rfc5958>.

   [RFC6402]  Schaad, J., "Certificate Management over CMS (CMC)
              Updates", RFC 6402, DOI 10.17487/RFC6402, November 2011,
              <https://www.rfc-editor.org/info/rfc6402>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8933]  Housley, R., "Update to the Cryptographic Message Syntax
              (CMS) for Algorithm Identifier Protection", RFC 8933,
              DOI 10.17487/RFC8933, October 2020,
              <https://www.rfc-editor.org/info/rfc8933>.

   [RFC9045]  Housley, R., "Algorithm Requirements Update to the
              Internet X.509 Public Key Infrastructure Certificate
              Request Message Format (CRMF)", RFC 9045,
              DOI 10.17487/RFC9045, June 2021,
              <https://www.rfc-editor.org/info/rfc9045>.

   [RFC9481]  Brockhaus, H., Aschauer, H., Ounsworth, M., and J. Gray,
              "Certificate Management Protocol (CMP) Algorithms",
              RFC 9481, DOI 10.17487/RFC9481, November 2023,
              <https://www.rfc-editor.org/info/rfc9481>.

   [RFC9629]  Housley, R., Gray, J., and T. Okubo, "Using Key
              Encapsulation Mechanism (KEM) Algorithms in the
              Cryptographic Message Syntax (CMS)", RFC 9629,
              DOI 10.17487/RFC9629, August 2024,
              <https://www.rfc-editor.org/info/rfc9629>.

10.2.  Informative References

   [AIS31]    Killmann, W. and W. Schindler, "A proposal for:
              Functionality classes for random number generators -
              Version 2.0", Federal Office for Information Security
              (BSI), September 2011,
              <https://www.bsi.bund.de/SharedDocs/Downloads/DE/BSI/
              Zertifizierung/Interpretationen/AIS_31_Functionality_class
              es_for_random_number_generators_e.pdf>.

   [CVE-2008-0166]
              National Institute of Science and Technology (NIST),
              "National Vulnerability Database - CVE-2008-0166", May
              2008, <https://nvd.nist.gov/vuln/detail/CVE-2008-0166>.

   [ETSI-3GPP.33.310]
              3GPP, "Network Domain Security (NDS); Authentication
              Framework (AF)", 3GPP TS 33.310 16.6.0, December 2020,
              <http://www.3gpp.org/ftp/Specs/html-info/33310.htm>.

   [Fujisaki] Fujisaki, E. and T. Okamoto, "Secure Integration of
              Asymmetric and Symmetric Encryption Schemes", Journal of
              Cryptology, vol. 26, no. 1, pp. 80-101,
              DOI 10.1007/s00145-011-9114-1, December 2011,
              <https://doi.org/10.1007/s00145-011-9114-1>.

   [Gueneysu] Gueneysu, T., Hodges, P., Land, G., Ounsworth, M.,
              Stebila, D., and G. Zaverucha, "Proof-of-possession for
              KEM certificates using verifiable generation", Cryptology
              ePrint Archive, Paper 2022/703, 2022,
              <https://eprint.iacr.org/2022/703>.

   [Hofheinz] Hofheinz, D., Hövelmanns, K., and E. Kiltz, "A Modular
              Analysis of the Fujisaki-Okamoto Transformation", Theory
              of Cryptography (TCC 2017), Lecture Notes in Computer
              Science, vol. 10677, pp. 341-371,
              DOI 10.1007/978-3-319-70500-2_12, November 2017,
              <https://doi.org/10.1007/978-3-319-70500-2_12>.

   [IEEE.802.1AR-2018]
              IEEE, "IEEE Standard for Local and Metropolitan Area
              Networks - Secure Device Identity", IEEE Std 802.1AR-2018,
              DOI 10.1109/ieeestd.2018.8423794, August 2018,
              <https://doi.org/10.1109/ieeestd.2018.8423794>.

   [ISO.20543-2019]
              ISO/IEC, "Information technology -- Security techniques --
              Test and analysis methods for random bit generators within
              ISO/IEC 19790 and ISO/IEC 15408", ISO/IEC 20543:2019,
              October 2019, <https://www.iso.org/standard/68296.html>.

   [MiningPsQs]
              Heninger, N., Durumeric, Z., Wustrow, E., and J. A.
              Halderman, "Mining Your Ps and Qs: Detection of Widespread
              Weak Keys in Network Devices", 21st USENIX Security
              Symposium (USENIX Security 12), August 2012,
              <https://www.usenix.org/conference/usenixsecurity12/
              technical-sessions/presentation/heninger>.

   [ML-KEM]   Turner, S., Kampanakis, P., Massimo, J., and B.
              Westerbaan, "Internet X.509 Public Key Infrastructure -
              Algorithm Identifiers for the Module-Lattice-Based Key-
              Encapsulation Mechanism (ML-KEM)", Work in Progress,
              Internet-Draft, draft-ietf-lamps-kyber-certificates-11, 22
              July 2025, <https://datatracker.ietf.org/doc/html/draft-
              ietf-lamps-kyber-certificates-11>.

   [NIST.SP.800_90Ar1]
              Barker, E. B. and J. M. Kelsey, "Recommendation for Random
              Number Generation Using Deterministic Random Bit
              Generators", NIST SP 800-90Ar1,
              DOI 10.6028/NIST.SP.800-90Ar1, June 2015,
              <https://nvlpubs.nist.gov/nistpubs/SpecialPublications/
              NIST.SP.800-90Ar1.pdf>.

   [RFC1847]  Galvin, J., Murphy, S., Crocker, S., and N. Freed,
              "Security Multiparts for MIME: Multipart/Signed and
              Multipart/Encrypted", RFC 1847, DOI 10.17487/RFC1847,
              October 1995, <https://www.rfc-editor.org/info/rfc1847>.

   [RFC2510]  Adams, C. and S. Farrell, "Internet X.509 Public Key
              Infrastructure Certificate Management Protocols",
              RFC 2510, DOI 10.17487/RFC2510, March 1999,
              <https://www.rfc-editor.org/info/rfc2510>.

   [RFC2585]  Housley, R. and P. Hoffman, "Internet X.509 Public Key
              Infrastructure Operational Protocols: FTP and HTTP",
              RFC 2585, DOI 10.17487/RFC2585, May 1999,
              <https://www.rfc-editor.org/info/rfc2585>.

   [RFC4210]  Adams, C., Farrell, S., Kause, T., and T. Mononen,
              "Internet X.509 Public Key Infrastructure Certificate
              Management Protocol (CMP)", RFC 4210,
              DOI 10.17487/RFC4210, September 2005,
              <https://www.rfc-editor.org/info/rfc4210>.

   [RFC4212]  Blinov, M. and C. Adams, "Alternative Certificate Formats
              for the Public-Key Infrastructure Using X.509 (PKIX)
              Certificate Management Protocols", RFC 4212,
              DOI 10.17487/RFC4212, October 2005,
              <https://www.rfc-editor.org/info/rfc4212>.

   [RFC4303]  Kent, S., "IP Encapsulating Security Payload (ESP)",
              RFC 4303, DOI 10.17487/RFC4303, December 2005,
              <https://www.rfc-editor.org/info/rfc4303>.

   [RFC4511]  Sermersheim, J., Ed., "Lightweight Directory Access
              Protocol (LDAP): The Protocol", RFC 4511,
              DOI 10.17487/RFC4511, June 2006,
              <https://www.rfc-editor.org/info/rfc4511>.

   [RFC5912]  Hoffman, P. and J. Schaad, "New ASN.1 Modules for the
              Public Key Infrastructure Using X.509 (PKIX)", RFC 5912,
              DOI 10.17487/RFC5912, June 2010,
              <https://www.rfc-editor.org/info/rfc5912>.

   [RFC6268]  Schaad, J. and S. Turner, "Additional New ASN.1 Modules
              for the Cryptographic Message Syntax (CMS) and the Public
              Key Infrastructure Using X.509 (PKIX)", RFC 6268,
              DOI 10.17487/RFC6268, July 2011,
              <https://www.rfc-editor.org/info/rfc6268>.

   [RFC6712]  Kause, T. and M. Peylo, "Internet X.509 Public Key
              Infrastructure -- HTTP Transfer for the Certificate
              Management Protocol (CMP)", RFC 6712,
              DOI 10.17487/RFC6712, September 2012,
              <https://www.rfc-editor.org/info/rfc6712>.

   [RFC7296]  Kaufman, C., Hoffman, P., Nir, Y., Eronen, P., and T.
              Kivinen, "Internet Key Exchange Protocol Version 2
              (IKEv2)", STD 79, RFC 7296, DOI 10.17487/RFC7296, October
              2014, <https://www.rfc-editor.org/info/rfc7296>.

   [RFC7299]  Housley, R., "Object Identifier Registry for the PKIX
              Working Group", RFC 7299, DOI 10.17487/RFC7299, July 2014,
              <https://www.rfc-editor.org/info/rfc7299>.

   [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
              <https://www.rfc-editor.org/info/rfc8446>.

   [RFC8572]  Watsen, K., Farrer, I., and M. Abrahamsson, "Secure Zero
              Touch Provisioning (SZTP)", RFC 8572,
              DOI 10.17487/RFC8572, April 2019,
              <https://www.rfc-editor.org/info/rfc8572>.

   [RFC8649]  Housley, R., "Hash Of Root Key Certificate Extension",
              RFC 8649, DOI 10.17487/RFC8649, August 2019,
              <https://www.rfc-editor.org/info/rfc8649>.

   [RFC8995]  Pritikin, M., Richardson, M., Eckert, T., Behringer, M.,
              and K. Watsen, "Bootstrapping Remote Secure Key
              Infrastructure (BRSKI)", RFC 8995, DOI 10.17487/RFC8995,
              May 2021, <https://www.rfc-editor.org/info/rfc8995>.

   [RFC9147]  Rescorla, E., Tschofenig, H., and N. Modadugu, "The
              Datagram Transport Layer Security (DTLS) Protocol Version
              1.3", RFC 9147, DOI 10.17487/RFC9147, April 2022,
              <https://www.rfc-editor.org/info/rfc9147>.

   [RFC9162]  Laurie, B., Messeri, E., and R. Stradling, "Certificate
              Transparency Version 2.0", RFC 9162, DOI 10.17487/RFC9162,
              December 2021, <https://www.rfc-editor.org/info/rfc9162>.

   [RFC9480]  Brockhaus, H., von Oheimb, D., and J. Gray, "Certificate
              Management Protocol (CMP) Updates", RFC 9480,
              DOI 10.17487/RFC9480, November 2023,
              <https://www.rfc-editor.org/info/rfc9480>.

