CBOR Web Token (CWT) Claims in COSE Headers
draft-ietf-cose-cwt-claims-in-headers-10

COSE                                                           T. Looker
Internet-Draft                                                     Mattr
Intended status: Standards Track                                M. Jones
Expires: 1 June 2024                              Self-Issued Consulting
                                                        29 November 2023

              CBOR Web Token (CWT) Claims in COSE Headers
                draft-ietf-cose-cwt-claims-in-headers-10

Abstract

   This document describes how to include CBOR Web Token (CWT) claims in
   the header parameters of any COSE structure.  This functionality
   helps to facilitate applications that wish to make use of CBOR Web
   Token (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.

Discussion Venues

   This note is to be removed before publishing as an RFC.

   Source for this draft and an issue tracker can be found at
   https://github.com/tplooker/draft-ietf-cose-cwt-claims-in-headers.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on 1 June 2024.

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Copyright Notice

   Copyright (c) 2023 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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   Please review these documents carefully, as they describe your rights
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   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Terminology  . . . . . . . . . . . . . . . .   3
   2.  Representation  . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .   4
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   4
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   5
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .   5
     6.2.  Informative References  . . . . . . . . . . . . . . . . .   6
   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . .   6
   Appendix B.  Document History . . . . . . . . . . . . . . . . . .   6
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   8

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.

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   Section 5.3 of JSON Web Token (JWT) [RFC7519] defined a similar
   mechanism for expressing selected JWT based claims as 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
   [I-D.ietf-cose-typ-header-parameter].  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:

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   +========+================+=======+=============+===================+
   | Name   | Label          | Value | Value       | Description       |
   |        |                | Type  | Registry    |                   |
   +========+================+=======+=============+===================+
   | CWT    | TBD (requested | map   | [IANA.COSE] | Location for      |
   | Claims | assignment 15) |       |             | CWT Claims in     |
   |        |                |       |             | 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 is used only
   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].

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   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 that are
   present both in the payload and the header of a CWT are identical,
   unless it defines 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 secured 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 is requested to register the new COSE header parameter "CWT
   Claims" in the table in Section 2 in the "COSE Header Parameters"
   registry [IANA.COSE].

6.  References

6.1.  Normative References

   [I-D.ietf-cose-typ-header-parameter]
              Jones, M. B. and O. Steele, "COSE "typ" (type) Header
              Parameter", Work in Progress, Internet-Draft, draft-ietf-
              cose-typ-header-parameter-01, 7 November 2023,
              <https://datatracker.ietf.org/doc/html/draft-ietf-cose-
              typ-header-parameter-01>.

   [IANA.COSE]
              IANA, "COSE Header Parameters",
              <https://www.iana.org/assignments/cose/cose.xhtml#header-
              parameters>.

   [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>.

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   [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>.

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>.

Appendix A.  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.

Appendix B.  Document History

   -09

   *  Described use cases where CWT claims can't be put in the payload
      in response to Hannes Tschofenig's IotDir review.
   *  Said that profiles specify the semantics of the CWT claims in
      response to Carsten Bormann's feedback.

   -08

   *  Added Security Consideration about profiles and processing CWT
      claims.

   -07

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   *  Added Privacy Consideration about unencrypted claims in header
      parameters.
   *  Added Security Consideration about detached content.
   *  Added Security Consideration about claims that are present both in
      the payload and the header of a CWT.
   *  Changed requested IANA COSE Header Parameter assignment number
      from 13 to 15 due to subsequent assignments of 13 and 14.
   *  Acknowledged last call reviewers.

   -06

   *  Changed requested IANA COSE Header Parameter assignment number
      from 11 to 13 due to Countersignature being allocated 11.
   *  Reference correct registry IANA COSE Header Parameters.

   -05

   *  Added Acknowledgements section.
   *  Addressed WGLC feedback.  Specifically...
   *  Added statement about being able to use the header parameter in
      any COSE object.
   *  Moved statment about verifing that claim values present in both
      the header and payload are identical from the Security
      Considerations to the body of the specification.

   -04

   *  Update author affiliation.
   *  Add standard reference to RFC terminology.
   *  Added reference to security considerations from RFC8392.

   -03

   *  Added recommendation around header treatment in protected vs
      unprotected.

   -02

   *  Added CDDL description for CWT claim value.

   -01

   *  Changed example from Key ID to Issuer.

   -00

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   *  Created draft-ietf-cose-cwt-claims-in-headers-00 from draft-
      looker-cose-cwt-claims-in-headers-00 following working group
      adoption.

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/

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