MARGINALIA · TERMS OF ART
A valid signature answers a narrower question than most applications ask.
Verification can establish that a signature corresponds to particular bytes under a particular public key.
Authorization decides what the system permits as a result.
Between those statements sit several questions that the signature cannot answer alone. Who is the key bound to? Is the credential current? Was the message intended for this protocol and this relying party? Is it fresh, or has a valid message been replayed? Is the asserted purpose allowed by local policy?
RFC 8224 states the separation precisely for SIP identity: verifying an Identity header does not prescribe how the request must be treated. The same discipline applies elsewhere.
- Determine the exact representation.
- Verify the cryptographic operation.
- Validate the key, credential, status and purpose.
- Establish freshness and protocol context.
- Apply authorization policy.
Collapsing these steps makes cryptography appear to decide something for which it was never given the necessary information.
Source: RFC 8224, Section 6.2.1.
Related research: Evidence is not a verdict.
