---
name: sigstore-slsa-provenance
description: "Keyless cosign signing, SLSA Build L3 provenance, and in-toto attestation verification at admission for CRA compliance. Production supply-chain security with Sigstore: keyless cosign signing via Fulcio short-lived certs and Rekor transparency-log entries (OIDC ambient creds in GitHub Actions), cosign attest for SPDX/CycloneDX SBOMs and in-toto attestations. Generates SLSA Build L3 provenance with the slsa-github-generator and verifies it with slsa-verifier. Enforces signature and provenance policy at admission via cosign verify-attestation, Kyverno, or Sigstore Policy Controller, mapped to EU CRA vulnerability-reporting obligations."
license: MIT
---

# Sigstore SLSA Provenance

Keyless signing, attestation, and verification for your container and artifact supply chain, built on Sigstore and the SLSA framework. This skill wires `cosign` keyless signing, SLSA Build L3 provenance generation, and in-toto attestation checks into your CI and admission flow so every image you ship carries a verifiable, transparency-logged chain of custody. It's aimed at platform and DevSecOps engineers who need to satisfy the EU Cyber Resilience Act (CRA), executive-order SBOM mandates, or internal "no unsigned images in prod" policies.

## When to use it

- You publish OCI images or release artifacts from GitHub Actions and want signatures without managing long-lived private keys.
- An auditor or the CRA requires you to demonstrate build provenance and SBOM attestations per release.
- You want a Kubernetes admission gate that rejects images lacking valid Sigstore signatures or SLSA provenance.
- You're migrating off GPG-based release signing toward keyless, OIDC-driven workflows.

## How it works

Signing is keyless: `cosign sign` requests a short-lived (~10 min) code-signing cert from Fulcio, bound to the OIDC identity of the workflow (in GitHub Actions, the ambient `id-token: write` token). The signature and cert are recorded in the Rekor transparency log, so no key material is stored. For supply-chain metadata, `cosign attest --type spdxjson` (or `cyclonedx`) wraps an SBOM as a DSSE-enveloped in-toto attestation.

SLSA Build L3 provenance comes from the `slsa-github-generator` reusable workflows (e.g. `generator_container_slsa3.yml`), which run in an isolated builder and emit a signed `provenance` predicate. Verification uses `cosign verify` / `cosign verify-attestation` with `--certificate-identity-regexp` and `--certificate-oidc-issuer`, or `slsa-verifier verify-image` to assert builder identity and source repo.

## Usage

```bash
curl -fsSL --create-dirs -o ~/.claude/skills/sigstore-slsa-provenance/SKILL.md https://claudeskil.com/skills/sigstore-slsa-provenance/SKILL.md

# In CI (GitHub Actions job needs: id-token: write, packages: write)
cosign sign --yes ghcr.io/acme/api@${DIGEST}
cosign attest --yes --type spdxjson \
  --predicate sbom.spdx.json ghcr.io/acme/api@${DIGEST}

# At admission / pre-deploy
cosign verify ghcr.io/acme/api@${DIGEST} \
  --certificate-identity-regexp '^https://github.com/acme/.+' \
  --certificate-oidc-issuer https://token.actions.githubusercontent.com

slsa-verifier verify-image ghcr.io/acme/api@${DIGEST} \
  --source-uri github.com/acme/api --builder-id 'https://github.com/slsa-framework/slsa-github-generator/.github/workflows/generator_container_slsa3.yml@refs/tags/v2.0.0'
```

## Tips & gotchas

- Always sign and verify by **digest**, never by tag — tags are mutable and a verified tag tells you nothing about what's running.
- `verify` without `--certificate-identity*` flags will pass any valid Fulcio cert; pin the identity regexp and issuer or the check is cosmetic.
- For air-gapped or self-hosted Sigstore, point cosign at your own Fulcio/Rekor via `--fulcio-url`/`--rekor-url` and a TUF root; the public Good instance won't be reachable.
- SLSA L3 requires the isolated reusable workflow — provenance generated in your own job tops out at L2.

## FAQ

**Do I need to store a private key?** No. Keyless signing derives ephemeral keys from your OIDC identity; only the Rekor log entry persists.

**Can policy controllers consume this?** Yes — Kyverno and the Sigstore Policy Controller both verify cosign signatures and in-toto attestations at admission.

**SPDX or CycloneDX?** Either; `cosign attest --type` supports both, and verifiers extract the predicate by type.