SD-JWT Verifier: Verify Holder Binding

[sandrask]

If Holder Binding is required, verify that the Holder Binding JWT is signed by the Holder and valid.

Validation steps:

1. If Holder Binding JWT is not provided, the Verifier MUST reject the Presentation.
2. Determine the public key for the Holder from the SD-JWT.
3. Ensure that a signing algorithm was used that was deemed secure for the application. Refer to [RFC8725], Sections 3.1 and 3.2 for details. The none algorithm MUST NOT be accepted.
4. Validate the signature over the Holder Binding JWT.
5. Check that the Holder Binding JWT is valid using nbf, iat, and exp claims, if provided in the Holder Binding JWT.
6. Determine that the Holder Binding JWT is bound to the current transaction and was created for this Verifier (replay protection). This is usually achieved by a nonce and aud field within the Holder Binding JWT.