



Internet Engineering Task Force (IETF)                       F. Driscoll
Request for Comments: 9794                                    M. Parsons
Category: Informational                UK National Cyber Security Centre
ISSN: 2070-1721                                                  B. Hale
                                               Naval Postgraduate School
                                                               June 2025


        Terminology for Post-Quantum Traditional Hybrid Schemes

Abstract

   One aspect of the transition to post-quantum algorithms in
   cryptographic protocols is the development of hybrid schemes that
   incorporate both post-quantum and traditional asymmetric algorithms.
   This document defines terminology for such schemes.  It is intended
   to be used as a reference and, hopefully, to ensure consistency and
   clarity across different protocols, standards, and organisations.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   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).  Not all documents
   approved by the IESG are candidates for any level of Internet
   Standard; see 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/rfc9794.

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
   2.  Primitives
   3.  Cryptographic Elements
   4.  Protocols
   5.  Properties
   6.  Certificates
   7.  Security Considerations
   8.  IANA Considerations
   9.  Informative References
   Acknowledgments
   Authors' Addresses

1.  Introduction

   The mathematical problems of integer factorisation and discrete
   logarithms over finite fields or elliptic curves underpin most of the
   asymmetric algorithms used for key establishment and digital
   signatures on the Internet.  These problems, and hence the algorithms
   based on them, will be vulnerable to attacks using Shor's Algorithm
   on a sufficiently large general-purpose quantum computer, known as a
   Cryptographically Relevant Quantum Computer (CRQC).  Current
   predictions vary on when, or if, such a device will exist.  However,
   it is necessary to anticipate and prepare to defend against such a
   development.  Data encrypted today (in 2025) with an algorithm
   vulnerable to a quantum computer can be stored for decryption by a
   future attacker with a CRQC.  Signing algorithms in products that are
   expected to be in use for many years, and that cannot be updated or
   replaced, are also at risk if a CRQC is developed during the
   operational lifetime of that product.

   Ongoing responses to the potential development of a CRQC include
   modifying established (or standardised) protocols to use asymmetric
   algorithms that are designed to be secure against quantum computers
   as well as today's classical computers.  These algorithms are called
   "post-quantum", while algorithms based on integer factorisation,
   finite-field discrete logarithms, or elliptic-curve discrete
   logarithms are called "traditional cryptographic algorithms".  In
   this document, "traditional algorithm" is also used to refer to this
   class of algorithms.

   At the time of publication, the term "post-quantum" is generally used
   to describe cryptographic algorithms that are designed to be secure
   against an adversary with access to a CRQC.  Post-quantum algorithms
   can also be referred to as "quantum-resistant" or "quantum-safe"
   algorithms.  There are merits to the different terms.  For example,
   some prefer to use the terms quantum-resistant or quantum-safe to
   explicitly indicate that these algorithms are designed to be secure
   against quantum computers.  Others disagree and prefer to use the
   term post-quantum, in case of compromises against such algorithms
   that could make the terms quantum-resistant or quantum-safe
   misleading.  Similarly, some prefer to refer specifically to Shor's
   Algorithm or to the mathematical problem that is being used to
   prevent attacks.  Post-Quantum Cryptography (PQC) is commonly used
   amongst the cryptography community, and so it will be used throughout
   this document.  Similarly, the term "traditional algorithm" will be
   used throughout the document as, at the time of publication, it is
   widely used in the community, though other terms, including
   classical, pre-quantum, or quantum-vulnerable, are preferred by some.

   To mitigate risks, there may be a requirement for protocols that use
   both algorithm types, either during the transition from traditional
   to post-quantum algorithms or as a general solution.  When the risk
   of deploying new algorithms is above the accepted threshold for their
   use case, a designer may combine a post-quantum algorithm with a
   traditional algorithm, with the goal of adding protection against an
   attacker with a CRQC to the security properties provided by the
   traditional algorithm.  They may also implement a post-quantum
   algorithm alongside a traditional algorithm for ease of migration
   from an ecosystem where only traditional algorithms are implemented
   and used, to one that only uses post-quantum algorithms.  Examples of
   solutions that could use both types of algorithm include, but are not
   limited to, [RFC9370], [HYBRID-TLS], [COMPOSITE-KEM], and [RFC9763].