   [RFC9482]  Sahni, M., Ed. and S. Tripathi, Ed., "Constrained
              Application Protocol (CoAP) Transfer for the Certificate
              Management Protocol", RFC 9482, DOI 10.17487/RFC9482,
              November 2023, <https://www.rfc-editor.org/info/rfc9482>.

   [RFC9483]  Brockhaus, H., von Oheimb, D., and S. Fries, "Lightweight
              Certificate Management Protocol (CMP) Profile", RFC 9483,
              DOI 10.17487/RFC9483, November 2023,
              <https://www.rfc-editor.org/info/rfc9483>.

   [RFC9733]  von Oheimb, D., Ed., Fries, S., and H. Brockhaus, "BRSKI
              with Alternative Enrollment (BRSKI-AE)", RFC 9733,
              DOI 10.17487/RFC9733, March 2025,
              <https://www.rfc-editor.org/info/rfc9733>.

   [RFC9811]  Brockhaus, H., von Oheimb, D., Ounsworth, M., and J. Gray,
              "Internet X.509 Public Key Infrastructure -- HTTP Transfer
              for the Certificate Management Protocol (CMP)", RFC 9811,
              July 2025, <https://www.rfc-editor.org/info/rfc9811>.

   [UNISIG.Subset-137]
              UNISIG, "ERTMS/ETCS On-line Key Management FFFIS", Subset-
              137, V1.0.0, December 2015,
              <https://www.era.europa.eu/system/files/2023-01/
              sos3_index083_-_subset-137_v100.pdf>.

   [X509.2019]
              ITU-T, "Information technology - Open Systems
              Interconnection - The Directory: Public-key and attribute
              certificate frameworks", ITU-T Recommendation X.509
              (10/2019), October 2019,
              <https://handle.itu.int/11.1002/1000/14033>.

Appendix A.  Reasons for the Presence of RAs

   The reasons that justify the presence of an RA can be split into
   those that are due to technical factors and those that are
   organizational in nature.  Technical reasons include the following.

   *  If hardware tokens are in use, then not all end entities will have
      the equipment needed to initialize these; the RA equipment can
      include the necessary functionality (this may also be a matter of
      policy).

   *  Some end entities may not have the capability to publish
      certificates; again, the RA may be suitably placed for this.

   *  The RA will be able to issue signed revocation requests on behalf
      of end entities associated with it, whereas the end entity may not
      be able to do this (if the key pair is completely lost).

   Some of the organizational reasons that argue for the presence of an
   RA are the following.

   *  It may be more cost effective to concentrate functionality in the
      RA equipment than to supply functionality to all end entities
      (especially if special token initialization equipment is to be
      used).

   *  Establishing RAs within an organization can reduce the number of
      CAs required, which is sometimes desirable.

   *  RAs may be better placed to identify people with their
      "electronic" names, especially if the CA is physically remote from
      the end entity.

   *  For many applications, there will already be some administrative
      structure in place so that candidates for the role of RA are easy
      to find (which may not be true of the CA).

   Further reasons relevant for automated machine-to-machine certificate
   lifecycle management are available in the Lightweight CMP Profile
   [RFC9483].

Appendix B.  The Use of Revocation Passphrase

   A revocation request must incorporate suitable security mechanisms,
   including proper authentication, in order to reduce the probability
   of successful denial-of-service attacks.  A digital signature or DH/
   KEM-based message protection on the request -- REQUIRED to support
   within this specification depending on the key type used if
   revocation requests are supported -- can provide the authentication
   required, but there are circumstances under which an alternative
   mechanism may be desirable (e.g., when the private key is no longer
   accessible and the entity wishes to request a revocation prior to re-
   certification of another key pair).  In order to accommodate such
   circumstances, a password-based MAC (see Section 6.1 of CMP
   Algorithms [RFC9481]) on the request is also REQUIRED to support
   within this specification (subject to local security policy for a
   given environment) if revocation requests are supported and if shared
   secret information can be established between the requester and the
   responder prior to the need for revocation.

   A mechanism that has seen use in some environments is "revocation
   passphrase", in which a value of sufficient entropy (i.e., a
   relatively long passphrase rather than a short password) is shared
   between (only) the entity and the CA/RA at some point prior to
   revocation; this value is later used to authenticate the revocation
   request.

   In this specification, the following technique to establish shared
   secret information (i.e., a revocation passphrase) is OPTIONAL to
   support.  Its precise use in CMP messages is as follows.

   *  The OID and value specified in Section 5.3.19.9 MAY be sent in a
      GenMsg message at any time or MAY be sent in the generalInfo field
      of the PKIHeader of any PKIMessage at any time.  (In particular,
      the EncryptedKey structure as described in Section 5.2.2 may be
      sent in the header of the certConf message that confirms
      acceptance of certificates requested in an initialization request
      or certificate request message.)  This conveys a revocation
      passphrase chosen by the entity to the relevant CA/RA.  When
      EnvelopedData is used, this is in the decrypted bytes of the
      encryptedContent field.  When EncryptedValue is used, this is in
      the decrypted bytes of the encValue field.  Furthermore, the
      transfer is accomplished with appropriate confidentiality
      characteristics.

   *  If a CA/RA receives the revocation passphrase (OID and value
      specified in Section 5.3.19.9) in a GenMsg, it MUST construct and
      send a GenRep message that includes the OID (with absent value)
      specified in Section 5.3.19.9.  If the CA/RA receives the
      revocation passphrase in the generalInfo field of a PKIHeader of
      any PKIMessage, it MUST include the OID (with absent value) in the
      generalInfo field of the PKIHeader of the corresponding response
      PKIMessage.  If the CA/RA is unable to return the appropriate
      response message for any reason, it MUST send an error message
      with a status of "rejection" and, optionally, a failInfo reason
      set.

   *  Either the localKeyId attribute of EnvelopedData as specified in
      [RFC2985] or the valueHint field of EncryptedValue MAY contain a
      key identifier (chosen by the entity, along with the passphrase
      itself) to assist in later retrieval of the correct passphrase
      (e.g., when the revocation request is constructed by the end
      entity and received by the CA/RA).

   *  The revocation request message is protected by a password-based
      MAC (see Section 6.1 of "CMP Algorithms" [RFC9481]) with the
      revocation passphrase as the key.  If appropriate, the senderKID
      field in the PKIHeader MAY contain the value previously
      transmitted in localKeyId or valueHint.

   Note: For a message transferring a revocation passphrase indicating
   cmp2021(3) in the pvno field of the PKIHeader, the encrypted
   passphrase MUST be transferred in the envelopedData choice of
   EncryptedKey as defined in Section 5.2.2.  When using cmp2000(2) in
   the message header for backward compatibility, the encryptedValue is
   used.  This allows the necessary conveyance and protection of the
   passphrase while maintaining bits-on-the-wire compatibility with
   [RFC4210].  The encryptedValue choice has been deprecated in favor of
   encryptedData.

   Using the technique specified above, the revocation passphrase may be
   initially established and updated at any time without requiring extra
   messages or out-of-band exchanges.  For example, the revocation
   request message itself (protected and authenticated through a MAC
   that uses the revocation passphrase as a key) may contain, in the
   PKIHeader, a new revocation passphrase to be used for authenticating
   future revocation requests for any of the entity's other
   certificates.  In some environments, this may be preferable to
   mechanisms that reveal the passphrase in the revocation request
   message, since this can allow a denial-of-service attack in which the
   revealed passphrase is used by an unauthorized third party to
   authenticate revocation requests on the entity's other certificates.
   However, because the passphrase is not revealed in the request
   message, there is no requirement that the passphrase must always be
   updated when a revocation request is made (that is, the same
   passphrase MAY be used by an entity to authenticate revocation
   requests for different certificates at different times).

   Furthermore, the above technique can provide strong cryptographic
   protection over the entire revocation request message even when a
   digital signature is not used.  Techniques that do authentication of
   the revocation request by simply revealing the revocation passphrase
   typically do not provide cryptographic protection over the fields of
   the request message (so that a request for revocation of one
   certificate may be modified by an unauthorized third party to a
   request for revocation of another certificate for that entity).

Appendix C.  PKI Management Message Profiles (REQUIRED)

   This appendix contains detailed profiles for those PKIMessages that
   MUST be supported by conforming implementations (see Section 6).

   Note: Appendices C and D focus on PKI management operations managing
   certificates for human end entities.  In contrast, the Lightweight
   CMP Profile [RFC9483] focuses on typical use cases of industrial and
   IoT scenarios supporting highly automated certificate lifecycle
   management scenarios.

   Profiles for the PKIMessages used in the following PKI management
   operations are provided:

   *  initial registration/certification

   *  basic authenticated scheme

   *  certificate request

   *  key update

C.1.  General Rules for Interpretation of These Profiles

   1.  Where OPTIONAL or DEFAULT fields are not mentioned in individual
       profiles, they SHOULD be absent from the relevant message (i.e.,
       a receiver can validly reject a message containing such fields as
       being syntactically incorrect).  Mandatory fields are not
       mentioned if they have an obvious value.  The protocol version
       number MUST be set as specified in Section 7).

   2.  Where structures occur in more than one message, they are
       separately profiled as appropriate.

   3.  The algorithmIdentifiers from PKIMessage structures are profiled
       separately.

   4.  A "special" X.500 DN is called the "NULL-DN"; this means a DN
       containing a zero-length SEQUENCE OF RelativeDistinguishedNames
       (its DER encoding is then '3000'H).

   5.  Where a GeneralName is required for a field, but no suitable
       value is available (e.g., an end entity produces a request before
       knowing its name), then the GeneralName is to be an X.500 NULL-DN
       (i.e., the Name field of the CHOICE is to contain a NULL-DN).

   6.  Where a profile omits to specify the value for a GeneralName,
       then the NULL-DN value is to be present in the relevant
       PKIMessage field.  This occurs with the sender field of the
       PKIHeader for some messages.

   7.  Where any ambiguity arises due to naming of fields, the profile
       names these using a "dot" notation (e.g., "certTemplate.subject"
       means the subject field within a field called certTemplate).

   8.  Where a "SEQUENCE OF types" is part of a message, a zero-based
       array notation is used to describe fields within the SEQUENCE OF
       (e.g., crm[0].certReq.certTemplate.subject refers to a subfield
       of the first CertReqMsg contained in a request message).

   9.  All PKI message exchanges in Appendices C.4 to C.6 require a
       certConf message to be sent by the initiating entity and a
       pkiconf message to be sent by the responding entity.  The pkiconf
       is not included in some of the profiles given since its body is
       NULL and its header contents are clear from the context.  Any
       authenticated means can be used for the protectionAlg (e.g.,
       password-based MAC, if shared secret information is known, or
       signature).

C.2.  Algorithm Use Profile

   For specifications of algorithm identifiers and respective
   conventions for conforming implementations, please refer to
   Section 7.1 of CMP Algorithms [RFC9481].

