



Internet Engineering Task Force (IETF)                         T. Looker
Request for Comments: 9597                                         Mattr
Category: Standards Track                                     M.B. Jones
ISSN: 2070-1721                                   Self-Issued Consulting
                                                               June 2024


              CBOR Web Token (CWT) Claims in COSE Headers

Abstract

   This document describes how to include CBOR Web Token (CWT) claims in
   the header parameters of any CBOR Object Signing and Encryption
   (COSE) structure.  This functionality helps to facilitate
   applications that wish to make use of CWT claims in encrypted COSE
   structures and/or COSE structures featuring detached signatures,
   while having some of those claims be available before decryption and/
   or without inspecting the detached payload.  Another use case is
   using CWT claims with payloads that are not CWT Claims Sets,
   including payloads that are not CBOR at all.

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/rfc9597.

Copyright Notice

   Copyright (c) 2024 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
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   Trust Legal Provisions and are provided without warranty as described
   in the Revised BSD License.

Table of Contents

   1.  Introduction
     1.1.  Requirements Terminology
   2.  Representation
   3.  Privacy Considerations
   4.  Security Considerations
   5.  IANA Considerations
   6.  References
     6.1.  Normative References
     6.2.  Informative References
   Acknowledgements
   Authors' Addresses

1.  Introduction

   In some applications of COSE, it is useful to have a standard
   representation of CWT claims [RFC8392] available in the header
   parameters.  These include encrypted COSE structures, which may or
   may not be an encrypted CWT, and/or those featuring a detached
   signature.  Another use case is using CWT claims with payloads that
   are not CWT Claims Sets, including payloads that are not CBOR at all.
   For instance, an application might want to include an "iss" (issuer)
   claim in a COSE_Sign1 structure when the payload being signed is a
   non-CBOR data structure, such as a bitmap image, and the issuer value
   is used for key discovery.

   Section 5.3 of [RFC7519], "JSON Web Token (JWT)", defined a similar
   mechanism for expressing selected JWT-based claims as JSON Object
   Signing and Encryption (JOSE) header parameters.  This JWT feature
   was motivated by the desire to have certain claims, such as the
   Issuer value, be visible to software processing the JWT, even though
   the JWT is encrypted.  No corresponding feature was standardized for
   CWTs, which was an omission that this specification corrects.

   Directly including CWT claim values as COSE header parameter values
   would not work, since there are conflicts between the numeric header
   parameter assignments and the numeric CWT claim assignments.
   Instead, this specification defines a single header parameter
   registered in the IANA "COSE Header Parameters" registry that creates
   a location to store CWT claims in a COSE header parameter.

   This specification does not define how to use CWT claims and their
   semantics for particular applications, whether they are in the COSE
   payload or the CWT Claims header parameter, or both.  Therefore,
   understanding how to process the CWT Claims header parameter requires
   unambiguously knowing the intended interpretation.  The necessary
   information about this MAY come from other header parameters.  Unless
   there already is a natural way of providing this information at an
   appropriate level of integrity protection and authentication, a
   RECOMMENDED way to include this information in the COSE structure is
   use of the "typ" (type) Header Parameter [RFC9596].  Other methods
   for determining the intended interpretation MAY also be used.
   Recipients of the CWT Claims header parameter MUST NOT use the
   information in the CWT Claims header parameter beyond the integrity
   protection or authentication afforded to the CWT Claims header and
   the information used to derive its intended interpretation.

1.1.  Requirements Terminology

   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.

2.  Representation

   This document defines the following COSE header parameter:

   +========+=======+=======+==============+===============+===========+
   | Name   | Label | Value | Value        | Description   | Reference |
   |        |       | Type  | Registry     |               |           |
   +========+=======+=======+==============+===============+===========+
   | CWT    | 15    | map   | map keys in  | Location      | Section 2 |
   | Claims |       |       | [CWT.Claims] | for CWT       | of RFC    |
   |        |       |       |              | Claims in     | 9597      |
   |        |       |       |              | COSE Header   |           |
   |        |       |       |              | Parameters    |           |
   +--------+-------+-------+--------------+---------------+-----------+

                                  Table 1

   The following is a non-normative description for the value type of
   the CWT claim header parameter using CDDL [RFC8610].

