Overview
Cosign, the Sigstore tool for signing, verifying, and attaching metadata to container images and other OCI artifacts. Helps developers implement supply chain security by signing images in CI/CD, verifying signatures before deployment, and attaching SBOMs and vulnerability scan results as attestations.
Instructions
Sign and Verify Images
bash
# Install
brew install cosign
# Generate a keypair
cosign generate-key-pair
# Creates cosign.key (private) and cosign.pub (public)
# Sign an image after building
docker build -t myregistry.com/myapp:v1.2.3 .
docker push myregistry.com/myapp:v1.2.3
cosign sign --key cosign.key myregistry.com/myapp:v1.2.3
# Verify before deploying
cosign verify --key cosign.pub myregistry.com/myapp:v1.2.3
# Keyless signing with Sigstore (no key management!)
# Uses OIDC identity (GitHub Actions, Google, etc.)
cosign sign myregistry.com/myapp:v1.2.3
# Opens browser for OIDC login, signs with ephemeral key,
# records signature in Rekor transparency log
# Keyless verification
cosign verify \
--certificate-identity=user@example.com \
--certificate-oidc-issuer=https://accounts.google.com \
myregistry.com/myapp:v1.2.3
CI/CD Integration
yaml
# .github/workflows/build.yml — Sign images in CI
jobs:
build-and-sign:
runs-on: ubuntu-latest
permissions:
contents: read
id-token: write # Required for keyless signing
packages: write
steps:
- uses: actions/checkout@v4
- name: Build and push
run: |
docker build -t ghcr.io/${{ github.repository }}:${{ github.sha }} .
docker push ghcr.io/${{ github.repository }}:${{ github.sha }}
- name: Install Cosign
uses: sigstore/cosign-installer@v3
- name: Sign image (keyless)
run: |
cosign sign \
--yes \
ghcr.io/${{ github.repository }}:${{ github.sha }}
env:
COSIGN_EXPERIMENTAL: 1
- name: Attach SBOM
run: |
# Generate SBOM with syft
syft ghcr.io/${{ github.repository }}:${{ github.sha }} -o spdx-json > sbom.spdx.json
# Attach SBOM as an attestation
cosign attest \
--yes \
--predicate sbom.spdx.json \
--type spdxjson \
ghcr.io/${{ github.repository }}:${{ github.sha }}
- name: Attach vulnerability scan
run: |
# Scan with grype
grype ghcr.io/${{ github.repository }}:${{ github.sha }} -o json > vuln-scan.json
# Attach scan results
cosign attest \
--yes \
--predicate vuln-scan.json \
--type vuln \
ghcr.io/${{ github.repository }}:${{ github.sha }}
Verify in Kubernetes (Kyverno Policy)
yaml
# Kubernetes policy: only deploy signed images
apiVersion: kyverno.io/v1
kind: ClusterPolicy
metadata:
name: verify-image-signatures
spec:
validationFailureAction: Enforce
background: false
rules:
- name: verify-cosign-signature
match:
any:
- resources:
kinds: ["Pod"]
verifyImages:
- imageReferences:
- "ghcr.io/myorg/*"
attestors:
- entries:
- keyless:
subject: "https://github.com/myorg/*"
issuer: "https://token.actions.githubusercontent.com"
rekor:
url: "https://rekor.sigstore.dev"
Attestations
bash
# Attest build provenance (SLSA)
cosign attest \
--yes \
--predicate provenance.json \
--type slsaprovenance \
myregistry.com/myapp:v1.2.3
# Verify attestation
cosign verify-attestation \
--type spdxjson \
--certificate-identity-regexp=".*@myorg.com" \
--certificate-oidc-issuer=https://accounts.google.com \
myregistry.com/myapp:v1.2.3
# Download and inspect attached SBOM
cosign download attestation myregistry.com/myapp:v1.2.3 | jq -r '.payload' | base64 -d | jq .
Installation
bash
brew install cosign
# Or: go install github.com/sigstore/cosign/v2/cmd/cosign@latest
# Or: Download from https://github.com/sigstore/cosign/releases
Examples
Example 1: Setting up Cosign for a microservices project
User request:
I have a Node.js API and a React frontend running in Docker. Set up Cosign for monitoring/deployment.
The agent creates the necessary configuration files based on patterns like # Install, sets up the integration with the existing Docker setup, configures appropriate defaults for a Node.js + React stack, and provides verification commands to confirm everything is working.
Example 2: Troubleshooting ci/cd integration issues
User request:
Cosign is showing errors in our ci/cd integration. Here are the logs: [error output]
The agent analyzes the error output, identifies the root cause by cross-referencing with common Cosign issues, applies the fix (updating configuration, adjusting resource limits, or correcting syntax), and verifies the resolution with appropriate health checks.
Guidelines
- Keyless signing in CI — Use Sigstore's keyless signing in GitHub Actions; no key management, signatures tied to OIDC identity
- Sign every image — Sign in CI/CD, verify before deployment; no unsigned image should reach production
- Attach SBOMs — Generate and attach SBOM with every build; required for compliance (Executive Order 14028) and vulnerability tracking
- Verify in admission control — Use Kyverno or OPA to enforce signature verification at the Kubernetes admission level
- Rekor transparency log — Keyless signatures are recorded in Rekor; provides an immutable audit trail of who signed what and when
- Attestations for provenance — Attach SLSA provenance attestations; prove where and how the image was built
- Pin by digest — Reference images by SHA256 digest, not tag; tags can be overwritten, digests are immutable
- Vulnerability scan attestations — Attach scan results as attestations; verify no critical CVEs before deployment