C.3.  POP Profile

   The table below describes the POP fields for use (in signature field
   of pop field of ProofOfPossession structure) when proving possession
   of a private signing key that corresponds to a public verification
   key for which a certificate has been requested.

     +=====================+=============+===========================+
     | Field               | Value       | Comment                   |
     +=====================+=============+===========================+
     | algorithmIdentifier | MSG_SIG_ALG | only signature protection |
     |                     |             | is allowed for this proof |
     +---------------------+-------------+---------------------------+
     | signature           | present     | bits calculated using     |
     |                     |             | MSG_SIG_ALG               |
     +---------------------+-------------+---------------------------+

                                  Table 2

   Note: For examples of MSG_SIG_ALG OIDs, see Section 3 of CMP
   Algorithms [RFC9481].

   POP of a private decryption key that corresponds to a public
   encryption key for which a certificate has been requested does not
   use this profile; the CertHash field of the certConf message is used
   instead.

   Not every CA/RA will do POP (of signing key, decryption key, or key
   agreement key) in the PKIX-CMP in-band certification request protocol
   (how POP is done MAY ultimately be a policy issue that is made
   explicit for any given CA in its publicized Policy OID and
   Certification Practice Statement).  However, this specification
   mandates that CA/RA entities MUST do POP (by some means) as part of
   the certification process.  All end entities MUST be prepared to
   provide POP (i.e., these components of the PKIX-CMP protocol MUST be
   supported).

C.4.  Initial Registration/Certification (Basic Authenticated Scheme)

   An (uninitialized) end entity requests a (first) certificate from a
   CA.  When the CA responds with a message containing a certificate,
   the end entity replies with a certificate confirmation.  The CA sends
   a pkiconf message back, closing the transaction.  All messages are
   authenticated.

   This scheme allows the end entity to request certification of a
   locally generated public key (typically a signature key).  The end
   entity MAY also choose to request the centralized generation and
   certification of another key pair (typically an encryption key pair).

   Certification may only be requested for one locally generated public
   key (for more, use separate PKIMessages).

   The end entity MUST support POP of the private key associated with
   the locally generated public key.

   Preconditions:

   1.  The end entity can authenticate the CA's signature based on out-
       of-band means.

   2.  The end entity and the CA share a symmetric MACing key.

   Message Flow:

   Step# End entity                           PKI
   ---------------------------------------------------------------------
     1   format ir
     2                     -->   ir      -->
     3                                        handle ir
     4                                        format ip
     5                     <--   ip      <--
     6   handle ip
     7   format certConf
     8                     -->  certConf -->
     9                                        handle certConf
    10                                        format pkiconf
    11                     <--  pkiconf  <--
    12   handle pkiconf

   For this profile, we mandate that the end entity MUST include all
   (i.e., one or two) CertReqMsg in a single PKIMessage and that the PKI
   (CA) MUST produce a single response PKIMessage that contains the
   complete response (i.e., including the OPTIONAL second key pair, if
   it was requested and if centralized key generation is supported).
   For simplicity, we also mandate that this message MUST be the final
   one (i.e., no use of "waiting" status value).

   The end entity has an out-of-band interaction with the CA/RA.  This
   transaction established the shared secret, the referenceNumber, and
   optionally the DN used for both the sender and subject name in the
   certificate template.  See Section 8.7 for security considerations on
   quality of shared secret information.

   Initialization Request -- ir


   Field                Value

   recipient            CA name
     -- the name of the CA who is being asked to produce a certificate
   protectionAlg        MSG_MAC_ALG
     -- only MAC protection is allowed for this request, based
     -- on initial authentication key
   senderKID            referenceNum
     -- the reference number that the CA has previously issued
     -- to the end entity (together with the MACing key)
   transactionID        present
     -- implementation-specific value, meaningful to end
     -- entity.
     -- [If already in use at the CA, then a rejection message MUST
     -- be produced by the CA]

   senderNonce          present
     -- 128 (pseudo-)random bits
   freeText             any valid value
   body                 ir (CertReqMessages)
                        only one or two CertReqMsg
                        are allowed
     -- if more certificates are required, requests MUST be
     -- packaged in separate PKIMessages

   CertReqMsg           one or two present
     -- see below for details, note: crm[0] means the first
     -- (which MUST be present), crm[1] means the second (which
     -- is OPTIONAL, and used to ask for a centrally generated key)

   crm[0].certReq.      fixed value of zero
      certReqId
     -- this is the index of the template within the message
   crm[0].certReq       present
      certTemplate
     -- MUST include subject public key value, otherwise unconstrained
   crm[0].pop...        optionally present if public key
      POPOSigningKey    from crm[0].certReq.certTemplate is
                        a signing key
     -- POP MAY be required in this exchange
     -- (see Appendix D.3 for details)
   crm[0].certReq.      optionally present
      controls.archiveOptions
     -- the end entity MAY request that the locally generated
     -- private key be archived

   crm[0].certReq.      optionally present
      controls.publicationInfo
     -- the end entity MAY ask for publication of resulting cert.

   crm[1].certReq       fixed value of one
         certReqId
        -- the index of the template within the message
      crm[1].certReq       present
         certTemplate
         -- MUST NOT include actual public key bits, otherwise
         -- unconstrained (e.g., the names need not be the same as in
         -- crm[0]).  Note that subjectPublicKeyInfo MAY be present
         -- and contain an AlgorithmIdentifier followed by a
         -- zero-length BIT STRING for the subjectPublicKey if it is
         -- desired to inform the CA/RA of algorithm and parameter
         -- preferences regarding the to-be-generated key pair.

      crm[1].certReq.      present [object identifier MUST be
                                    PROT_ENC_ALG]

         controls.protocolEncrKey
        -- if centralized key generation is supported by this CA,
        -- this short-term asymmetric encryption key (generated by
        -- the end entity) will be used by the CA to encrypt (a
        -- symmetric key used to encrypt) a private key generated by
        -- the CA on behalf of the end entity

   crm[1].certReq.      optionally present
      controls.archiveOptions
   crm[1].certReq.      optionally present
      controls.publicationInfo
   protection           present
     -- bits calculated using MSG_MAC_ALG

   Initialization Response -- ip

   Field                Value

   sender               CA name
     -- the name of the CA who produced the message
   messageTime          present
     -- time at which CA produced message
   protectionAlg        MSG_MAC_ALG
     -- only MAC protection is allowed for this response
   senderKID             referenceNum
     -- the reference number that the CA has previously issued to the
     -- end entity (together with the MACing key)
   transactionID        present
     -- value from corresponding ir message
   senderNonce          present
     -- 128 (pseudo-)random bits
   recipNonce           present
     -- value from senderNonce in corresponding ir message
   freeText             any valid value
   body                 ip (CertRepMessage)
                        contains exactly one response
                        for each request
     -- The PKI (CA) responds to either one or two requests as
     -- appropriate.  crc[0] denotes the first (always present);
     -- crc[1] denotes the second (only present if the ir message
     -- contained two requests and if the CA supports centralized
     -- key generation).
   crc[0].              fixed value of zero
      certReqId
     -- MUST contain the response to the first request in the
     -- corresponding ir message
   crc[0].status.       present, positive values allowed:
      status               "accepted", "grantedWithMods"
                        negative values allowed:
                           "rejection"
   crc[0].status.       present if and only if
      failInfo          crc[0].status.status is "rejection"
   crc[0].              present if and only if
      certifiedKeyPair  crc[0].status.status is
                           "accepted" or "grantedWithMods"
   certificate          present unless end entity's public
                        key is an encryption key and POP
                        is done in this in-band exchange
   encryptedCert        present if and only if end entity's
                        public key is an encryption key and
                        POP done in this in-band exchange
   publicationInfo      optionally present

     -- indicates where certificate has been published (present
     -- at discretion of CA)

   crc[1].              fixed value of one
      certReqId
     -- MUST contain the response to the second request in the
     -- corresponding ir message
   crc[1].status.       present, positive values allowed:
      status               "accepted", "grantedWithMods"
                        negative values allowed:
                           "rejection"
   crc[1].status.       present if and only if
      failInfo          crc[0].status.status is "rejection"
   crc[1].              present if and only if
      certifiedKeyPair  crc[0].status.status is "accepted"
                        or "grantedWithMods"
   certificate          present
   privateKey           present
     -- Use EnvelopedData; if backward compatibility is required,
     -- use EncryptedValue, see Section 5.2.2
   publicationInfo      optionally present
     -- indicates where certificate has been published (present
     -- at discretion of CA)

   protection           present
     -- bits calculated using MSG_MAC_ALG
   extraCerts           optionally present
     -- the CA MAY provide additional certificates to the end
     -- entity

   Certificate confirm -- certConf

   Field                Value

   sender               present
     -- same as in ir
   recipient            CA name
     -- the name of the CA who was asked to produce a certificate
   transactionID        present
     -- value from corresponding ir and ip messages
   senderNonce          present
     -- 128 (pseudo-)random bits
   recipNonce           present
     -- value from senderNonce in corresponding ip message
   protectionAlg        MSG_MAC_ALG
     -- only MAC protection is allowed for this message.  The
     -- MAC is based on the initial authentication key shared
     -- between the end entity and the CA.

   senderKID            referenceNum
     -- the reference number that the CA has previously issued
     -- to the end entity (together with the MACing key)

   body                 certConf
     -- see Section 5.3.18, "PKI Confirmation Content", for the
     -- contents of the certConf fields.
     -- Note: two CertStatus structures are required if both an
     -- encryption and a signing certificate were sent.

   protection           present
     -- bits calculated using MSG_MAC_ALG

   Confirmation -- pkiconf

   Field                Value

   sender               present
     -- same as in ip
   recipient            present
     -- sender name from certConf
   transactionID        present
     -- value from certConf message
   senderNonce          present
     -- 128 (pseudo-)random bits
   recipNonce           present
     -- value from senderNonce from certConf message
   protectionAlg        MSG_MAC_ALG
     -- only MAC protection is allowed for this message.
   senderKID            referenceNum
   body                 pkiconf
   protection           present
     -- bits calculated using MSG_MAC_ALG

C.5.  Certificate Request

   An (initialized) end entity requests a certificate from a CA (for any
   reason).  When the CA responds with a message containing a
   certificate, the end entity replies with a certificate confirmation.
   The CA replies with a pkiconf message to close the transaction.  All
   messages are authenticated.

   The profile for this exchange is identical to that given in
   Appendix C.4, with the following exceptions:

   *  sender name SHOULD be present;

   *  protectionAlg of MSG_SIG_ALG MUST be supported (MSG_MAC_ALG MAY
      also be supported) in request, response, certConf, and pkiconf
      messages;

   *  senderKID and recipKID are only present if required for message
      verification;

   *  body is cr or cp;

   *  body may contain one or two CertReqMsg structures, but either
      CertReqMsg may be used to request certification of a locally
      generated public key or a centrally generated public key (i.e.,
      the position-dependence requirement of Appendix C.4 is removed);
      and

   *  protection bits are calculated according to the protectionAlg
      field.