   Schemes that combine post-quantum and traditional algorithms for key
   establishment or digital signatures are often called "hybrids".  For
   example:

   *  The National Institute of Standards and Technology (NIST) defines
      hybrid key establishment to be a "scheme that is a combination of
      two or more components that are themselves cryptographic key-
      establishment schemes" [NIST_PQC_FAQ].

   *  The European Telecommunications Standards Institute (ETSI) defines
      hybrid key exchanges to be "constructions that combine a
      traditional key exchange ... with a post-quantum key exchange ...
      into a single key exchange" [ETSI_TS103774].

   The word "hybrid" is also used in cryptography to describe encryption
   schemes that combine asymmetric and symmetric algorithms [RFC9180],
   so using it in the post-quantum context overloads it and risks
   misunderstandings.  However, this terminology is well-established
   amongst the Post-Quantum Cryptography (PQC) community.  Therefore, an
   attempt to move away from its use for PQC could lead to multiple
   definitions for the same concept, resulting in confusion and lack of
   clarity.  At the time of publication, hybrid is generally used for
   schemes that combine post-quantum and traditional algorithms; it will
   be so used throughout this document, though some have alternative
   preferences such as double-algorithm or multi-algorithm.

   This document provides language for constructions that combine
   traditional and post-quantum algorithms.  Specific solutions for
   enabling the use of multiple asymmetric algorithms in cryptographic
   schemes may be more general than this, allowing the use of solely
   traditional or solely post-quantum algorithms.  However, where
   relevant, we focus on post-quantum traditional combinations as these
   are the motivation for the wider work in the IETF.  This document is
   intended as a reference terminology guide for other documents, in
   order to add clarity and consistency across different protocols,
   standards, and organisations.  Additionally, this document aims to
   reduce misunderstandings about the use of the word "hybrid" and to
   define a shared language for different types of post-quantum and
   traditional hybrid constructions.

   In this document, a "cryptographic algorithm" is defined, as in
   [NIST_SP_800-152], to be a "well-defined computational procedure that
   takes variable inputs, often including a cryptographic key, and
   produces an output".  Examples include RSA, Elliptic Curve Diffie-
   Hellman (ECDH), Module-Lattice-Based Key-Encapsulation Mechanism (ML-
   KEM) (formerly known as Kyber), and Module-Lattice-Based Digital
   Signature Algorithm (ML-DSA) (formerly known as Dilithium).  The
   expression "cryptographic scheme" is used to refer to a construction
   that uses a cryptographic algorithm or a group of cryptographic
   algorithms to achieve a particular cryptographic outcome, e.g., key
   agreement.  A cryptographic scheme may be made up of a number of
   functions.  For example, a Key Encapsulation Mechanism (KEM) is a
   cryptographic scheme consisting of three functions: Key Generation,
   Encapsulation, and Decapsulation.  A cryptographic protocol
   incorporates one or more cryptographic schemes.  For example, TLS
   [RFC8446] is a cryptographic protocol that includes schemes for key
   agreement, record layer encryption, and server authentication.

2.  Primitives

   This section introduces terminology related to cryptographic
   algorithms and to hybrid constructions for cryptographic schemes.

   Traditional asymmetric cryptographic algorithm:
      An asymmetric cryptographic algorithm based on integer
      factorisation, finite field discrete logarithms, elliptic curve
      discrete logarithms, or related mathematical problems.

      A related mathematical problem is one that can be solved by
      solving the integer factorisation, finite field discrete
      logarithm, or elliptic curve discrete logarithm problem.

      Where there is little risk of confusion, traditional asymmetric
      cryptographic algorithms can also be referred to as "traditional
      algorithms" for brevity.  Traditional algorithms can also be
      called "classical" or "conventional" algorithms.

   Post-quantum asymmetric cryptographic algorithm:
      An asymmetric cryptographic algorithm that is intended to be
      secure against attacks using quantum computers as well as
      classical computers.

      Where there is little risk of confusion, post-quantum asymmetric
      cryptographic algorithms can also be referred to as "post-quantum
      algorithms" for brevity.  Post-quantum algorithms can also be
      called "quantum-resistant" or "quantum-safe" algorithms.

      As with all cryptography, it always remains the case that attacks,
      either quantum or classical, may be found against post-quantum
      algorithms.  Therefore, it should not be assumed that an algorithm
      will not be compromised just because it is designed to provide
      post-quantum cryptography.  Should an attack be found against a
      post-quantum algorithm, it is commonly still referred to as a
      "post-quantum algorithm", as they were designed to protect against
      an adversary with access to a CRQC, and the labels are referring
      to the designed or desired properties.

