CBOR Object Signing and Encryption (COSE): Header Parameters for Carrying and Referencing X.509 Certificates

Note

IETF specification extending COSE by defining attributes that reference or contain X.509 certificates to identify and convey keys.

Summary

Purpose

IETF RFC 9360 (February 2023) supplements COSE (CBOR Object Signing and Encryption), the signing and encryption mechanism for data in the binary CBOR format. It adds header parameters that refer to or carry X.509 certificates:

Using X.509 certificates makes it possible to reuse an existing trust infrastructure with COSE: trust anchors, certificate chains, revocation checking RFC 9360, [2.](#section-2) [X.509 COSE Header Parameters](#name-x509-cose-header-parameters).

Who it applies to

The RFC itself imposes nothing under eIDAS. It applies by reference to providers of person identification data (PID), who ensure that the PID issued comply with the annex to Implementing Regulation 2024/2977 Art. 3(3).

The reference concerns the signing of PID in the ISO/IEC-mdoc format: the protected header of the CB-AdES digital signature that signs an mdoc PID must contain the x5u and x5t parameters, both specified in RFC 9360 Annex.

Key points

• x5t, the certificate thumbprint. This parameter identifies the signer’s certificate by a hash value computed over the DER-encoded certificate; it consists of an algorithm identifier and the value itself RFC 9360, [2.](#section-2) [X.509 COSE Header Parameters](#name-x509-cose-header-parameters).
• SHA-256. The RFC requires applications to support SHA-256, without excluding other algorithms RFC 9360, [2.](#section-2) [X.509 COSE Header Parameters](#name-x509-cose-header-parameters). The act goes further: the algorithm used in x5t must be SHA-256 Annex.
• x5u, the certificate location. This parameter designates the certificate by a URI; the resource may be served under several media types RFC 9360, [2.](#section-2) [X.509 COSE Header Parameters](#name-x509-cose-header-parameters).
• Protecting the certificate identification. The RFC requires the signer’s certificate to be integrity-protected by COSE, for example by combining x5u with an x5t placed in the protected header RFC 9360, [2.](#section-2) [X.509 COSE Header Parameters](#name-x509-cose-header-parameters). The act places both parameters in the protected header Annex.
• Validation is still needed. Whoever relies on the certificate must always validate it, for example according to the rules of RFC 5280 RFC 9360, [2.](#section-2) [X.509 COSE Header Parameters](#name-x509-cose-header-parameters).
• The other parameters. The RFC also defines x5bag (an unordered bag of certificates) and x5chain (an ordered chain) RFC 9360, [2.](#section-2) [X.509 COSE Header Parameters](#name-x509-cose-header-parameters); the annex does not mention them for PID Annex.
• The SD-JWT VC parallel. For PID in SD-JWT VC format, the annex likewise requires the x5u and x5t#S256 parameters, but those of RFC 7515 Annex.

Clauses referred to by the act

• x5u and x5t parameters of the RFC, without a clause number, with one adaptation: SHA-256 mandatory for x5t Annex.
• The annex to Implementing Regulation 2024/2977 was replaced by Implementing Regulation 2026/1731 Implementing Regulation (EU) 2026/1731; this summary cites the consolidated version that incorporates it.

Links

• Act that makes it applicable: Implementing Regulation 2024/2977 Implementing Regulation (EU) 2024/2977 — consolidated version of 11/08/2026.
• Related standards: RFC 7515, JWS RFC 7515; RFC 8949, CBOR RFC 8949; RFC 5280 RFC 5280.
• Learning path: step 3, the European wallet.

Written by Asphalia.