C.6.  Key Update Request

   An (initialized) end entity requests a certificate from a CA (to
   update the key pair and/or corresponding certificate that it already
   possesses).  When the CA responds with a message containing a
   certificate, the end entity replies with a certificate confirmation.
   The CA replies with a PKIConfirm to close the transaction.  All
   messages are authenticated.

   The profile for this exchange is identical to that given in
   Appendix C.4, with the following exceptions:

   *  sender name SHOULD be present;

   *  protectionAlg of MSG_SIG_ALG MUST be supported (MSG_MAC_ALG MAY
      also be supported) in request, response, certConfirm, and
      PKIConfirm messages;

   *  senderKID and recipKID are only present if required for message
      verification;

   *  body is kur or kup;

   *  body may contain one or two CertReqMsg structures, but either
      CertReqMsg may be used to request certification of a locally
      generated public key or a centrally generated public key (i.e.,the
      position-dependence requirement of Appendix C.4 is removed);

   *  protection bits are calculated according to the protectionAlg
      field; and

   *  regCtrl OldCertId SHOULD be used (unless it is clear to both the
      sender and receiver -- by means not specified in this document --
      that it is not needed).

Appendix D.  PKI Management Message Profiles (OPTIONAL)

   This appendix contains detailed profiles for those PKIMessages that
   MAY be supported by implementations.

   Profiles for the PKIMessages used in the following PKI management
   operations are provided:

   *  root CA key update

   *  information request/response

   *  cross-certification request/response (1-way)

   *  in-band initialization using external identity certificate

   Future versions of this document may extend the above to include
   profiles for the operations listed below (along with other
   operations, if desired).

   *  revocation request

   *  certificate publication

   *  CRL publication

D.1.  General Rules for Interpretation of These Profiles

   Identical to Appendix C.1.

D.2.  Algorithm Use Profile

   Identical to Appendix C.2.

D.3.  Self-Signed Certificates

   The table below is a profile of how a certificate structure may be
   "self-signed".  These structures are used for distribution of new
   root CA public keys.  This can occur in one of three ways (see
   Section 4.4 above for a description of the use of these structures):

       +============+=============================================+
       | Type       | Function                                    |
       +============+=============================================+
       | newWithNew | a "self-signed" certificate; the contained  |
       |            | public key MUST be usable to verify the     |
       |            | signature (though this provides only        |
       |            | integrity and no authentication whatsoever) |
       +------------+---------------------------------------------+
       | oldWithNew | previous root CA public key signed with new |
       |            | private key                                 |
       +------------+---------------------------------------------+
       | newWithOld | new root CA public key signed with previous |
       |            | private key                                 |
       +------------+---------------------------------------------+

                                 Table 3

   A newWithNew certificate (including relevant extensions) must contain
   "sensible" values for all fields.  For example, when present,
   subjectAltName MUST be identical to issuerAltName, and, when present,
   keyIdentifiers must contain appropriate values, et cetera.

D.4.  Root CA Key Update

   A root CA updates its key pair.  It then produces a CA key update
   announcement message that can be made available (via some transport
   mechanism) to the relevant entities.  A confirmation message is not
   required from the end entities.

   ckuann message:

   +============+================================+=====================+
   | Field      | Value                          | Comment             |
   +============+================================+=====================+
   | sender     | CA name CA name                |                     |
   +------------+--------------------------------+---------------------+
   | body       | ckuann(RootCaKeyUpdateContent) |                     |
   +------------+--------------------------------+---------------------+
   | newWithNew | optionally present             | see Appendix D.3    |
   |            |                                | above               |
   +------------+--------------------------------+---------------------+
   | newWithOld | optionally present             | see Appendix D.3    |
   |            |                                | above               |
   +------------+--------------------------------+---------------------+
   | oldWithNew | optionally present             | see Appendix D.3    |
   |            |                                | above               |
   +------------+--------------------------------+---------------------+
   | extraCerts | optionally present             | can be used to      |
   |            |                                | "publish"           |
   |            |                                | certificates        |
   |            |                                | (e.g.,              |
   |            |                                | certificates        |
   |            |                                | signed using the    |
   |            |                                | new private key)    |
   +------------+--------------------------------+---------------------+

                                  Table 4

D.5.  PKI Information Request/Response

   The end entity sends a general message to the PKI requesting details
   that will be required for later PKI management operations.  The RA/CA
   responds with a general response.  If an RA generates the response,
   then it will simply forward the equivalent message that it previously
   received from the CA, with the possible addition of certificates to
   the extraCerts fields of the PKIMessage.  A confirmation message is
   not required from the end entity.

   Message Flows:

   Step# End entity                        PKI
   ---------------------------------------------------------------------
     1   format genm
     2                -->   genm   -->
     3                                     handle genm
     4                                     produce genp
     5                <--   genp   <--
     6   handle genp

   genM:

   Field               Value

   recipient           CA name
     -- the name of the CA as contained in issuerAltName
     -- extensions or issuer fields within certificates
   protectionAlg       MSG_MAC_ALG or MSG_SIG_ALG
     -- any authenticated protection alg.
   SenderKID           present if required
     -- must be present if required for verification of message
     -- protection
   freeText            any valid value
   body                genr (GenReqContent)
   GenMsgContent       empty SEQUENCE
     -- all relevant information requested
   protection          present
     -- bits calculated using MSG_MAC_ALG or MSG_SIG_ALG

   genP:

   Field                Value

   sender               CA name
     -- name of the CA that produced the message
   protectionAlg        MSG_MAC_ALG or MSG_SIG_ALG
     -- any authenticated protection alg.
   senderKID            present if required
     -- must be present if required for verification of message
     -- protection
   body                 genp (GenRepContent)
   CAProtEncCert        present (object identifier one
                        of PROT_ENC_ALG), with relevant
                        value
     -- to be used if end entity needs to encrypt information for
     -- the CA (e.g., private key for recovery purposes)

   SignKeyPairTypes     present, with relevant value
     -- the set of signature algorithm identifiers that this CA will
     -- certify for subject public keys
   EncKeyPairTypes      present, with relevant value
     -- the set of encryption / key agreement algorithm identifiers that
     -- this CA will certify for subject public keys
   PreferredSymmAlg     present (object identifier one
                        of PROT_SYM_ALG) , with relevant
                        value
     -- the symmetric algorithm that this CA expects to be used
     -- in later PKI messages (for encryption)
   RootCaKeyUpdate      optionally present, with
                        relevant value
     -- Use RootCaKeyUpdate; if backward compatibility with cmp2000 is
     -- required, use CAKeyUpdateInfo.
     -- The CA MAY provide information about a relevant root CA
     -- key pair using this field (note that this does not imply
     -- that the responding CA is the root CA in question)
   CurrentCRL           optionally present, with relevant value
     -- the CA MAY provide a copy of a complete CRL (i.e.,
     -- fullest possible one)
   protection           present
     -- bits calculated using MSG_MAC_ALG or MSG_SIG_ALG
   extraCerts           optionally present
     -- can be used to send some certificates to the end
     -- entity. An RA MAY add its certificate here.

D.6.  Cross-Certification Request/Response (1-way)

   This section describes the creation of a single cross-certificate
   (i.e., not two at once).  The requesting CA MAY choose who is
   responsible for publication of the cross-certificate created by the
   responding CA through use of the PKIPublicationInfo control.

   Preconditions:

   1.  Responding CA can verify the origin of the request (possibly
       requiring out-of-band means) before processing the request.

   2.  Requesting CA can authenticate the authenticity of the origin of
       the response (possibly requiring out-of-band means) before
       processing the response.

   The use of certificate confirmation and the corresponding server
   confirmation is determined by the generalInfo field in the PKIHeader
   (see Section 5.1.1).  The following profile does not mandate support
   for either confirmation.

   Message Flows:

   Step# Requesting CA                       Responding CA
   ---------------------------------------------------------------------
     1   format ccr
     2                  -->    ccr    -->
     3                                       handle ccr
     4                                       produce ccp
     5                  <--    ccp    <--
     6   handle ccp

   ccr:

   Field                 Value

   sender                Requesting CA name
     -- the name of the CA who produced the message
   recipient             Responding CA name
     -- the name of the CA who is being asked to produce a certificate
   messageTime           time of production of message
     -- current time at requesting CA
   protectionAlg         MSG_SIG_ALG
     -- only signature protection is allowed for this request
   senderKID             present if required
     -- must be present if required for verification of message
     -- protection
   recipKID             present if required
     -- must be present if required for verification of message
     -- protection
   transactionID         present
     -- implementation-specific value, meaningful to requesting CA.
     -- [If already in use at responding CA, then a rejection message
     -- MUST be produced by responding CA]
   senderNonce           present
     -- 128 (pseudo-)random bits
   freeText              any valid value
   body                  ccr (CertReqMessages)
                         only one CertReqMsg
                         allowed
     -- if multiple cross-certificates are required, they MUST be
     -- packaged in separate PKIMessages
   certTemplate          present
     -- details follow
   version               v1 or v3
     -- v3 STRONGLY RECOMMENDED
   signingAlg            present
     -- the requesting CA must know in advance with which algorithm it
     -- wishes the certificate to be signed

   subject               present
     -- may be NULL-DN only if subjectAltNames extension value proposed
   validity              present
     -- MUST be completely specified (i.e., both fields present)
   issuer                present
     -- may be NULL-DN only if issuerAltNames extension value proposed
   publicKey             present
     -- the key to be certified (which must be for a signing algorithm)
   extensions            optionally present
     -- a requesting CA must propose values for all extensions
     -- that it requires to be in the cross-certificate
   POPOSigningKey        present
     -- see Appendix C.3: POP Profile
   protection            present
     -- bits calculated using MSG_SIG_ALG
   extraCerts            optionally present
     -- MAY contain any additional certificates that requester wishes
     -- to include

   ccp:

   Field                 Value

   sender                Responding CA name
     -- the name of the CA who produced the message
   recipient             Requesting CA name
     -- the name of the CA who asked for production of a certificate
   messageTime           time of production of message
     -- current time at responding CA
   protectionAlg         MSG_SIG_ALG
     -- only signature protection is allowed for this message
   senderKID             present if required
     -- must be present if required for verification of message
     -- protection
   recipKID              present if required
   transactionID         present
     -- value from corresponding ccr message
   senderNonce           present
     -- 128 (pseudo-)random bits
   recipNonce            present
   -- senderNonce from corresponding ccr message
   freeText              any valid value
   body                  ccp (CertRepMessage)
                         only one CertResponse allowed
     -- if multiple cross-certificates are required, they MUST be
     -- packaged in separate PKIMessages
   response              present
   status                present