   There may be asymmetric cryptographic constructions that are neither
   post-quantum nor asymmetric traditional algorithms according to the
   definitions above.  These are out of scope of this document.

   Component asymmetric algorithm:
      Each cryptographic algorithm that forms part of a cryptographic
      scheme.

      An asymmetric component algorithm operates on the input of the
      cryptographic operation and produces a cryptographic output that
      can be used by itself or jointly to complete the operation.  Where
      there is little risk of confusion, component asymmetric algorithms
      can also be referred to as "component algorithms" for brevity, as
      is done in the following definitions.

   Single-algorithm scheme:
      A cryptographic scheme with one component algorithm.

      A single-algorithm scheme could use either a traditional algorithm
      or a post-quantum algorithm.

   Multi-algorithm scheme:
      A cryptographic scheme that incorporates more than one component
      algorithm, where the component algorithms have the same
      cryptographic purpose as each other and as the multi-algorithm
      scheme.

      For example, a multi-algorithm signature scheme may include
      multiple signature algorithms, or a multi-algorithm Public Key
      Encryption (PKE) scheme may include multiple PKE algorithms.
      Component algorithms could be all traditional, all post-quantum,
      or a mixture of the two.

   Post-Quantum Traditional (PQ/T) hybrid scheme:
      A multi-algorithm scheme where at least one component algorithm is
      a post-quantum algorithm and at least one is a traditional
      algorithm.

      Components of a PQ/T hybrid scheme operate on the same input
      message and their output is used together to complete the
      cryptographic operation either serially or in parallel.  PQ/T
      hybrid scheme design is aimed at requiring successful breaking of
      all component algorithms to break the PQ/T hybrid scheme's
      security properties.

   PQ/T hybrid Key Encapsulation Mechanism (KEM):
      A multi-algorithm KEM made up of two or more component algorithms
      where at least one is a post-quantum algorithm and at least one is
      a traditional algorithm.  The component algorithms could be KEMs
      or other key establishment algorithms.

   PQ/T hybrid Public Key Encryption (PKE):
      A multi-algorithm PKE scheme made up of two or more component
      algorithms where at least one is a post-quantum algorithm and at
      least one is a traditional algorithm.  The component algorithms
      could be PKE algorithms or other key establishment algorithms.

      The standard security property for a PKE scheme is
      indistinguishability under chosen-plaintext attack (IND-CPA)
      [BDPR].  IND-CPA security is not sufficient for secure
      communication in the presence of an active attacker.  Therefore,
      in general, PKE schemes are not appropriate for use on the
      Internet, and KEMs, which provide indistinguishability under
      chosen-ciphertext attack (IND-CCA) [BDPR], are required.

   PQ/T hybrid digital signature:
      A multi-algorithm digital signature scheme made up of two or more
      component digital signature algorithms where at least one is a
      post-quantum algorithm and at least one is a traditional
      algorithm.

      Note that there are many possible ways of constructing a PQ/T
      hybrid digital signature.  Examples include parallel signatures,
      composite signatures, or nested signatures.

   PQ/T hybrid KEMs, PQ/T hybrid PKE, and PQ/T hybrid digital signatures
   are all examples of PQ/T hybrid schemes.

   Post-Quantum Traditional (PQ/T) hybrid composite scheme:
      A multi-algorithm scheme where at least one component algorithm is
      a post-quantum algorithm and at least one is a traditional
      algorithm, and where the resulting composite scheme is exposed as
      a singular interface of the same type as the component algorithms.

      A PQ/T hybrid composite can be referred to as a "PQ/T composite".
      An example of a PQ/T hybrid composite is a single KEM algorithm
      comprised of a PQ KEM component and a traditional KEM component,
      for which the result presents as a KEM output.

   PQ/T hybrid combiner:
      A method that takes two or more component algorithms and combines
      them to form a PQ/T hybrid scheme.

   PQ/PQ hybrid scheme:
      A multi-algorithm scheme where all components are post-quantum
      algorithms.

      The definitions for types of PQ/T hybrid schemes can be adapted to
      define types of PQ/PQ hybrid schemes, which are multi-algorithm
      schemes where all component algorithms are post-quantum
      algorithms.  These are designed to mitigate risks when the two
      post-quantum algorithms are based on different mathematical
      problems.  Some prefer to refer to these as PQ/PQ multi-algorithm
      schemes, and reserve the term "hybrid" for PQ/T hybrids.

