attestation_tier
Term URI: https://w3id.org/agent-evidence/vocab/v1/attestation_tier
Section: evidence_dimensions. Criticality: critical.
Whether the subject a platform signed was RE-DERIVED by that platform from evidence the caller does not control (authoritative), or was accepted from the caller and signed as supplied (voluntary). A voluntary record is non-repudiable for exactly what was signed: this platform signed these bytes, at this time, under this key. It is not a capture and it is not unbypassable. A verifier MUST NOT read a voluntary attestation as evidence that the attested content corresponds to any independently observed fact. The tier is set by the PLATFORM and read by a verifier from the record. A caller MUST NOT select it. A caller-selected tier is the whole attack: the one party with an interest in the answer would be choosing how much weight the answer carries. Closed and fail-closed, on the same rule as containment_posture: an absent, non-string, or unregistered value makes the enclosing statement malformed, and a missing tier is never read as the stronger of the two. The bounds of what was re-derived are part of the claim. Where a caller supplied any bound of the re-derivation input, the record MUST declare that, and a verifier MUST NOT read an authoritative tier as coverage of everything the subject did. SELECTION is part of the same question. A caller that chooses which subject to request an attestation over, and when, has bounded the claim without supplying any bound, so the rule above is not satisfied by a record that merely had no bound to declare. coverage_denominator is the field that closes that gap, and an authoritative record making any claim about a population needs it. An authoritative tier is a claim about a re-derivation, which is a claim about bytes a platform fetched. It composes with observation_vantage and never substitutes for it: whether anyone observed the execution is a separate question this field does not answer. So a record declaring authoritative MUST also declare observation_vantage, and a verifier MUST reject an authoritative record that does not. Without that pairing the composition rule has nothing to compose with, and a platform the observed party operates can set the stronger tier on its own records forever.
Registered values
https://w3id.org/agent-evidence/vocab/v1/attestation_tier#voluntary