   PKIStatusInfo.status  present
     -- if PKIStatusInfo.status is one of:
     --   accepted, or
     --   grantedWithMods,
     -- then certifiedKeyPair MUST be present and failInfo MUST
     -- be absent

   failInfo              present depending on
                         PKIStatusInfo.status
     -- if PKIStatusInfo.status is:
     --   rejection,
     -- then certifiedKeyPair MUST be absent and failInfo MUST be
     -- present and contain appropriate bit settings

   certifiedKeyPair      present depending on
                         PKIStatusInfo.status
   certificate           present depending on
                         certifiedKeyPair
     -- content of actual certificate must be examined by requesting CA
     -- before publication
   protection            present
     -- bits calculated using MSG_SIG_ALG
   extraCerts            optionally present
     -- MAY contain any additional certificates that responder wishes
     -- to include

D.7.  In-Band Initialization Using External Identity Certificate

   An (uninitialized) end entity wishes to initialize into the PKI with
   a CA, CA-1.  It uses, for authentication purposes, a pre-existing
   identity certificate issued by another (external) CA, CA-X.  A trust
   relationship must already have been established between CA-1 and CA-X
   so that CA-1 can validate the end entity identity certificate signed
   by CA-X.  Furthermore, some mechanism must already have been
   established within the TEE, also known as PSE, of the end entity that
   would allow it to authenticate and verify PKIMessages signed by CA-1
   (as one example, the TEE may contain a certificate issued for the
   public key of CA-1, signed by another CA that the end entity trusts
   on the basis of out-of-band authentication techniques).

   The end entity sends an initialization request to start the
   transaction.  When CA-1 responds with a message containing the new
   certificate, the end entity replies with a certificate confirmation.
   CA-1 replies with a pkiconf message to close the transaction.  All
   messages are signed (the end entity messages are signed using the
   private key that corresponds to the public key in its external
   identity certificate; the CA-1 messages are signed using the private
   key that corresponds to the public key in a certificate that can be
   chained to a trust anchor in the end entity's TEE).

   The profile for this exchange is identical to that given in
   Appendix C.4, with the following exceptions:

   *  the end entity and CA-1 do not share a symmetric MACing key (i.e.,
      there is no out-of-band shared secret information between these
      entities);

   *  sender name in ir MUST be present (and identical to the subject
      name present in the external identity certificate);

   *  protectionAlg of MSG_SIG_ALG MUST be used in all messages;

   *  external identity certificate MUST be carried in ir extraCerts
      field

   *  senderKID and recipKID are not used;

   *  body is ir or ip; and

   *  protection bits are calculated according to the protectionAlg
      field.

Appendix E.  Variants of Using KEM Keys for PKI Message Protection

   As described in Section 5.1.3.4, any party in a PKI management
   operation may wish to use a KEM key pair for message protection.
   Possible cases are described below.

   For any PKI management operation started by a PKI entity with any
   type of request message, the following message flows describe the use
   of a KEM key.  There are two cases to distinguish, namely whether the
   PKI entity or the PKI management entity owns a KEM key pair.  If both
   sides own KEM key pairs, the flows need to be combined such that for
   each direction a shared secret key is established.

   In the following message flows, Alice indicates the PKI entity that
   uses a KEM key pair for message authentication and Bob provides the
   KEM ciphertext using Alice's public KEM key, as described in
   Section 5.1.3.4.

   Step# PKI entity                           PKI management entity
         (Alice)                              (Bob)
   ---------------------------------------------------------------------
     1   format unprotected genm
           of type
           KemCiphertextInfo
           without value, and
           KEM certificate in
           extraCerts
     2                     -->   genm    -->
     3                                        validate KEM certificate
     4                                        perform KEM Encapsulate
     5                                        format unprotected genp
                                                of type
                                                KemCiphertextInfo
                                                providing KEM ciphertext
     6                     <--   genp    <--
     7   perform KEM Decapsulate
     8   perform key derivation
           to get ssk
     9   format request with
           MAC-based protection
    10                     -->  request  -->
    11                                        perform key derivation
                                                to get ssk
    12                                        verify MAC-based
                                                protection

   --------  PKI entity authenticated by PKI management entity  --------

    13                                        format response with
                                                protection depending on
                                                available key material
    14                     <-- response  <--
    15   verify protection
           provided by the
           PKI management entity

    16       Further messages of this PKI management operation
           can be exchanged with MAC-based protection by the PKI
            entity using the established shared secret key (ssk)

     Figure 3: Message Flow When the PKI Entity Has a KEM Key Pair and
                                Certificate

   Step# PKI entity                           PKI management entity
         (Bob)                                (Alice)
   ---------------------------------------------------------------------
     1   perform KEM Encapsulate
     2   format request providing
           KEM ciphertext in
           generalInfo of type
           KemCiphertextInfo,
           and with protection
           depending on available
           key material
     3                     -->  request  -->
     4                                        perform KEM Decapsulate
     5                                        perform key derivation
                                                to get ssk
     6                                        format response with
                                                MAC-based protection
     7                     <-- response  <--
     8   perform key derivation
           to get ssk
     9   verify MAC-based
           protection

   --------  PKI management entity authenticated by PKI entity  --------

    10       Further messages of this PKI management operation
             can be exchanged with MAC-based protection by the
                PKI management entity using the established
                           shared secret key (ssk)

       Figure 4: Message Flow When the PKI Entity Knows That the PKI
        Management Entity Uses a KEM Key Pair and Has the Authentic
                                 Public Key

   Note: Figure 4 describes the situation where KEM-based message
   protection may not require more than one message exchange.  In this
   case, the transactionID MUST also be used by the PKI entity (Bob) to
   ensure domain separation between different PKI management operations.

   Step# PKI entity                           PKI management entity
         (Bob)                                (Alice)
   ---------------------------------------------------------------------
     1   format request with
           protection depending
           on available key
           material
     2                     -->  request  -->
     3                                        format unprotected error
                                                with status "rejection"
                                                and failInfo
                                                "wrongIntegrity" and KEM
                                                certificate in
                                                extraCerts
     4                     <--   error   <--
     5   validate KEM certificate

     6              proceed as shown in the figure before

   Figure 5: Message Flow When the PKI Entity Does Not Know That the PKI
                   Management Entity Uses a KEM Key Pair

Appendix F.  Compilable ASN.1 Definitions

   This section contains the updated 2002 ASN.1 module from [RFC5912],
   which was updated in [RFC9480].  This module replaces the module in
   Section 9 of [RFC5912].  The module contains those changes to the
   normative ASN.1 module from Appendix F of [RFC4210] that were
   specified in [RFC9480], as well as changes made in this document.
   This module makes reference to ASN.1 structures defined in [RFC6268],
   as well as the UTF-8 encoding defined in [RFC3629].

   PKIXCMP-2023
       { iso(1) identified-organization(3) dod(6) internet(1)
       security(5) mechanisms(5) pkix(7) id-mod(0)
       id-mod-cmp2023-02(116) }
   DEFINITIONS EXPLICIT TAGS ::=
   BEGIN
   IMPORTS

   AttributeSet{}, SingleAttribute{}, Extensions{}, EXTENSION, ATTRIBUTE
   FROM PKIX-CommonTypes-2009
       {iso(1) identified-organization(3) dod(6) internet(1) security(5)
       mechanisms(5) pkix(7) id-mod(0) id-mod-pkixCommon-02(57)}

   AlgorithmIdentifier{}, SIGNATURE-ALGORITHM, ALGORITHM,
       DIGEST-ALGORITHM, MAC-ALGORITHM, KEY-DERIVATION
   FROM AlgorithmInformation-2009
       {iso(1) identified-organization(3) dod(6) internet(1) security(5)
       mechanisms(5) pkix(7) id-mod(0)
       id-mod-algorithmInformation-02(58)}

   Certificate, CertificateList, Time, id-kp
   FROM PKIX1Explicit-2009
       {iso(1) identified-organization(3) dod(6) internet(1) security(5)
       mechanisms(5) pkix(7) id-mod(0) id-mod-pkix1-explicit-02(51)}

   DistributionPointName, GeneralNames, GeneralName, KeyIdentifier
   FROM PKIX1Implicit-2009
       {iso(1) identified-organization(3) dod(6) internet(1) security(5)
       mechanisms(5) pkix(7) id-mod(0) id-mod-pkix1-implicit-02(59)}

   CertTemplate, PKIPublicationInfo, EncryptedKey, CertId,
       CertReqMessages, Controls, RegControlSet, id-regCtrl
   FROM PKIXCRMF-2009
       { iso(1) identified-organization(3) dod(6) internet(1)
       security(5) mechanisms(5) pkix(7) id-mod(0)
       id-mod-crmf2005-02(55) }
       -- The import of EncryptedKey is added due to the updates made
       -- in [RFC9480]. EncryptedValue does not need to be imported
       -- anymore and is therefore removed here.

   CertificationRequest
   FROM PKCS-10
       {iso(1) identified-organization(3) dod(6) internet(1) security(5)
       mechanisms(5) pkix(7) id-mod(0) id-mod-pkcs10-2009(69)}
   -- (specified in [RFC2986] with 1993 ASN.1 syntax and IMPLICIT
   -- tags).  Alternatively, implementers may directly include
   -- the syntax of [RFC2986] in this module.

   localKeyId
   FROM PKCS-9
       {iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-9(9)
       modules(0) pkcs-9(1)}
       -- The import of localKeyId is added due to the updates made in
       -- [RFC9480]

   EnvelopedData, SignedData
   FROM CryptographicMessageSyntax-2010
       {iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-9(9)
       smime(16) modules(0) id-mod-cms-2009(58)}
       -- The import of EnvelopedData and SignedData from [RFC6268] is
       -- added due to the updates made in CMP Updates [RFC9480]

   KEM-ALGORITHM
   FROM KEMAlgorithmInformation-2023  -- [RFC9629]
       { iso(1) identified-organization(3) dod(6) internet(1)
       security(5) mechanisms(5) pkix(7) id-mod(0)
       id-mod-kemAlgorithmInformation-2023(109) }
       -- The import of KEM-ALGORITHM was added due to the updates made
       -- in [RFC9810]
   ;

