Code change:
sense for their operational use cases. At least one identifier for an identity document MUST be included in <del> the protected header of the COSE envelope, either "x5t" or "kid". </del> <ins> the protected header of the COSE envelope, as one of "x5t", "x5chain" or "kid". </ins> Support for "x5t" is mandatory to implement. <del> Support for "kid" is optional. </del> <ins> Support for "kid" and "x5chain" is optional. </ins> <del> When "x5t" is present, "iss" MUST be a string with a value between 1 and 8192 characters in length that fits the regular expression of a distinguished name. </del> <ins> When "x5t" or "x5chain" is present, "iss" MUST be a string with a value between 1 and 8192 characters in length that fits the regular expression of a distinguished name. </ins> The mechanisms for how Transparency Services obtain identity documents is out-of-scope of this document. <del> The "kid" header parameter MUST be present when the "x5t" header parameter is not present.  Key discovery protocols are out-of-scope of this document. </del> <ins> The "kid" header parameter MUST be present when neither "x5t" nor "x5chain" are present.  Key discovery protocols are out-of-scope of this document. </ins> The protected header of a Signed Statement and a Receipt MUST include the "CWT Claims" header parameter as specified in CWT_CLAIMS_COSE.

Related discussion:
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Since the refactor and the shift to X.509-based claim signature, the authentication of issuers is no longer auditable - i.e. it is no longer possible for verifiers and auditors to re-validate the claim signature and issuer authentication performed by a TS at registration, which massively increases the trust in the TS (which may now try to register forged claims with plausible deniability - by claiming the TS lost the issuer's certificate used at registration even if none existed). Part of the issue is that claims use x5t instead of x5c. This means there is no mechanism for auditors to obtain the certificate chain used to sign a claim from the TS at registration. Even if x5c was used instead of x5t, only the leaf certificate is required to appear in x5c. Reconstructing the full issuer certificate chain requires at least the TS configuration to link to the configured root CAs used by the TS. Since roots and intermediate certificates expire (or can be updated, as is often the case with PKIX root programs from Mozilla, Google and Microsoft), it is not possible to assume that verifiers can just store the root configuration with the TS's certificate. There are further issues with issuer authentication: >When x5t is present, iss MUST be a string with a value between 1 and 8192 characters in length that fits the regular expression of a distinguished name. This is not well-specified without a reference to such a regular expression. Is this supposed to mean RFC4514 string representation of a distinguished name? The language of RFC4514 doesn't appear to be regular (it is context free), so this must be clarified. There is also no requirement for the issuer's serialized distinguished name to be produced from the claim's X.509 leaf subject. A malicious issuer can put a different issuer's serialized distinguished name in the iss field and is likely to confuse a verifier who cannot detect the mismatch since issuer authentication is not auditable. This makes iss essentially useless for verifiers, and it may be preferable to set iss to nil if x5t/x5c is used.
I propose we require in the architecture that the full certificate chain used to identify the issuer and verify its signature at the time of registration be authenticated by the TS via the resulting receipt.