CheckSeal

A check-receipt for AI artifacts: which verification checks passed, and how strongly each one binds.

Provenance standards tell you who made an artifact and that it hasn't changed since. They do not tell you which checks it passed and whether those checks were mechanically enforced or merely advisory. CheckSeal is the reference implementation of that check-assertion predicate. It rides in-toto Attestation v1 and Sigstore keyless; it does not invent a competing format.

The differentiator is the enforced / advisory / observed grade on every check, and the honesty machinery behind it:

• evidence is a digest, recomputed — a verifier checks the evidence, it does not trust the claim.
• an enforced grade needs a proof — enforced_proof resolves to a HarnessBench report that empirically measured the gate, and the corpus's threat class must actually cover the check (a destructive-execution corpus cannot prove a content rights-gate).
• consumers display trust_floor, never a bare "enforced" — the weaker of how strongly a check binds and how strong its evidence is, so a seal can't over-claim in either dimension.
• Sigstore + Rekor make backdating detectable (a not-after bound; Rekor does not prove not-before, and CheckSeal says so).

A seal asserts presence with evidence. It cannot prove a check was not run. That limit is stated, not hidden.

Install

pip install checkseal            # stdlib-only core
pip install checkseal[sign]      # + T1 local-key signing (cryptography)
pip install checkseal[keyless]   # + T2 Sigstore keyless (public seals)

Quickstart (T1, offline)

checkseal keygen --out key.pem --pub key.pub.pem

# after your checks run and land in a T0 store (t0.jsonl), seal one subject:
checkseal seal --store t0.jsonl --subject ./artifact --name my/artifact \
  --key key.pem --out artifact.intoto.jsonl

# verify against the live artifact (exits non-zero if the seal does not pass):
checkseal verify artifact.intoto.jsonl --subject ./artifact --pubkey key.pub.pem

Public seals must be T2 (Sigstore keyless); that path runs in CI where an OIDC credential is available.

The Verifier Contract

A verification is valid only if the verifier (1) recomputes the live subject digest, (2) checks Rekor inclusion for a freshness bound, (3) re-executes enforced Grade-A checks, (4) resolves enforced_proof against HarnessBench with a corpus-relevance check, (5) renders trust_floor, and (6) treats all sealed content as untrusted. See DESIGN.md.

Client-side verification (/receipts)

js/checkseal_verify.mjs is the honest browser subset: it recomputes the subject digest, checks the Statement/predicate subject coupling, verifies the Ed25519 signature over the DSSE PAE, and renders trust_floor — and it states loudly what it does NOT check (re-execution, enforced_proof resolution, full Rekor proof), which are CLI-only. A Python-signed seal verifies in this JS verifier (node --test js/), proving the format is language-agnostic.

Public T2 seals are minted in CI: checkseal seal-keyless plus .github/workflows/seal.yml (GitHub OIDC → Fulcio → Rekor).

Status

Phases 0-2 complete (format, producer/sealer, verifier CLI); Phase 3 in progress (client-side verifier + T2 keyless CI). Part of the Verification Chain program (HarnessBench + Verification Ledger + CheckSeal on one schema). MIT.