   -- History of the PKIXCMP ASN.1 modules
   -- [RFC2510]
   --    1988 Syntax, PKIXCMP, 1.3.6.1.5.5.7.0.9 (id-mod-cmp)
   --    Obsoleted by RFC 4210 PKIXCMP, 1.3.6.1.5.5.7.0.16
   --                                   (id-mod-cmp2000)
   -- [RFC4210]
   --    1988 Syntax, PKIXCMP, 1.3.6.1.5.5.7.0.16 (id-mod-cmp2000)
   --    Replaced by RFC 9480 PKIXCMP, 1.3.6.1.5.5.7.0.99
   --                                  (id-mod-cmp2021-88)
   -- [RFC5912]
   --    2002 Syntax, PKIXCMP-2009, 1.3.6.1.5.5.7.0.50
   --                               (id-mod-cmp2000-02)
   --    Replaced by RFC 9480 PKIXCMP-2021, 1.3.6.1.5.5.7.0.100
   --                                       (id-mod-cmp2021-02)
   -- [RFC9480]
   --    1988 Syntax, PKIXCMP, 1.3.6.1.5.5.7.0.99 (id-mod-cmp2021-88)
   --    2002 Syntax, PKIXCMP-2021, 1.3.6.1.5.5.7.0.100
   --                               (id-mod-cmp2021-02)
   --    Obsoleted by [RFC9810] PKIXCMP-2023, 1.3.6.1.5.5.7.0.116
   --                                         (id-mod-cmp2023-02)
   -- [RFC9810]
   --    2002 Syntax, PKIXCMP-2023, 1.3.6.1.5.5.7.0.116
   --                               (id-mod-cmp2023-02)


   -- The rest of the module contains locally defined OIDs and
   -- constructs:

   CMPCertificate ::= CHOICE { x509v3PKCert Certificate, ... }
   -- This syntax, while bits-on-the-wire compatible with the
   -- standard X.509 definition of "Certificate", allows the
   -- possibility of future certificate types (such as X.509
   -- attribute certificates, card-verifiable certificates, or other
   -- kinds of certificates) within this Certificate Management
   -- Protocol, should a need ever arise to support such generality.
   -- Those implementations that do not foresee a need to ever support
   -- other certificate types MAY, if they wish, comment out the
   -- above structure and "uncomment" the following one prior to
   -- compiling this ASN.1 module.  (Note that interoperability
   -- with implementations that don't do this will be unaffected by
   -- this change.)

   -- CMPCertificate ::= Certificate

   PKIMessage ::= SEQUENCE {
       header           PKIHeader,
       body             PKIBody,
       protection   [0] PKIProtection OPTIONAL,
       extraCerts   [1] SEQUENCE SIZE (1..MAX) OF CMPCertificate
                     OPTIONAL }

   PKIMessages ::= SEQUENCE SIZE (1..MAX) OF PKIMessage

   PKIHeader ::= SEQUENCE {
       pvno                INTEGER     { cmp1999(1), cmp2000(2),
                                         cmp2021(3) },
       sender              GeneralName,
       -- identifies the sender
       recipient           GeneralName,
       -- identifies the intended recipient
       messageTime     [0] GeneralizedTime         OPTIONAL,
       -- time of production of this message (used when sender
       -- believes that the transport will be "suitable", i.e.,
       -- that the time will still be meaningful upon receipt)
       protectionAlg   [1] AlgorithmIdentifier{ALGORITHM, {...}}
                               OPTIONAL,
       -- algorithm used for calculation of protection bits
       senderKID       [2] KeyIdentifier           OPTIONAL,
       recipKID        [3] KeyIdentifier           OPTIONAL,
       -- to identify specific keys used for protection
       transactionID   [4] OCTET STRING            OPTIONAL,
       -- identifies the transaction, i.e., this will be the same in
       -- corresponding request, response, certConf, and pkiconf
       -- messages
       senderNonce     [5] OCTET STRING            OPTIONAL,
       recipNonce      [6] OCTET STRING            OPTIONAL,
       -- nonces used to provide replay protection, senderNonce
       -- is inserted by the creator of this message; recipNonce
       -- is a nonce previously inserted in a related message by
       -- the intended recipient of this message.
       freeText        [7] PKIFreeText             OPTIONAL,
       -- this may be used to indicate context-specific instructions
       -- (this field is intended for human consumption)
       generalInfo     [8] SEQUENCE SIZE (1..MAX) OF
                           InfoTypeAndValue     OPTIONAL
       -- this may be used to convey context-specific information
       -- (this field is not primarily intended for human consumption)
   }

   PKIFreeText ::= SEQUENCE SIZE (1..MAX) OF UTF8String
       -- text encoded as UTF-8 string [RFC3629]

   PKIBody ::= CHOICE {       -- message-specific body elements
       ir       [0]  CertReqMessages,        --Initialization Request
       ip       [1]  CertRepMessage,         --Initialization Response
       cr       [2]  CertReqMessages,        --Certification Request
       cp       [3]  CertRepMessage,         --Certification Response
       p10cr    [4]  CertificationRequest,   --imported from [RFC2986]
       popdecc  [5]  POPODecKeyChallContent, --pop Challenge
       popdecr  [6]  POPODecKeyRespContent,  --pop Response
       kur      [7]  CertReqMessages,        --Key Update Request
       kup      [8]  CertRepMessage,         --Key Update Response
       krr      [9]  CertReqMessages,        --Key Recovery Request
       krp      [10] KeyRecRepContent,       --Key Recovery Response
       rr       [11] RevReqContent,          --Revocation Request
       rp       [12] RevRepContent,          --Revocation Response
       ccr      [13] CertReqMessages,        --Cross-Cert. Request
       ccp      [14] CertRepMessage,         --Cross-Cert. Response
       ckuann   [15] CAKeyUpdContent,        --CA Key Update Ann.
       cann     [16] CertAnnContent,         --Certificate Ann.
       rann     [17] RevAnnContent,          --Revocation Ann.
       crlann   [18] CRLAnnContent,          --CRL Announcement
       pkiconf  [19] PKIConfirmContent,      --Confirmation
       nested   [20] NestedMessageContent,   --Nested Message
       genm     [21] GenMsgContent,          --General Message
       genp     [22] GenRepContent,          --General Response
       error    [23] ErrorMsgContent,        --Error Message
       certConf [24] CertConfirmContent,     --Certificate Confirm
       pollReq  [25] PollReqContent,         --Polling Request
       pollRep  [26] PollRepContent          --Polling Response
   }

   PKIProtection ::= BIT STRING

   ProtectedPart ::= SEQUENCE {
       header    PKIHeader,
       body      PKIBody }

   id-PasswordBasedMac OBJECT IDENTIFIER ::= { iso(1) member-body(2)
       usa(840) nt(113533) nsn(7) algorithms(66) 13 }
   PBMParameter ::= SEQUENCE {
       salt                OCTET STRING,
       -- Note:  Implementations MAY wish to limit acceptable sizes
       -- of this string to values appropriate for their environment
       -- in order to reduce the risk of denial-of-service attacks.
       owf                 AlgorithmIdentifier{DIGEST-ALGORITHM, {...}},
       -- AlgId for the OWF
       iterationCount      INTEGER,
       -- number of times the OWF is applied
       -- Note:  Implementations MAY wish to limit acceptable sizes
       -- of this integer to values appropriate for their environment
       -- in order to reduce the risk of denial-of-service attacks.
       mac                 AlgorithmIdentifier{MAC-ALGORITHM, {...}}
       -- AlgId of the MAC algorithm
   }

   id-DHBasedMac OBJECT IDENTIFIER ::= { iso(1) member-body(2)
       usa(840) nt(113533) nsn(7) algorithms(66) 30 }
   DHBMParameter ::= SEQUENCE {
       owf                 AlgorithmIdentifier{DIGEST-ALGORITHM, {...}},
       -- AlgId for an OWF
       mac                 AlgorithmIdentifier{MAC-ALGORITHM, {...}}
       -- AlgId of the MAC algorithm
   }

   -- id-KemBasedMac and KemBMParameter were added in [RFC9810]

   id-KemBasedMac OBJECT IDENTIFIER ::= { iso(1) member-body(2)
       usa(840) nt(113533) nsn(7) algorithms(66) 16 }
   KemBMParameter ::= SEQUENCE {
       kdf              AlgorithmIdentifier{KEY-DERIVATION, {...}},
       -- AlgId of the Key Derivation Function algorithm
       kemContext   [0] OCTET STRING OPTIONAL,
       -- MAY contain additional algorithm-specific context information
       len              INTEGER (1..MAX),
       -- Defines the length of the keying material output of the KDF
       -- SHOULD be the maximum key length of the MAC function
       mac              AlgorithmIdentifier{MAC-ALGORITHM, {...}}
       -- AlgId of the MAC algorithm
   }

   PKIStatus ::= INTEGER {
       accepted               (0),
       -- you got exactly what you asked for
       grantedWithMods        (1),
       -- you got something like what you asked for; the
       -- requester is responsible for ascertaining the differences
       rejection              (2),
       -- you don't get it, more information elsewhere in the message
       waiting                (3),
       -- the request body part has not yet been processed; expect to
       -- hear more later (note: proper handling of this status
       -- response MAY use the polling req/rep PKIMessages specified
       -- in Section 5.3.22; alternatively, polling in the underlying
       -- transport layer MAY have some utility in this regard)
       revocationWarning      (4),
       -- this message contains a warning that a revocation is
       -- imminent
       revocationNotification (5),
       -- notification that a revocation has occurred
       keyUpdateWarning       (6)
       -- update already done for the oldCertId specified in
       -- CertReqMsg
   }

   PKIFailureInfo ::= BIT STRING {
   -- since we can fail in more than one way!
   -- More codes may be added in the future if/when required.
       badAlg              (0),
       -- unrecognized or unsupported algorithm identifier
       badMessageCheck     (1),
       -- integrity check failed (e.g., signature did not verify)
       badRequest          (2),
       -- transaction not permitted or supported
       badTime             (3),
       -- messageTime was not sufficiently close to the system time,
       -- as defined by local policy
       badCertId           (4),
       -- no certificate could be found matching the provided criteria
       badDataFormat       (5),
       -- the data submitted has the wrong format
       wrongAuthority      (6),
       -- the authority indicated in the request is different from the
       -- one creating the response token
       incorrectData       (7),
       -- the requester's data is incorrect (for notary services)
       missingTimeStamp    (8),
       -- when the timestamp is missing but should be there
       -- (by policy)
       badPOP              (9),
       -- the POP failed
       certRevoked         (10),
       -- the certificate has already been revoked
       certConfirmed       (11),
       -- the certificate has already been confirmed
       wrongIntegrity      (12),
       -- KEM ciphertext missing for MAC-based protection of response,
       -- or not valid integrity of message received (password based
       -- instead of signature or vice versa)
       badRecipientNonce   (13),
       -- not valid recipient nonce, either missing or wrong value
       timeNotAvailable    (14),
       -- the TSA's time source is not available
       unacceptedPolicy    (15),
       -- the requested TSA policy is not supported by the TSA
       unacceptedExtension (16),
       -- the requested extension is not supported by the TSA
       addInfoNotAvailable (17),
       -- the additional information requested could not be
       -- understood or is not available
       badSenderNonce      (18),
       -- not valid sender nonce, either missing or wrong size
       badCertTemplate     (19),
       -- not valid cert. template or missing mandatory information
       signerNotTrusted    (20),
       -- signer of the message unknown or not trusted
       transactionIdInUse  (21),
       -- the transaction identifier is already in use
       unsupportedVersion  (22),
       -- the version of the message is not supported
       notAuthorized       (23),
       -- the sender was not authorized to make the preceding
       -- request or perform the preceding action
       systemUnavail       (24),
       -- the request cannot be handled due to system unavailability
       systemFailure       (25),
       -- the request cannot be handled due to system failure
       duplicateCertReq    (26)
       -- certificate cannot be issued because a duplicate
       -- certificate already exists
   }

