Cosign — Sign and Verify Container Images and Artifacts

What it is

Cosign is a tool from the Sigstore project for cryptographically signing container images and other software artifacts. It supports both traditional key-pair signing and keyless signing via OIDC identity providers (GitHub Actions, Google, Microsoft). Signatures can be verified in CI/CD pipelines and Kubernetes admission controllers to ensure only trusted images run in production.

Cosign targets DevOps engineers, security teams, and platform operators who need to establish software supply chain trust without managing long-lived signing keys.

How it saves time or tokens

Keyless signing eliminates key management overhead. Instead of generating, rotating, and securing private keys, Cosign uses your existing OIDC identity (e.g., GitHub Actions workflow identity) to produce short-lived certificates. Verification is automated in CI/CD, so no manual signature checks are needed.

How to use

1. Install Cosign: go install github.com/sigstore/cosign/v2/cmd/cosign@latest or download the binary.
2. Sign an image with keyless signing (requires OIDC identity).
3. Verify the signature before deploying.

Example

# Keyless signing (opens browser for OIDC auth)
cosign sign myregistry.io/myapp:v1.0.0

# Verify the signature
cosign verify myregistry.io/myapp:v1.0.0 \
  --certificate-identity=user@example.com \
  --certificate-oidc-issuer=https://accounts.google.com

# Key-pair signing (traditional)
cosign generate-key-pair
cosign sign --key cosign.key myregistry.io/myapp:v1.0.0
cosign verify --key cosign.pub myregistry.io/myapp:v1.0.0

Related on TokRepo

• AI tools for security -- Security tooling and supply chain protection.
• AI tools for DevOps -- CI/CD and infrastructure automation tools.

Common pitfalls

• Keyless signing requires network access to Sigstore's public infrastructure (Fulcio CA, Rekor transparency log). Air-gapped environments need key-pair signing instead.
• Verification requires specifying the expected certificate identity and OIDC issuer. Omitting these checks weakens the trust chain.
• Cosign stores signatures as OCI artifacts alongside the image in the registry. Some registries do not support OCI artifacts; check compatibility.
• Kubernetes policy enforcement requires an admission controller like Kyverno or Connaisseur to reject unsigned images at deploy time.
• Signing multi-architecture images requires signing the manifest list, not individual platform images.