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Third-party audit-checkpoint anchoring

Steward can optionally send the SHA-256 digest of each canonical Ed25519 audit checkpoint payload to an RFC 3161 timestamp authority (TSA). A verified token shows that the checkpoint existed no later than the TSA’s signed time. This narrows the window in which an operator could rewrite pre-anchor history and produce a new checkpoint. Anchoring is not tamper-proof or operator-proof. It does not prove that the export is complete, protect events created after the anchor, prevent TSA and operator collusion, or replace the auditor’s out-of-band trust in Steward’s Ed25519 signing-key fingerprint and the TSA certificate authority.

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Configuration

No setting means off: Steward constructs no sink, makes no network request, and returns the existing v1 bundle shape without an anchor field.
  • best-effort logs a TSA failure and returns the ordinary signed bundle without an anchor proof.
  • required returns HTTP 503 and does not emit an evidence bundle when the TSA is missing, rejects the request, times out, returns a malformed response, fails cryptographic verification, or the verified proof cannot be persisted.
  • production TSA URLs must use HTTPS, may not embed credentials, do not follow redirects, and responses are capped at 1 MiB.
The reference sink sends a DER RFC 3161 TimeStampReq with a SHA-256 MessageImprint, a new unpredictable 128-bit nonce, and certReq=true. Before accepting the response, Steward invokes OpenSSL’s RFC 3161 verifier against the original query and operator-supplied CA. That verifies the CMS SignedData, signed TSTInfo, imprint, nonce, timestamping EKU, signature, and certificate path at the signed genTime (so a legitimately expired TSA signing certificate does not invalidate older evidence). Steward also enforces the signed policy OID when configured and checks genTime freshness using the TSA’s signed accuracy interval. The complete uncertainty interval must fit the configured acquisition window; an imprecise timestamp cannot pass merely because its interval overlaps the window. The bundle contains the DER TimeStampResp, checkpoint digest, nonce, policy, time/accuracy, verification time, and acquisition trust-anchor fingerprint. The same proof is stored append-only beside the exact signed checkpoint. It never contains TSA credentials or a trust root. best-effort may still return an anchored bundle if later database persistence fails; required may not. Anchoring occurs when an audit bundle or governed evidence case produces a checkpoint. Steward does not claim a periodic anchor cadence: operators who need hourly/daily bounds must schedule those evidence exports externally and monitor for successful persisted proofs. Self-hosted API compositions can implement AuditCheckpointAnchorSink and register a named provider with registerAuditCheckpointAnchorSink before serving evidence routes. A custom registration must include both a sink and a proof verifier. Its sink returns a discriminated type: "custom" proof with the configured provider/sink IDs, exact checkpoint digest, and provider-native JSON evidence. Steward validates those bindings and runs the registered verifier before persisting or returning the proof; missing or rejected verification fails closed in required mode. This preserves a custom witness’s native proof instead of representing it as RFC 3161 evidence. Selecting an unregistered provider fails closed.

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Offline verification

Trust the TSA only through CA material obtained independently from the bundle:
The verifier reconstructs the exact checkpoint digest, invokes OpenSSL’s RFC 3161 verifier entirely offline, validates the CMS/TSTInfo signature, timestamping certificate path, message imprint, persisted nonce/policy/time metadata, and reports genTime + accuracy as the conservative “existed no later than” bound. An intermediate chain can be supplied with --tsa-untrusted. Verification is deliberately offline and does not fetch CRLs or OCSP responses. For long-term validation, auditors must retain the independently obtained CA and intermediate set applicable at genTime, plus any time-applicable revocation evidence required by their policy. The persisted CA fingerprint and TSA policy identify the acquisition trust policy; they are not themselves revocation proof. To enforce a compliance cutoff:
Without --tsa-ca, an included token is reported as present but untrusted. The bundle-carried token and any bundle-carried identity are never treated as their own trust root. The bundled CLI verifies RFC 3161 proofs only. A type: "custom" proof remains bound to the signed checkpoint and was verified by the registered application verifier at acquisition, but auditors must independently verify its evidence with the corresponding witness-specific tooling and trust material.