   PKIStatusInfo ::= SEQUENCE {
       status        PKIStatus,
       statusString  PKIFreeText     OPTIONAL,
       failInfo      PKIFailureInfo  OPTIONAL }

   OOBCert ::= CMPCertificate

   OOBCertHash ::= SEQUENCE {
       hashAlg     [0] AlgorithmIdentifier{DIGEST-ALGORITHM, {...}}
                           OPTIONAL,
       certId      [1] CertId                  OPTIONAL,
       hashVal         BIT STRING
       -- hashVal is calculated over the DER encoding of the
       -- self-signed certificate with the identifier certID.
   }

   POPODecKeyChallContent ::= SEQUENCE OF Challenge
   -- One Challenge per encryption or key agreement key certification
   -- request (in the same order as these requests appear in
   -- CertReqMessages).

   -- encryptedRand was added in [RFC9810]

   Challenge ::= SEQUENCE {
      owf                 AlgorithmIdentifier{DIGEST-ALGORITHM, {...}}
                               OPTIONAL,
      -- MUST be present in the first Challenge; MAY be omitted in
      -- any subsequent Challenge in POPODecKeyChallContent (if
      -- omitted, then the owf used in the immediately preceding
      -- Challenge is to be used).
      witness             OCTET STRING,
      -- the result of applying the OWF to a
      -- randomly generated INTEGER, A. (Note that a different
      -- INTEGER MUST be used for each Challenge.)
      challenge           OCTET STRING,
      -- MUST be used for cmp2000(2) popdecc messages and MUST be
      -- the encryption of Rand (using a mechanism depending on the
      -- private key type).
      -- MUST be an empty OCTET STRING for cmp2021(3) popdecc messages.
      -- Note: Using challenge omitting the optional encryptedRand is
      -- bit-compatible to the syntax without adding this optional
      -- field.
      encryptedRand   [0] EnvelopedData OPTIONAL
      -- MUST be omitted for cmp2000(2) popdecc messages.
      -- MUST be used for cmp2021(3) popdecc messages and MUST contain
      -- the encrypted value of Rand using CMS EnvelopedData using the
      -- key management technique depending on the private key type as
      -- defined in Section 5.2.2.
   }

   -- Rand was added in [RFC9480]

   Rand ::= SEQUENCE {
   -- Rand is encrypted involving the public key to form the content of
   -- challenge or encryptedRand in POPODecKeyChallContent
      int                  INTEGER,
      -- the randomly generated INTEGER A (above)
      sender               GeneralName
      -- the sender's name (as included in PKIHeader)
   }

   POPODecKeyRespContent ::= SEQUENCE OF INTEGER
   -- One INTEGER per encryption or key agreement key certification
   -- request (in the same order as these requests appear in
   -- CertReqMessages). The retrieved INTEGER A (above) is returned to
   -- the sender of the corresponding Challenge.

   CertRepMessage ::= SEQUENCE {
       caPubs       [1] SEQUENCE SIZE (1..MAX) OF CMPCertificate
                     OPTIONAL,
       response         SEQUENCE OF CertResponse }

   CertResponse ::= SEQUENCE {
       certReqId           INTEGER,
       -- to match this response with the corresponding request (a value
       -- of -1 is to be used if certReqId is not specified in the
       -- corresponding request, which can only be a p10cr)
       status              PKIStatusInfo,
       certifiedKeyPair    CertifiedKeyPair    OPTIONAL,
       rspInfo             OCTET STRING        OPTIONAL
       -- analogous to the id-regInfo-utf8Pairs string defined
       -- for regInfo in CertReqMsg [RFC4211]
   }

   CertifiedKeyPair ::= SEQUENCE {
       certOrEncCert       CertOrEncCert,
       privateKey      [0] EncryptedKey      OPTIONAL,
       -- See [RFC4211] for comments on encoding.
       -- Changed from EncryptedValue to EncryptedKey as a CHOICE of
       -- EncryptedValue and EnvelopedData due to the changes made in
       -- [RFC9480].
       -- Using the choice EncryptedValue is bit-compatible to the
       -- syntax without this change.
       publicationInfo [1] PKIPublicationInfo  OPTIONAL }

   CertOrEncCert ::= CHOICE {
       certificate     [0] CMPCertificate,
       encryptedCert   [1] EncryptedKey
       -- Changed from Encrypted Value to EncryptedKey as a CHOICE of
       -- EncryptedValue and EnvelopedData due to the changes made in
       -- [RFC9480].
       -- Using the choice EncryptedValue is bit-compatible to the
       -- syntax without this change.
   }

   KeyRecRepContent ::= SEQUENCE {
       status                  PKIStatusInfo,
       newSigCert          [0] CMPCertificate OPTIONAL,
       caCerts             [1] SEQUENCE SIZE (1..MAX) OF
                                        CMPCertificate OPTIONAL,
       keyPairHist         [2] SEQUENCE SIZE (1..MAX) OF
                                        CertifiedKeyPair OPTIONAL }

   RevReqContent ::= SEQUENCE OF RevDetails

   RevDetails ::= SEQUENCE {
       certDetails         CertTemplate,
       -- allows requester to specify as much as they can about
       -- the cert. for which revocation is requested
       -- (e.g., for cases in which serialNumber is not available)
       crlEntryDetails     Extensions{{...}}    OPTIONAL
       -- requested crlEntryExtensions
   }

   RevRepContent ::= SEQUENCE {
       status       SEQUENCE SIZE (1..MAX) OF PKIStatusInfo,
       -- in same order as was sent in RevReqContent
       revCerts [0] SEQUENCE SIZE (1..MAX) OF CertId OPTIONAL,
       -- IDs for which revocation was requested
       -- (same order as status)
       crls     [1] SEQUENCE SIZE (1..MAX) OF CertificateList OPTIONAL
       -- the resulting CRLs (there may be more than one)
   }

   CAKeyUpdAnnContent ::= SEQUENCE {
       oldWithNew   CMPCertificate, -- old pub signed with new priv
       newWithOld   CMPCertificate, -- new pub signed with old priv
       newWithNew   CMPCertificate  -- new pub signed with new priv
   }

   -- CAKeyUpdContent was added in [RFC9810]
   CAKeyUpdContent ::= CHOICE {
       cAKeyUpdAnnV2      CAKeyUpdAnnContent, -- deprecated
       cAKeyUpdAnnV3  [0] RootCaKeyUpdateContent
   }
   -- With cmp2021, the use of CAKeyUpdAnnContent is deprecated, use
   -- RootCaKeyUpdateContent instead.

   CertAnnContent ::= CMPCertificate

   RevAnnContent ::= SEQUENCE {
       status              PKIStatus,
       certId              CertId,
       willBeRevokedAt     GeneralizedTime,
       badSinceDate        GeneralizedTime,
       crlDetails          Extensions{{...}}  OPTIONAL
       -- extra CRL details (e.g., crl number, reason, location, etc.)
   }

   CRLAnnContent ::= SEQUENCE OF CertificateList

   PKIConfirmContent ::= NULL

   NestedMessageContent ::= PKIMessages

   -- CertReqTemplateContent, AttributeTypeAndValue,
   -- ExpandedRegControlSet, id-regCtrl-altCertTemplate,
   -- AltCertTemplate, regCtrl-algId, id-regCtrl-algId, AlgIdCtrl,
   -- regCtrl-rsaKeyLen, id-regCtrl-rsaKeyLen, and RsaKeyLenCtrl
   -- were added in [RFC9480]

   CertReqTemplateContent ::= SEQUENCE {
      certTemplate           CertTemplate,
      -- prefilled certTemplate structure elements
      -- The SubjectPublicKeyInfo field in the certTemplate MUST NOT
      -- be used.
      keySpec                Controls OPTIONAL
      -- MAY be used to specify supported algorithms
      -- Controls  ::= SEQUENCE SIZE (1..MAX) OF AttributeTypeAndValue
      -- as specified in CRMF [RFC4211]
      }

   AttributeTypeAndValue ::= SingleAttribute{{ ... }}

   ExpandedRegControlSet ATTRIBUTE ::= { RegControlSet |
      regCtrl-altCertTemplate | regCtrl-algId | regCtrl-rsaKeyLen, ... }

   regCtrl-altCertTemplate ATTRIBUTE ::=
      { TYPE AltCertTemplate IDENTIFIED BY id-regCtrl-altCertTemplate }

   id-regCtrl-altCertTemplate OBJECT IDENTIFIER ::= { id-regCtrl 7 }

   AltCertTemplate ::= AttributeTypeAndValue
      -- specifies a template for a certificate other than an X.509v3
      -- public key certificate

   regCtrl-algId ATTRIBUTE ::=
      { TYPE AlgIdCtrl IDENTIFIED BY id-regCtrl-algId }

   id-regCtrl-algId OBJECT IDENTIFIER ::= { id-regCtrl 11 }

   AlgIdCtrl ::= AlgorithmIdentifier{ALGORITHM, {...}}
      -- SHALL be used to specify supported algorithms other than RSA

   regCtrl-rsaKeyLen ATTRIBUTE ::=
      { TYPE RsaKeyLenCtrl IDENTIFIED BY id-regCtrl-rsaKeyLen }

   id-regCtrl-rsaKeyLen OBJECT IDENTIFIER ::= { id-regCtrl 12 }

   RsaKeyLenCtrl ::= INTEGER (1..MAX)
      -- SHALL be used to specify supported RSA key lengths

   -- RootCaKeyUpdateContent, CRLSource, and CRLStatus were added in
   -- [RFC9480]

   RootCaKeyUpdateContent ::= SEQUENCE {
      newWithNew       CMPCertificate,
      -- new root CA certificate
      newWithOld   [0] CMPCertificate OPTIONAL,
      -- X.509 certificate containing the new public root CA key
      -- signed with the old private root CA key
      oldWithNew   [1] CMPCertificate OPTIONAL
      -- X.509 certificate containing the old public root CA key
      -- signed with the new private root CA key
      }

