Anthropic-Cybersecurity-Skills/skills/implementing-image-provenance-verification-with-cosign/SKILL.md

name, description, domain, subdomain, tags, version, author, license, nist_csf, mitre_attack

name	description	domain	subdomain	tags	version	author	license	nist_csf	mitre_attack
implementing-image-provenance-verification-with-cosign	Signs and verifies container image provenance with Sigstore Cosign, covering key-based and keyless OIDC signing (Fulcio, Rekor transparency log), SLSA attestations, and enforcing signature verification through Kubernetes admission control. Use when signing images for supply chain security, setting up keyless OIDC signing, attaching attestations, or enforcing a verified-images-only policy at admission. Keywords: Cosign, Sigstore, Fulcio, Rekor, keyless, attestation, cosign verify, admission policy. Do not use for in-toto layout-based pipeline attestation - use implementing-supply-chain-security-with-in-toto.	cybersecurity	container-security	cosign sigstore image-signing supply-chain provenance keyless slsa	1.0	mahipal	Apache-2.0	PR.PS-01 PR.IR-01 ID.AM-08 DE.CM-01	T1610 T1611 T1609 T1525 T1195

Implementing Image Provenance Verification with Cosign

Overview

Cosign is a Sigstore tool for signing, verifying, and attaching metadata to container images and OCI artifacts. It supports both key-based and keyless (OIDC) signing, integrates with Fulcio (certificate authority) and Rekor (transparency log), and enables supply chain security for container images.

When to Use

• When deploying or configuring implementing image provenance verification with cosign capabilities in your environment
• When establishing security controls aligned to compliance requirements
• When building or improving security architecture for this domain
• When conducting security assessments that require this implementation

Prerequisites

• Cosign CLI installed
• Docker or Podman for building images
• OCI-compliant container registry (Docker Hub, GHCR, GCR, ECR)
• OIDC provider account (GitHub, Google, Microsoft) for keyless signing

Installing Cosign

# Install via Go
go install github.com/sigstore/cosign/v2/cmd/cosign@latest

# Install via Homebrew
brew install cosign

# Install via script
curl -O -L "https://github.com/sigstore/cosign/releases/latest/download/cosign-linux-amd64"
sudo mv cosign-linux-amd64 /usr/local/bin/cosign
sudo chmod +x /usr/local/bin/cosign

# Verify installation
cosign version

Key-Based Signing

Generate Key Pair

# Generate cosign key pair (creates cosign.key and cosign.pub)
cosign generate-key-pair

# Generate key pair stored in KMS
cosign generate-key-pair --kms awskms:///alias/cosign-key
cosign generate-key-pair --kms gcpkms://projects/PROJECT/locations/LOCATION/keyRings/KEYRING/cryptoKeys/KEY
cosign generate-key-pair --kms hashivault://transit/keys/cosign

Sign Image with Key

# Sign an image
cosign sign --key cosign.key ghcr.io/myorg/myapp:v1.0.0

# Sign with annotations
cosign sign --key cosign.key \
  -a "build-id=12345" \
  -a "git-sha=$(git rev-parse HEAD)" \
  ghcr.io/myorg/myapp:v1.0.0

Verify Image with Key

# Verify signature
cosign verify --key cosign.pub ghcr.io/myorg/myapp:v1.0.0

# Verify with annotation check
cosign verify --key cosign.pub \
  -a "build-id=12345" \
  ghcr.io/myorg/myapp:v1.0.0

Keyless Signing (OIDC)

Sign with Keyless (Interactive)

# Keyless sign - opens browser for OIDC auth
cosign sign ghcr.io/myorg/myapp:v1.0.0

# The signature, certificate, and Rekor entry are created automatically

Sign with Keyless (CI/CD - Non-Interactive)

# GitHub Actions (uses OIDC token automatically)
cosign sign ghcr.io/myorg/myapp:v1.0.0 \
  --yes

# With explicit identity token
cosign sign ghcr.io/myorg/myapp:v1.0.0 \
  --identity-token=$(cat /var/run/sigstore/cosign/oidc-token) \
  --yes