   In cases where there is little chance of confusion between other
   types of hybrid cryptography (e.g., as defined in [RFC4949]) and
   where the component algorithms of a multi-algorithm scheme could be
   either post-quantum or traditional, it may be appropriate to use the
   phrase "hybrid scheme" without PQ/T or PQ/PQ preceding it.

   Component scheme:
      Each cryptographic scheme that makes up a PQ/T hybrid scheme or
      PQ/T hybrid protocol.

3.  Cryptographic Elements

   This section introduces terminology related to cryptographic elements
   and their inclusion in hybrid schemes.

   Cryptographic element:
      Any data type (private or public) that contains an input or output
      value for a cryptographic algorithm or for a function making up a
      cryptographic algorithm.

      Types of cryptographic elements include public keys, private keys,
      plaintexts, ciphertexts, shared secrets, and signature values.

   Component cryptographic element:
      A cryptographic element of a component algorithm in a multi-
      algorithm scheme.

      For example, in [HYBRID-TLS], the client's keyshare contains two
      component public keys: one for a post-quantum algorithm and one
      for a traditional algorithm.

   Composite cryptographic element:
      A cryptographic element that incorporates multiple component
      cryptographic elements of the same type for use in a multi-
      algorithm scheme, such that the resulting composite cryptographic
      element is exposed as a singular interface of the same type as the
      component cryptographic elements.

      For example, a composite cryptographic public key is made up of
      two component public keys.

   PQ/T hybrid composite cryptographic element:
      A cryptographic element that incorporates multiple component
      cryptographic elements of the same type for use in a multi-
      algorithm scheme, such that the resulting composite cryptographic
      element is exposed as a singular interface of the same type as the
      component cryptographic elements, where at least one component
      cryptographic element is post-quantum and at least one is
      traditional.

   Cryptographic element combiner:
      A method that takes two or more component cryptographic elements
      of the same type and combines them to form a composite
      cryptographic element.

      A cryptographic element combiner could be concatenation, such as
      where two component public keys are concatenated to form a
      composite public key as in [HYBRID-TLS], or something more
      involved such as the dualPRF defined in [BINDEL].

4.  Protocols

   This section introduces terminology related to the use of post-
   quantum and traditional algorithms together in protocols.

   PQ/T hybrid protocol:
      A protocol that uses two or more component algorithms providing
      the same cryptographic functionality, where at least one is a
      post-quantum algorithm and at least one is a traditional
      algorithm.

      For example, a PQ/T hybrid protocol providing confidentiality
      could use a PQ/T hybrid KEM such as in [HYBRID-TLS], or it could
      combine the output of a post-quantum KEM and a traditional KEM at
      the protocol level to generate a single shared secret, such as in
      [RFC9370].  Similarly, a PQ/T hybrid protocol providing
      authentication could use a PQ/T hybrid digital signature scheme,
      or it could include both post-quantum and traditional single-
      algorithm digital signature schemes.

      A protocol that can negotiate the use of either a traditional
      algorithm or a post-quantum algorithm, but not the use of both
      types of algorithm, is not a PQ/T hybrid protocol.  Protocols that
      use two or more component algorithms but with different
      cryptographic functionalities, for example, a post-quantum KEM and
      a Pre-Shared Key (PSK), are also not PQ/T hybrid protocols.

   PQ/T hybrid protocol with composite key establishment:
      A PQ/T hybrid protocol that incorporates a PQ/T hybrid composite
      scheme to achieve key establishment, in such a way that the
      protocol fields and message flow are the same as those in a
      version of the protocol that uses a single-algorithm scheme.

      For example, a PQ/T hybrid protocol with composite key
      establishment could include a single PQ/T hybrid KEM, such as in
      [HYBRID-TLS].

   PQ/T hybrid protocol with composite data authentication:
      A PQ/T hybrid protocol that incorporates a PQ/T hybrid composite
      scheme to achieve data authentication, in such a way that the
      protocol fields and message flow are the same as those in a
      version of the protocol that uses a single-algorithm scheme.

      For example, a PQ/T hybrid protocol with composite data
      authentication could include data authentication through the use
      of a PQ/T composite hybrid digital signature, exposed as a single
      interface for PQ signature and traditional signature components.

