CBOR Web Token (CWT) Claims in COSE 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 Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at https://datatracker.ietf.org/drafts/current/.¶

Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."¶

This Internet-Draft will expire on 10 May 2024.¶

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 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

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.¶

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.¶

2. Representation

This document defines the following COSE header parameter:¶

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].¶

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.¶

Profiles define how to use CWT claims 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 requires unambiguously knowing the profile being used. 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 profile can also be used.¶

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].¶

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

-08¶

• Added Security Consideration about profiles and processing CWT claims.¶

-07¶

• 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¶

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

Authors' Addresses