---
name: "sigstore-cosign-image-signing"
description: "Sign and verify container images and OCI artifacts using Sigstore Cosign."
metadata:
  author: "Sigstore"
  version: "1.0.0"
  runtime_compatibility:
    - "codex"
    - "claude-code"
    - "cursor"
    - "windsurf"
triggers:
  - "sign container image cosign"
  - "verify image signature cosign"
  - "attach sbom attestation cosign"
permissions:
  fs_read: true
  fs_write: "workspace"
  shell_exec: false
  net_access: false
---

# Cosign Container Image Keyless Signing

Implements container supply chain integrity with Cosign: keyless signing via OIDC (Fulcio/Rekor), public key verification, SBOM attestation, and Kubernetes Kyverno admission policies.

## When to Invoke
Invoke this skill whenever you need to:
- sign container image cosign
- verify image signature cosign
- attach sbom attestation cosign

## Execution SOP & Rules
1. Execute cosign sign <image_uri> using GitHub Actions OIDC token permissions (id-token: write).
2. Verify signatures with cosign verify --certificate-identity=<identity> --certificate-oidc-issuer=<issuer> <image_uri>.
3. Attach signed attestations using cosign attest --predicate sbom.json --type cyclonedx <image_uri>.
4. Reject unsigned or untrusted container images at the cluster admission boundary.

## Verification Checklist
- Run local linting, syntax validation, and type-checks before reporting completion.
- Ensure all created or modified files adhere to workspace boundaries and avoid uncommitted secrets.
- Verify tests or build commands exit cleanly with return code 0.