   PQ/T hybrid protocol with composite entity authentication:
      A PQ/T hybrid protocol that incorporates a PQ/T hybrid composite
      scheme to achieve entity authentication, in such a way that the
      protocol fields and message flow are the same as those in a
      version of the protocol that uses a single-algorithm scheme.

      For example, a PQ/T hybrid protocol with composite entity
      authentication could include entity authentication through the use
      of PQ/T Composite Hybrid certificates.

   In a PQ/T hybrid protocol with a composite construction, changes are
   primarily made to the formats of the cryptographic elements, while
   the protocol fields and message flow remain largely unchanged.  In
   implementations, most changes are likely to be made to the
   cryptographic libraries, with minimal changes to the protocol
   libraries.

   PQ/T hybrid protocol with non-composite key establishment:
      A PQ/T hybrid protocol that incorporates multiple single-algorithm
      schemes to achieve key establishment, where at least one uses a
      post-quantum algorithm and at least one uses a traditional
      algorithm, in such a way that the formats of the component
      cryptographic elements are the same as when they are used as a
      part of a single-algorithm scheme.

      For example, a PQ/T hybrid protocol with non-composite key
      establishment could include a traditional key exchange scheme and
      a post-quantum KEM.  A construction like this for the Internet Key
      Exchange Protocol Version 2 (IKEv2) is enabled by [RFC9370].

   PQ/T hybrid protocol with non-composite authentication:
      A PQ/T hybrid protocol that incorporates multiple single-algorithm
      schemes to achieve authentication, where at least one uses a post-
      quantum algorithm and at least one uses a traditional algorithm,
      in such a way that the formats of the component cryptographic
      elements are the same as when they are used as part of a single-
      algorithm scheme.

      For example, a PQ/T hybrid protocol with non-composite
      authentication could use a PQ/T parallel PKI with one traditional
      certificate chain and one post-quantum certificate chain.

   In a PQ/T hybrid protocol with a non-composite construction, changes
   are primarily made to the protocol fields, the message flow, or both,
   while changes to cryptographic elements are minimised.  In
   implementations, most changes are likely to be made to the protocol
   libraries, with minimal changes to the cryptographic libraries.

   It is possible for a PQ/T hybrid protocol to be designed with both
   composite and non-composite constructions.  For example, a protocol
   that offers both confidentiality and authentication could have
   composite key agreement and non-composite authentication.  Similarly,
   it is possible for a PQ/T hybrid protocol to achieve certain
   cryptographic outcomes in a non-hybrid manner.  For example,
   [HYBRID-TLS] describes a PQ/T hybrid protocol with composite key
   agreement, but with single-algorithm authentication.

   PQ/T hybrid protocols may not specify non-composite aspects, but can
   choose to do so for clarity, in particular, if including both
   composite and non-composite aspects.

   PQ/T hybrid composite protocol:
      A PQ/T hybrid protocol that only uses composite constructions can
      be referred to as a "PQ/T hybrid composite protocol".

      An example of this is a protocol that only provides entity
      authentication, and achieves this using PQ/T hybrid composite
      entity authentication.  Similarly, another example is a protocol
      that offers both key establishment and data authentication, and
      achieves this using both PQ/T hybrid composite key establishment
      and PQ/T hybrid composite data authentication.

   PQ/T hybrid non-composite protocol:
      A PQ/T hybrid protocol that does not use only composite
      constructions can be referred to as a "PQ/T hybrid non-composite
      protocol".

      For example, a PQ/T hybrid protocol that offers both
      confidentiality and authentication and uses composite key
      agreement and non-composite authentication would be referred to as
      a "PQ/T hybrid non-composite protocol".

5.  Properties

   This section describes some properties that may be desired from or
   achieved by a PQ/T hybrid scheme or a PQ/T hybrid protocol.
   Properties of PQ/T hybrid schemes are still an active area of
   research and development, e.g., in [BINDELHALE].  This section does
   not attempt to be comprehensive, but rather covers a basic set of
   properties.

   It is not possible for one PQ/T hybrid scheme or PQ/T hybrid protocol
   to achieve all of the properties in this section.  To understand what
   properties are required, a designer or implementer will think about
   why they are using a PQ/T hybrid scheme.  For example, a scheme that
   is designed for implementation security will likely require PQ/T
   hybrid confidentiality or PQ/T hybrid authentication, while a scheme
   for interoperability will require PQ/T hybrid interoperability.

