Internet-Draft CBOR Web Token (CWT) Claims in COSE Head October 2023
Looker & Jones Expires 24 April 2024 [Page]
Intended Status:
Standards Track
T. Looker
M. Jones
Self-Issued Consulting

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 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.

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 15) map [IANA.COSE] Location for CWT Claims in COSE Header Parameters

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.

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. Normative References

IANA, "COSE Header Parameters", <>.
Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, , <>.
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, , <>.

7. Informative References

Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token (JWT)", RFC 7519, DOI 10.17487/RFC7519, , <>.
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, , <>.
Schaad, J., "CBOR Object Signing and Encryption (COSE): Structures and Process", STD 96, RFC 9052, DOI 10.17487/RFC9052, , <>.

Appendix A. Acknowledgements

We would like to thank Daisuke Ajitomi, Claudio Allocchio, 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









Authors' Addresses

Tobias Looker
Michael B. Jones
Self-Issued Consulting