Signing and attestation audit — 2026-05-30¶

• Branch: chore/signing-and-attestation-audit
• Trigger: audit CLAUDE.md §11 ("Signing is keyless via Sigstore") against the actual implementation in .github/workflows/.
• Disposition: existing setup is genuinely strong; three closeable residual gaps. Tracked in ADR-0902.

TL;DR¶

"NEW" = added by this PR.

Methodology¶

1. Inventoried .github/workflows/ for cosign|sigstore|slsa|attest.
2. Read every workflow that produces an artefact (release blob, wheel, container image).
3. Cross-checked docs/development/release.md, ADR-0166 (MCP release channel), ADR-0698 (production Dockerfile), CLAUDE.md §11.
4. Checked .git/hooks/, scripts/git-hooks/, .pre-commit-config.yaml, Makefile for any DCO / sign-off enforcement.

Workflow inventory¶

Gaps¶

G1. Git tag signing — DEFERRED¶

release-please-action creates unsigned annotated tags via the GitHub Actions bot identity. Signing tags would require:

• A GitHub App with attached GPG (no first-class release-please-action support today), OR
• A post-tag workflow that re-signs the existing tag (rewrites tag refs, breaks consumers who pin by tag SHA).

The cosign-signed blobs + GitHub-native attestation cover artefact-level provenance. The branch-protection + linear-history invariants in ADR-0037 ensure the tagged commit itself can only have been added via reviewed PR. Tag-level signing adds little ROI on top.

Decision: deferred. Revisit if googleapis/release-please-action ships GPG-via-GitHub-App support.

G2. No GitHub-native build attestation on container images — FIXED THIS PR¶

cosign attest already publishes the SBOM as an in-toto predicate. actions/attest-build-provenance@v3 publishes the GitHub-native attestation that gh attestation verify reads. Both formats are useful to different consumer toolchains; neither replaces the other.

Action: add actions/attest-build-provenance@v4.1.0 to all five build jobs in docker-publish-production.yml. Native attestation runs in parallel with cosign sign so wall-clock impact is negligible.

G3. Smoke test runs unverified container image — FIXED THIS PR¶

The smoke-test job pulls the freshly-built CPU image by digest and runs --version. The cosign signature step ran in the upstream build-cpu job — but the smoke test never checks it. A compromised CI token could push an image, skip the signing step, and still pass the smoke test.

Action: install cosign in the smoke-test job; run cosign verify --certificate-identity-regexp … against the image digest before pulling. Failure fails the workflow (signature missing or mismatched identity).

G4. Release docs lack a copy-pasteable verification recipe — FIXED THIS PR¶

docs/development/release.md advertises verification with cosign verify-blob --certificate-identity-regexp … but elides the regex and never shows a container example. Consumers wanting to verify a download have to reverse-engineer the identity from CI logs.

Action: expand the "Signing" section with the literal identity regex, a release-blob verification example, a container-image verification example, and a gh attestation verify example for the new GitHub-native attestation.

G5. DCO sign-off enforcement — OUT OF SCOPE¶

The brief mentioned "DCO compliance per CLAUDE.md §12 r6". §12 r6 reads:

Every commit message is Conventional Commits (type(scope): subject). Enforced by the commit-msg git hook.

No DCO sign-off rule exists in CLAUDE.md, AGENTS.md, CONTRIBUTING.md, .pre-commit-config.yaml, or any git hook. Adding DCO would be a new policy, not an audit gap.

Decision: out of scope. Would require a separate ADR proposing the policy.

G6. Helm chart signing — DEFERRED¶

The chart in deploy/helm/vmafx/ has no publishing workflow. Signing follows once a chart-release channel is decided (chart-releaser-action + ghcr.io/OCI chart push, or helm-publish to a GitHub Pages site, etc.).

Decision: deferred. Tracked separately.

G7. Go binary standalone releases — DEFERRED¶

cmd/vmafx-* Go binaries ship inside the signed container images. No standalone tarball/zip release surface today. If one is added later (e.g. via goreleaser), it should sit alongside the existing supply-chain.yml sign-blob loop.

Decision: deferred. Add when standalone Go binary releases land.

G8. GPU SBOM is best-effort — ACCEPTED AS-IS¶

The CUDA + server container builds run their SBOM step with continue-on-error: true, meaning syft failures don't fail the workflow. This is a deliberate trade-off because the GPU base images sometimes break syft's package inspection (CUDA's package manifest layout is non-standard).

Decision: accepted as-is. The new GitHub-native attestation (actions/attest-build-provenance) does NOT have this fallback and will fail closed, so the loss of the cosign-attested SBOM is partly compensated.

Consumer verification — quick reference¶

# Release blob, verbatim from the bundle layout.
cosign verify-blob --bundle vmaf.bundle vmaf \
  --certificate-identity-regexp 'https://github.com/VMAFx/vmafx/.github/workflows/supply-chain.yml@.*' \
  --certificate-oidc-issuer https://token.actions.githubusercontent.com

# Container image, cosign route. Replace DIGEST with the actual sha256 digest.
cosign verify ghcr.io/vmafx/vmafx@sha256:DIGEST \
  --certificate-identity-regexp 'https://github.com/VMAFx/vmafx/.github/workflows/docker-publish-production.yml@.*' \
  --certificate-oidc-issuer https://token.actions.githubusercontent.com

# Container image, GitHub-native attestation, NEW.
gh attestation verify oci://ghcr.io/vmafx/vmafx@sha256:DIGEST --repo VMAFx/vmafx

The release docs carry the same recipe, kept in sync via the same PR.

References¶

• CLAUDE.md §11 — release policy advertising Sigstore.
• ADR-0166 — MCP server release channel.
• ADR-0698 — production Dockerfile.
• ADR-0037 — master branch protection (linear history + no force-push).
• Sigstore: cosign.
• SLSA L3 generator.
• actions/attest-build-provenance.
• PEP 740 — index support for digital attestations.