   CWT-Claims = {
    * Claim-Label => any
   }

   Claim-Label = int / text

   In cases where CWT claims are present both in the payload and the
   header of a CWT, an application receiving such a structure MUST
   verify that their values are identical, unless the application
   defines other specific processing rules for these claims.

   It is RECOMMENDED that the CWT Claims header parameter only be used
   in a protected header to avoid the contents being malleable.  The
   header parameter MUST only occur once in either the protected or
   unprotected header of a COSE structure.

   The CWT Claims header parameter MAY be used in any COSE object using
   header parameters, such as COSE_Sign objects.  Its use is not
   restricted to CWTs.

3.  Privacy Considerations

   Some of the registered CWT claims may contain privacy-sensitive
   information.  Since CWT claims in COSE headers are not encrypted,
   when privacy-sensitive information is present in these claims,
   applications and protocols using them should ensure that these COSE
   objects are only made visible to parties for which it is appropriate
   for them to have access to this sensitive information.

4.  Security Considerations

   Implementers should also review the security considerations for CWT,
   which are documented in Section 8 of [RFC8392].

   As described in [RFC9052], if the COSE payload is transported
   separately ("detached content"), then it is the responsibility of the
   application to ensure that it will be transported without changes.

   The reason for applications to verify that CWT claims present in both
   the payload and the header of a CWT are identical, unless they define
   other specific processing rules for these claims, is to eliminate
   potential confusion that might arise by having different values for
   the same claim, which could result in inconsistent processing of such
   claims.

   Processing information in claims prior to validating that their
   integrity is cryptographically secure can pose security risks.  This
   is true whether the claims are in the payload or a header parameter.
   Implementers must ensure that any tentative decisions made based on
   previously unverified information are confirmed once the
   cryptographic processing has been completed.  This includes any
   information that was used to derive the intended interpretation of
   the CWT claims parameter.

5.  IANA Considerations

   IANA has registered the new COSE header parameter "CWT Claims"
   defined in Table 1 in the "COSE Header Parameters" registry
   [COSE.HeaderParameters].

6.  References

6.1.  Normative References

   [COSE.HeaderParameters]
              IANA, "COSE Header Parameters",
              <https://www.iana.org/assignments/cose/>.

   [CWT.Claims]
              IANA, "CBOR Web Token (CWT) Claims",
              <https://www.iana.org/assignments/cwt/>.

   [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>.

   [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>.

   [RFC8392]  Jones, M., Wahlstroem, E., Erdtman, S., and H. Tschofenig,
              "CBOR Web Token (CWT)", RFC 8392, DOI 10.17487/RFC8392,
              May 2018, <https://www.rfc-editor.org/info/rfc8392>.

   [RFC9596]  Jones, M.B. and O. Steele, "CBOR Object Signing and
              Encryption (COSE) "typ" (type) Header Parameter",
              RFC 9596, DOI 10.17487/RFC9596, June 2024,
              <https://www.rfc-editor.org/info/rfc9596>.

6.2.  Informative References

   [RFC7519]  Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token
              (JWT)", RFC 7519, DOI 10.17487/RFC7519, May 2015,
              <https://www.rfc-editor.org/info/rfc7519>.

   [RFC8610]  Birkholz, H., Vigano, C., and C. Bormann, "Concise Data
              Definition Language (CDDL): A Notational Convention to
              Express Concise Binary Object Representation (CBOR) and
              JSON Data Structures", RFC 8610, DOI 10.17487/RFC8610,
              June 2019, <https://www.rfc-editor.org/info/rfc8610>.

   [RFC9052]  Schaad, J., "CBOR Object Signing and Encryption (COSE):
              Structures and Process", STD 96, RFC 9052,
              DOI 10.17487/RFC9052, August 2022,
              <https://www.rfc-editor.org/info/rfc9052>.

Acknowledgements

   We would like to thank Daisuke Ajitomi, Claudio Allocchio, Carsten
   Bormann, Laurence Lundblade, Ivaylo Petrov, Ines Robles, Orie Steele,
   Hannes Tschofenig, Paul Wouters, and Peter Yee for their valuable
   contributions to this specification.

Authors' Addresses

   Tobias Looker
   Mattr
   Email: tobias.looker@mattr.global


   Michael B. Jones
   Self-Issued Consulting
   Email: michael_b_jones@hotmail.com
   URI:   https://self-issued.info/