   PQ/T hybrid confidentiality:
      The property that confidentiality is achieved by a PQ/T hybrid
      scheme or a PQ/T hybrid protocol as long as at least one component
      algorithm that aims to provide this property remains secure.

   PQ/T hybrid authentication:
      The property that authentication is achieved by a PQ/T hybrid
      scheme or a PQ/T hybrid protocol as long as at least one component
      algorithm that aims to provide this property remains secure.

   The security properties of a PQ/T hybrid scheme or protocol depend on
   the security of its component algorithms, the choice of PQ/T hybrid
   combiner, and the capability of an attacker.  Changes to the security
   of a component algorithm can impact the security properties of a PQ/T
   hybrid scheme providing hybrid confidentiality or hybrid
   authentication.  For example, if the post-quantum component algorithm
   of a PQ/T hybrid scheme is broken, the scheme will remain secure
   against an attacker with a classical computer, but will be vulnerable
   to an attacker with a CRQC.

   PQ/T hybrid protocols that offer both confidentiality and
   authentication do not necessarily offer both hybrid confidentiality
   and hybrid authentication.  For example, [HYBRID-TLS] provides hybrid
   confidentiality but does not address hybrid authentication.
   Therefore, if the design in [HYBRID-TLS] is used with single-
   algorithm X.509 certificates as defined in [RFC5280], only
   authentication with a single algorithm is achieved.

   PQ/T hybrid interoperability:
      The property that a PQ/T hybrid scheme or a PQ/T hybrid protocol
      can be completed successfully provided that both parties share
      support for at least one component algorithm.

      For example, a PQ/T hybrid digital signature might achieve hybrid
      interoperability if the signature can be verified by either
      verifying the traditional or the post-quantum component, such as
      the approach defined in Section 7.2.2 of [ITU-T-X509-2019].  In
      this example, a verifier that has migrated to support post-quantum
      algorithms is required to verify only the post-quantum signature,
      while a verifier that has not migrated will verify only the
      traditional signature.

   In the case of a protocol that aims to achieve both authentication
   and confidentiality, PQ/T hybrid interoperability requires that at
   least one component authentication algorithm and at least one
   component algorithm for confidentiality is supported by both parties.

   It is not possible for a PQ/T hybrid scheme to achieve both PQ/T
   hybrid interoperability and PQ/T hybrid confidentiality without
   additional functionality at a protocol level.  For PQ/T hybrid
   interoperability, a scheme needs to work whenever one component
   algorithm is supported by both parties, while to achieve PQ/T hybrid
   confidentiality, all component algorithms need to be used.  However,
   both properties can be achieved in a PQ/T hybrid protocol by building
   in downgrade protection external to the cryptographic schemes.  For
   example, in [HYBRID-TLS], the client uses the TLS supported groups
   extension to advertise support for a PQ/T hybrid scheme, and the
   server can select this group if it supports the scheme.  This is
   protected using TLS's existing downgrade protection, so it achieves
   PQ/T hybrid confidentiality, but the connection can still be made if
   either the client or server does not support the PQ/T hybrid scheme,
   so PQ/T hybrid interoperability is achieved.

   The same is true for PQ/T hybrid interoperability and PQ/T hybrid
   authentication.  It is not possible to achieve both with a PQ/T
   hybrid scheme alone, but it is possible with a PQ/T hybrid protocol
   that has appropriate downgrade protection.

   PQ/T hybrid backwards compatibility:
      The property that a PQ/T hybrid scheme or a PQ/T hybrid protocol
      can be completed successfully provided that both parties support
      the traditional component algorithm, while also using both
      algorithms if both are supported by both parties.

   PQ/T hybrid forwards compatibility:
      The property that a PQ/T hybrid scheme or a PQ/T hybrid protocol
      can be completed successfully using a post-quantum component
      algorithm provided that both parties support it, while also having
      the option to use both post-quantum and traditional algorithms if
      both are supported by both parties.

      Note that PQ/T hybrid forwards compatibility is a protocol or
      scheme property only.

6.  Certificates

   This section introduces terminology related to the use of
   certificates in hybrid schemes.

   PQ/T hybrid certificate:
      A digital certificate that contains public keys for two or more
      component algorithms where at least one is a traditional algorithm
      and at least one is a post-quantum algorithm.

