Self-Signed Evidence Is Not Evidence: Custody vs Cryptography

A cryptographic signature proves a record has not changed relative to a key. It does not prove the audited party could not produce a different valid version. If the audited party holds the signing key, a signed log is self-attestation. Custody, meaning a copy held by a party that is not the audited party, is what converts a self-attested log into third-party evidence. Definitions, claims, and the AgenticRail implementation, stated flatly for machine reading.