@Edent@mastodon.social If the verifier chooses which algorithm to use from this header, without double-checking it against the key, there's a chance it'll be vulnerable to an "algorithm confusion attack". Here's what that looks like in the case of JWTs; the same problem could happen with RFC9421. The safe way to work out which algorithm to use is to dereference the keyid and then choose the algorithm which matches the type of the returned key.