   CRLSource ::= CHOICE {
      dpn          [0] DistributionPointName,
      issuer       [1] GeneralNames }

   CRLStatus ::= SEQUENCE {
      source       CRLSource,
      thisUpdate   Time OPTIONAL }

   -- KemCiphertextInfo and KemOtherInfo were added in [RFC9810]

   KemCiphertextInfo ::= SEQUENCE {
      kem              AlgorithmIdentifier{KEM-ALGORITHM, {...}},
      -- AlgId of the KEM algorithm
      ct               OCTET STRING
      -- Ciphertext output from the Encapsulate function
      }

   KemOtherInfo ::= SEQUENCE {
      staticString     PKIFreeText,
      -- MUST be "CMP-KEM"
      transactionID    OCTET STRING,
      -- MUST contain the values from the message previously received
      -- containing the ciphertext (ct) in KemCiphertextInfo
      kemContext   [0] OCTET STRING OPTIONAL
      -- MAY contain additional algorithm-specific context information
     }

   INFO-TYPE-AND-VALUE ::= TYPE-IDENTIFIER

   InfoTypeAndValue ::= SEQUENCE {
       infoType    INFO-TYPE-AND-VALUE.
                       &id({SupportedInfoSet}),
       infoValue   INFO-TYPE-AND-VALUE.
                       &Type({SupportedInfoSet}{@infoType}) }

   SupportedInfoSet INFO-TYPE-AND-VALUE ::= { ... }

   -- Example InfoTypeAndValue contents include, but are not limited
   -- to, the following (uncomment in this ASN.1 module and use as
   -- appropriate for a given environment):
   --
   --   id-it-caProtEncCert    OBJECT IDENTIFIER ::= {id-it 1}
   --      CAProtEncCertValue      ::= CMPCertificate
   --   id-it-signKeyPairTypes OBJECT IDENTIFIER ::= {id-it 2}
   --      SignKeyPairTypesValue   ::= SEQUENCE SIZE (1..MAX) OF
   --                                      AlgorithmIdentifier{{...}}
   --   id-it-encKeyPairTypes  OBJECT IDENTIFIER ::= {id-it 3}
   --      EncKeyPairTypesValue    ::= SEQUENCE SIZE (1..MAX) OF
   --                                      AlgorithmIdentifier{{...}}
   --   id-it-preferredSymmAlg OBJECT IDENTIFIER ::= {id-it 4}
   --      PreferredSymmAlgValue   ::= AlgorithmIdentifier{{...}}
   --   id-it-caKeyUpdateInfo  OBJECT IDENTIFIER ::= {id-it 5}
   --      CAKeyUpdateInfoValue    ::= CAKeyUpdAnnContent
   --      - id-it-caKeyUpdateInfo was deprecated with cmp2021
   --   id-it-currentCRL       OBJECT IDENTIFIER ::= {id-it 6}
   --      CurrentCRLValue         ::= CertificateList
   --   id-it-unsupportedOIDs  OBJECT IDENTIFIER ::= {id-it 7}
   --      UnsupportedOIDsValue    ::= SEQUENCE SIZE (1..MAX) OF
   --                                          OBJECT IDENTIFIER
   --   id-it-keyPairParamReq  OBJECT IDENTIFIER ::= {id-it 10}
   --      KeyPairParamReqValue    ::= OBJECT IDENTIFIER
   --   id-it-keyPairParamRep  OBJECT IDENTIFIER ::= {id-it 11}
   --      KeyPairParamRepValue    ::= AlgorithmIdentifier{{...}}
   --   id-it-revPassphrase    OBJECT IDENTIFIER ::= {id-it 12}
   --      RevPassphraseValue      ::= EncryptedKey
   --      - Changed from Encrypted Value to EncryptedKey as a CHOICE
   --      - of EncryptedValue and EnvelopedData due to the changes
   --      - made in [RFC9480]
   --      - Using the choice EncryptedValue is bit-compatible to
   --      - the syntax without this change
   --   id-it-implicitConfirm  OBJECT IDENTIFIER ::= {id-it 13}
   --      ImplicitConfirmValue    ::= NULL
   --   id-it-confirmWaitTime  OBJECT IDENTIFIER ::= {id-it 14}
   --      ConfirmWaitTimeValue    ::= GeneralizedTime
   --   id-it-origPKIMessage   OBJECT IDENTIFIER ::= {id-it 15}
   --      OrigPKIMessageValue     ::= PKIMessages
   --   id-it-suppLangTags     OBJECT IDENTIFIER ::= {id-it 16}
   --      SuppLangTagsValue       ::= SEQUENCE OF UTF8String
   --   id-it-caCerts          OBJECT IDENTIFIER ::= {id-it 17}
   --      CaCertsValue            ::= SEQUENCE SIZE (1..MAX) OF
   --                                             CMPCertificate
   --      - id-it-caCerts added in [RFC9480]
   --   id-it-rootCaKeyUpdate  OBJECT IDENTIFIER ::= {id-it 18}
   --      RootCaKeyUpdateValue    ::= RootCaKeyUpdateContent
   --      - id-it-rootCaKeyUpdate added in [RFC9480]
   --   id-it-certReqTemplate  OBJECT IDENTIFIER ::= {id-it 19}
   --      CertReqTemplateValue    ::= CertReqTemplateContent
   --      - id-it-certReqTemplate added in [RFC9480]
   --   id-it-rootCaCert       OBJECT IDENTIFIER ::= {id-it 20}
   --      RootCaCertValue         ::= CMPCertificate
   --      - id-it-rootCaCert added in [RFC9480]
   --   id-it-certProfile      OBJECT IDENTIFIER ::= {id-it 21}
   --      CertProfileValue        ::= SEQUENCE SIZE (1..MAX) OF
   --                                                 UTF8String
   --      - id-it-certProfile added in [RFC9480]
   --   id-it-crlStatusList    OBJECT IDENTIFIER ::= {id-it 22}
   --      CRLStatusListValue      ::= SEQUENCE SIZE (1..MAX) OF
   --                                                  CRLStatus
   --      - id-it-crlStatusList added in [RFC9480]
   --   id-it-crls             OBJECT IDENTIFIER ::= {id-it 23}
   --      CRLsValue               ::= SEQUENCE SIZE (1..MAX) OF
   --                                            CertificateList
   --      - id-it-crls added in [RFC9480]
   --   id-it-KemCiphertextInfo    OBJECT IDENTIFIER ::= {id-it 24}
   --      KemCiphertextInfoValue  ::= KemCiphertextInfo
   --      - id-it-KemCiphertextInfo was added in [RFC9810]
   --
   -- where
   --
   --   id-pkix OBJECT IDENTIFIER ::= {
   --      iso(1) identified-organization(3)
   --      dod(6) internet(1) security(5) mechanisms(5) pkix(7)}
   -- and
   --   id-it   OBJECT IDENTIFIER ::= {id-pkix 4}
   --
   --
   -- This construct MAY also be used to define new PKIX Certificate
   -- Management Protocol request and response messages or
   -- general-purpose (e.g., announcement) messages for future needs
   -- or for specific environments.

   GenMsgContent ::= SEQUENCE OF InfoTypeAndValue

   -- May be sent by end entity, RA, or CA (depending on message
   -- content).  The OPTIONAL infoValue parameter of InfoTypeAndValue
   -- will typically be omitted for some of the examples given above.
   -- The receiver is free to ignore any contained OIDs that it
   -- does not recognize.  If sent from end entity to CA, the empty set
   -- indicates that the CA may send
   -- any/all information that it wishes.

   GenRepContent ::= SEQUENCE OF InfoTypeAndValue
   -- The receiver MAY ignore any contained OIDs that it does not
   -- recognize.

   ErrorMsgContent ::= SEQUENCE {
       pKIStatusInfo          PKIStatusInfo,
       errorCode              INTEGER           OPTIONAL,
       -- implementation-specific error codes
       errorDetails           PKIFreeText       OPTIONAL
       -- implementation-specific error details
   }

   CertConfirmContent ::= SEQUENCE OF CertStatus

   CertStatus ::= SEQUENCE {
       certHash    OCTET STRING,
       -- the hash of the certificate, using the same hash algorithm
       -- as is used to create and verify the certificate signature
       certReqId   INTEGER,
       -- to match this confirmation with the corresponding req/rep
       statusInfo  PKIStatusInfo OPTIONAL,
       hashAlg [0] AlgorithmIdentifier{DIGEST-ALGORITHM, {...}} OPTIONAL
       -- the hash algorithm to use for calculating certHash
       -- SHOULD NOT be used in all cases where the AlgorithmIdentifier
       -- of the certificate signature specifies a hash algorithm
      }

   PollReqContent ::= SEQUENCE OF SEQUENCE {
       certReqId              INTEGER }

   PollRepContent ::= SEQUENCE OF SEQUENCE {
       certReqId              INTEGER,
       checkAfter             INTEGER,  -- time in seconds
       reason                 PKIFreeText OPTIONAL }

   --
   -- EKU extension for PKI entities used in CMP
   -- operations, added due to the changes made in [RFC9480]
   -- The EKUs for the CA and RA are reused from CMC, as defined in
   -- [RFC6402]
   --

   -- id-kp-cmcCA OBJECT IDENTIFIER ::= { id-kp 27 }
   -- id-kp-cmcRA OBJECT IDENTIFIER ::= { id-kp 28 }
   id-kp-cmKGA OBJECT IDENTIFIER ::= { id-kp 32 }

   END

Acknowledgements

   The authors of this document wish to thank Carlisle Adams, Stephen
   Farrell, Tomi Kause, and Tero Mononen, the original authors of
   [RFC4210], for their work.

   We also thank all reviewers of this document for their valuable
   feedback.

   Adding KEM support to this document was partly funded by the German
   Federal Ministry of Education and Research in the project Quoryptan
   through grant number 16KIS2033.

Authors' Addresses

   Hendrik Brockhaus
   Siemens
   Werner-von-Siemens-Strasse 1
   80333 Munich
   Germany
   Email: hendrik.brockhaus@siemens.com
   URI:   https://www.siemens.com


   David von Oheimb
   Siemens
   Werner-von-Siemens-Strasse 1
   80333 Munich
   Germany
   Email: david.von.oheimb@siemens.com
   URI:   https://www.siemens.com


   Mike Ounsworth
   Entrust
   1187 Park Place
   Minneapolis, MN 55379
   United States of America
   Email: mike.ounsworth@entrust.com
   URI:   https://www.entrust.com


   John Gray
   Entrust
   1187 Park Place
   Minneapolis, MN 55379
   United States of America
   Email: john.gray@entrust.com
   URI:   https://www.entrust.com
