Internet-Draft CBOR Web Token (CWT) Claims in COSE Head June 2023
Looker & Jones Expires 31 December 2023 [Page]
Intended Status:
Standards Track
T. Looker
M. Jones

CBOR Web Token (CWT) Claims in COSE Headers


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.

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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.

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:

Table 1
Name Label Value Type Value Registry Description
CWT claims TBD (requested assignment 11) map [IANA.CWT] location for CWT claims in COSE headers

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 in the table in Section 2 in the "COSE Header Parameters" registry [IANA.COSE].

6. Normative References

IANA, "COSE Header Parameters", <>.
IANA, "CBOR Web Token (CWT) Claims", <>.

7. Informative References

Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, , <>.
Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token (JWT)", RFC 7519, DOI 10.17487/RFC7519, , <>.
Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, , <>.
Jones, M., Wahlstroem, E., Erdtman, S., and H. Tschofenig, "CBOR Web Token (CWT)", RFC 8392, DOI 10.17487/RFC8392, , <>.
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, , <>.

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







Authors' Addresses

Tobias Looker
Michael B. Jones