The promise of a tamper-evident, auditable development lifecycle starts with Cryptographically-Verifiable Code Reviews: using signatures and attestations to ensure every commit, review, and CI-built artifact has provable provenance. This article explains why a zero-trust review chain matters, which standards and tools to adopt, and a pragmatic step-by-step implementation you can apply in GitHub, GitLab, or similar platforms.
Why a Zero-Trust Review Chain Matters
Traditional code review processes assume reviewers and CI behave honestly; they rarely guard against insider threats or supply-chain compromises. A zero-trust review chain replaces blind trust with cryptographic proof—every action that changes code or produces an artifact is signed, logged, and verifiable. That makes malicious tampering detectable, simplifies audits, and raises the bar for attackers.
Core Principles
- Sign what matters: Authors sign commits, integrators sign merges, and CI signs build outputs and provenance metadata.
- Automate attestations: Reviewer approvals become signed attestations (not just UI flags) recorded in a provenance store.
- Enforce in CI: Pipelines validate signatures and attestations and refuse to produce or promote artifacts that lack provable provenance.
- Store transparently: Use append-only transparency logs (e.g., Rekor) or artifact registries so signatures and attestations are tamper-evident and auditable.
Standards and Tools to Know
- Git commit signing: git commit -S (GPG/OpenPGP) or SSH-signed commits (platform-dependent)
- Sigstore (Fulcio + Rekor + Cosign): for keyless signing and public transparency logs
- in-toto / SLSA: standards for supply-chain provenance and attestations
- DSSE (Binary Authorization format): standardized envelope for attestations
- Policy engines: Open Policy Agent (OPA) / conftest for CI enforcement
Step-by-step Implementation
1. Begin with commit signing
Require developers to sign commits. GPG is widely supported and Git has native support: git commit -S -m "feat: add X". On hosting platforms enable “Require signed commits” so unsigned authoring is blocked at push or merge time. Encourage hardware-backed keys (YubiKey) or short-lived keyless signing via sigstore for reduced key-management risk.
2. Turn reviews into cryptographic attestations
Rather than relying only on a UI approval, have the reviewer’s approval produce a signed attestation that references the PR/commit SHA and the reviewer identity. Two practical approaches:
- CI-based attestation: When a reviewer clicks “approve,” trigger a GitHub Action or webhook that runs with the reviewer’s OIDC/short-lived credentials to create a signed attestation (DSSE/in-toto) and publish it to a provenance store.
- Developer-signed attestations: Reviewers sign a JSON attestation locally or via a small web app and the CI consumes and validates it before merge.
3. Record attestations in a transparency log
Publish attestations to an append-only log (Rekor) or attach them to the repository as attachable artifacts in your artifact registry. This makes attestations discoverable and tamper-evident for auditors.
4. Enforce verification in CI
Implement mandatory verification steps early in pipelines so builds, releases, and promotions are conditional on valid provenance:
- Verify commit signatures:
git verify-commit <sha>or use gpg to check signatures. - Verify reviewer attestations: check DSSE/in-toto envelope signature and referenced commit SHA.
- Reject if any signature is missing, invalid, or the attestation log is absent.
Example CI stage pseudo-steps:
checkout
verify-commit-signature.sh $COMMIT_SHA
verify-attestation.sh $PR_NUMBER
if verification fails: exit 1
build-and-sign-artifact.sh
publish-provenance.sh (to Rekor or registry)5. Sign CI outputs and produce provenance
When CI builds artifacts, sign them and generate SLSA/in-toto attestations that include the build recipe, inputs, environment, and responsible CI job. Use cosign or sigstore to produce signatures and push attestations to Rekor or an artifact registry so downstream consumers can verify origin and integrity.
Policy and Enforcement
Use a policy-as-code engine (OPA or conftest) in the pipeline to codify rules such as “every merge must have an author signature and at least two reviewer attestations” or “build artifacts must include a SLSA provenance entry.” Policies run as part of CI and produce human-readable failures and audit logs.
Operational Best Practices
- Use short-lived credentials and OIDC-based signing (sigstore Fulcio) to reduce key exposure risk.
- Mandate hardware-backed keys for privileged integrators.
- Rotate keys and automate revocation checks against the transparency log.
- Educate reviewers: attestations are legal-grade evidence of approval—make expectations clear.
- Monitor logs and alert on signature anomalies or missing attestations.
Putting it all together: a tamper-evident chain
A complete zero-trust review chain looks like this: developer signs commit → reviewer(s) produce signed attestations referencing commits → CI verifies signatures and attestations before building → CI signs build outputs and writes provenance to Rekor → policy engine enforces rules at each checkpoint. Each step adds cryptographic links so an auditor can trace any artifact back to its signed source and see whether any proof is missing or altered.
Common Pitfalls and How to Avoid Them
- Avoid brittle manual steps: automate attestation generation and verification to prevent human error.
- Don’t store private keys in CI: use OIDC/keyless signing or hardware tokens.
- Beware of UI-only approvals: surface approvals as cryptographic attestations to make them verifiable outside the hosting UI.
Adopting cryptographically-verifiable code reviews and a zero-trust review chain takes planning but yields a measurable reduction in risk: auditable history, tamper-evidence, and provable supply-chain integrity across commits, reviews, and builds.
Conclusion: Start small—require signed commits and add CI attestation checks—then expand to reviewer attestations and full provenance logging to lock in a tamper-evident, auditable pipeline.
Ready to harden your code review pipeline? Start by enabling signed commits and adding a CI step to verify signatures on your next sprint.