      A PQ/T hybrid certificate could be used to facilitate a PQ/T
      hybrid authentication protocol.  However, a PQ/T hybrid
      authentication protocol does not need to use a PQ/T hybrid
      certificate; separate certificates could be used for individual
      component algorithms.

      The component public keys in a PQ/T hybrid certificate could be
      included as a composite public key or as individual component
      public keys.

      The use of a PQ/T hybrid certificate does not necessarily achieve
      hybrid authentication of the identity of the sender; this is
      determined by properties of the chain of trust.  For example, an
      end-entity certificate that contains a composite public key, but
      which is signed using a single-algorithm digital signature scheme,
      could be used to provide hybrid authentication of the source of a
      message, but would not achieve hybrid authentication of the
      identity of the sender.

   Post-quantum certificate:
      A digital certificate that contains a single public key for a
      post-quantum digital signature algorithm.

   Traditional certificate:
      A digital certificate that contains a single public key for a
      traditional digital signature algorithm.

   X.509 certificates as defined in [RFC5280] could be either
   traditional or post-quantum certificates depending on the algorithm
   in the Subject Public Key Info.  For example, a certificate
   containing a ML-DSA public key, as defined in [ML-DSA], would be a
   post-quantum certificate.

   Post-quantum certificate chain:
      A certificate chain where all certificates include a public key
      for a post-quantum algorithm and are signed using a post-quantum
      digital signature scheme.

   Traditional certificate chain:
      A certificate chain where all certificates include a public key
      for a traditional algorithm and are signed using a traditional
      digital signature scheme.

   PQ/T hybrid certificate chain:
      A certificate chain where all certificates are PQ/T hybrid
      certificates and each certificate is signed with two or more
      component algorithms with at least one being a traditional
      algorithm and at least one being a post-quantum algorithm.

   A PQ/T hybrid certificate chain is one way of achieving hybrid
   authentication of the identity of a sender in a protocol, but it is
   not the only way.  An alternative is to use a PQ/T parallel PKI as
   defined below.

   PQ/T mixed certificate chain:
      A certificate chain containing at least two of the three
      certificate types defined in this document (PQ/T hybrid
      certificates, post-quantum certificates, and traditional
      certificates).

      For example, a traditional end-entity certificate could be signed
      by a post-quantum intermediate certificate, which in turn could be
      signed by a post-quantum root certificate.  This may be desirable
      due to the lifetimes of the certificates, the relative difficulty
      of rotating keys, or for efficiency reasons.  The security
      properties of a certificate chain that mixes post-quantum and
      traditional algorithms would need to be analysed on a case-by-case
      basis.

   PQ/T parallel PKI:
      Two certificate chains, one that is a post-quantum certificate
      chain and one that is a traditional certificate chain, and that
      are used together in a protocol.

      A PQ/T parallel PKI might be used to achieve hybrid authentication
      or hybrid interoperability depending on the protocol
      implementation.

   Multi-certificate authentication:
      Authentication that uses two or more end-entity certificates.

      For example, multi-certificate authentication may be achieved
      using a PQ/T parallel PKI.

7.  Security Considerations

   This document defines security-relevant terminology to be used in
   documents specifying PQ/T hybrid protocols and schemes.  However, the
   document itself does not have a security impact on Internet
   protocols.  The security considerations for each PQ/T hybrid protocol
   are specific to that protocol and should be discussed in the relevant
   specification documents.  More general guidance about the security
   considerations, timelines, and benefits and drawbacks of the use of
   PQ/T hybrids is also out of scope of this document.

8.  IANA Considerations

   This document has no IANA actions.

9.  Informative References

   [BDPR]     Bellare, M., Desai, A., Pointcheval, D., and P. Rogaway,
              "Relations Among Notions of Security for Public-Key
              Encryption Schemes", June 2001,
              <https://www.cs.ucdavis.edu/~rogaway/papers/
              relations.pdf>.

   [BINDEL]   Bindel, N., Brendel, J., Fischlin, M., Goncalves, B., and
              D. Stebila, "Hybrid Key Encapsulation Mechanisms and
              Authenticated Key Exchange", Post-Quantum Cryptography,
              PQCrypto 2019, Lecture Notes in Computer Science, vol.
              11505, pp. 206-226, DOI 10.1007/978-3-030-25510-7_12, July
              2019, <https://doi.org/10.1007/978-3-030-25510-7_12>.

