Signing and verifying

The signature algorithms themselves (ECDSA, EdDSA, RSA, the fully-specified identifiers of RFC 9864, ML-DSA) are listed in Algorithms; the keys they take in Keys.

• COSE_Sign1 — a message with a single signer, created and verified
• What the application must check — the two checks RFC 9052 §3.1 leaves to the caller
• COSE_Sign — several signers, each covering the Signature structure
• The verification contract — what verify() returns and what it throws
• Verifying with a certificate — verifying with the key of an X.509 certificate
• Countersignatures — a signature added after the message was finalized