CBOR Web Token (CWT) Claims in COSE Headers

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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 the JWT RFC [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. Therefore care must be taken when expressing CWT claims in COSE headers.¶

4. Security Considerations

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

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, Laurence Lundblade, Ivaylo Petrov, Orie Steele, and Hannes Tschofenig for their valuable contributions to this specification.¶

Appendix B. Document History

-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