   [BINDELHALE]
              Bindel, N. and B. Hale, "A Note on Hybrid Signature
              Schemes", Cryptology ePrint Archive, Paper 2023/423, 23
              July 2023, <https://eprint.iacr.org/2023/423.pdf>.

   [COMPOSITE-KEM]
              Ounsworth, M., Gray, J., Pala, M., Klaussner, J., and S.
              Fluhrer, "Composite ML-KEM for use in X.509 Public Key
              Infrastructure and CMS", Work in Progress, Internet-Draft,
              draft-ietf-lamps-pq-composite-kem-06, 18 March 2025,
              <https://datatracker.ietf.org/doc/html/draft-ietf-lamps-
              pq-composite-kem-06>.

   [ETSI_TS103774]
              European Telecommunications Standards Institute (ETSI),
              "CYBER; Quantum-safe Hybrid Key Exchanges", ETSI TS 103
              744 v1.1.1, December 2020, <https://www.etsi.org/deliver/
              etsi_ts/103700_103799/103744/01.01.01_60/
              ts_103744v010101p.pdf>.

   [HYBRID-TLS]
              Stebila, D., Fluhrer, S., and S. Gueron, "Hybrid key
              exchange in TLS 1.3", Work in Progress, Internet-Draft,
              draft-ietf-tls-hybrid-design-12, 14 January 2025,
              <https://datatracker.ietf.org/doc/html/draft-ietf-tls-
              hybrid-design-12>.

   [ITU-T-X509-2019]
              ITU-T, "Information Technology - Open Systems
              Interconnection - The Directory: Public-key and attribute
              certificate frameworks", ITU-T Recommendation X.509,
              October 2019,
              <https://www.itu.int/rec/T-REC-X.509-201910-I>.

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

   [NIST_PQC_FAQ]
              NIST, "Post-Quantum Cryptography (PQC) FAQs", 31 January
              2025, <https://csrc.nist.gov/Projects/post-quantum-
              cryptography/faqs>.

   [NIST_SP_800-152]
              Barker, E., Smid, M., and D. Branstad, "A Profile for U.
              S. Federal Cryptographic Key Management Systems", NIST
              SP 800-152, DOI 10.6028/NIST.SP.800-15, October 2015,
              <https://doi.org/10.6028/NIST.SP.800-152>.

   [RFC4949]  Shirey, R., "Internet Security Glossary, Version 2",
              FYI 36, RFC 4949, DOI 10.17487/RFC4949, August 2007,
              <https://www.rfc-editor.org/info/rfc4949>.

   [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>.

   [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>.

   [RFC9180]  Barnes, R., Bhargavan, K., Lipp, B., and C. Wood, "Hybrid
              Public Key Encryption", RFC 9180, DOI 10.17487/RFC9180,
              February 2022, <https://www.rfc-editor.org/info/rfc9180>.

   [RFC9370]  Tjhai, CJ., Tomlinson, M., Bartlett, G., Fluhrer, S., Van
              Geest, D., Garcia-Morchon, O., and V. Smyslov, "Multiple
              Key Exchanges in the Internet Key Exchange Protocol
              Version 2 (IKEv2)", RFC 9370, DOI 10.17487/RFC9370, May
              2023, <https://www.rfc-editor.org/info/rfc9370>.

   [RFC9763]  Becker, A., Guthrie, R., and M. Jenkins, "Related
              Certificates for Use in Multiple Authentications within a
              Protocol", RFC 9763, DOI 10.17487/RFC9763, June 2025,
              <https://www.rfc-editor.org/info/rfc9763>.

Acknowledgments

   This document is the product of numerous fruitful discussions in the
   IETF PQUIP group.  Thank you in particular to Mike Ounsworth, John
   Gray, Tim Hollebeek, Wang Guilin, Rebecca Guthrie, Stephen Farrell,
   Paul Hoffman, and Sofía Celi for their contributions.  This document
   is inspired by many others from the IETF and elsewhere.

Authors' Addresses

   Florence Driscoll
   UK National Cyber Security Centre
   Email: florence.d@ncsc.gov.uk


   Michael Parsons
   UK National Cyber Security Centre
   Email: michael.p1@ncsc.gov.uk


   Britta Hale
   Naval Postgraduate School
   Email: britta.hale@nps.edu