Verify Keyless Signature

# Verify by email identity
cosign verify ghcr.io/myorg/myapp:v1.0.0 \
  --certificate-identity=builder@example.com \
  --certificate-oidc-issuer=https://accounts.google.com

# Verify by GitHub Actions workflow
cosign verify ghcr.io/myorg/myapp:v1.0.0 \
  --certificate-identity=https://github.com/myorg/myrepo/.github/workflows/build.yml@refs/heads/main \
  --certificate-oidc-issuer=https://token.actions.githubusercontent.com

# Verify with regex matching
cosign verify ghcr.io/myorg/myapp:v1.0.0 \
  --certificate-identity-regexp=".*@example.com" \
  --certificate-oidc-issuer=https://accounts.google.com

Attestations (SLSA Provenance)

Attach SBOM Attestation

# Generate SBOM
syft ghcr.io/myorg/myapp:v1.0.0 -o cyclonedx-json > sbom.cdx.json

# Attach SBOM as attestation
cosign attest --key cosign.key \
  --type cyclonedx \
  --predicate sbom.cdx.json \
  ghcr.io/myorg/myapp:v1.0.0

# Verify attestation
cosign verify-attestation --key cosign.pub \
  --type cyclonedx \
  ghcr.io/myorg/myapp:v1.0.0

Attach Vulnerability Scan Attestation

# Run scan and save results
grype ghcr.io/myorg/myapp:v1.0.0 -o json > vuln-scan.json

# Attach scan results as attestation
cosign attest --key cosign.key \
  --type vuln \
  --predicate vuln-scan.json \
  ghcr.io/myorg/myapp:v1.0.0

SLSA Provenance Attestation

# Attach SLSA provenance
cosign attest --key cosign.key \
  --type slsaprovenance \
  --predicate provenance.json \
  ghcr.io/myorg/myapp:v1.0.0

# Verify SLSA provenance
cosign verify-attestation --key cosign.pub \
  --type slsaprovenance \
  ghcr.io/myorg/myapp:v1.0.0

CI/CD Integration

GitHub Actions

name: Sign and Publish
on:
  push:
    tags: ['v*']

permissions:
  contents: read
  packages: write
  id-token: write  # Required for keyless signing

jobs:
  build-sign:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v4

      - uses: sigstore/cosign-installer@v3

      - name: Login to GHCR
        uses: docker/login-action@v3
        with:
          registry: ghcr.io
          username: ${{ github.actor }}
          password: ${{ secrets.GITHUB_TOKEN }}

      - name: Build and push
        id: build
        uses: docker/build-push-action@v5
        with:
          push: true
          tags: ghcr.io/${{ github.repository }}:${{ github.ref_name }}

      - name: Sign image (keyless)
        run: |
          cosign sign --yes \
            ghcr.io/${{ github.repository }}@${{ steps.build.outputs.digest }}

      - name: Generate and attach SBOM
        run: |
          syft ghcr.io/${{ github.repository }}@${{ steps.build.outputs.digest }} -o cyclonedx-json > sbom.json
          cosign attest --yes \
            --type cyclonedx \
            --predicate sbom.json \
            ghcr.io/${{ github.repository }}@${{ steps.build.outputs.digest }}

Kubernetes Admission Enforcement

Policy Controller (Sigstore)

# Install policy-controller
helm repo add sigstore https://sigstore.github.io/helm-charts
helm install policy-controller sigstore/policy-controller \
  --namespace cosign-system --create-namespace

# Enforce signed images in namespace
apiVersion: policy.sigstore.dev/v1beta1
kind: ClusterImagePolicy
metadata:
  name: require-signed-images
spec:
  images:
    - glob: "ghcr.io/myorg/**"
  authorities:
    - keyless:
        url: https://fulcio.sigstore.dev
        identities:
          - issuer: https://token.actions.githubusercontent.com
            subjectRegExp: "https://github.com/myorg/.*"
      ctlog:
        url: https://rekor.sigstore.dev

Kyverno Integration

apiVersion: kyverno.io/v1
kind: ClusterPolicy
metadata:
  name: verify-image-signature
spec:
  validationFailureAction: Enforce
  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

Transparency Log (Rekor)

# Search Rekor for image signatures
rekor-cli search --email builder@example.com

# Get specific entry
rekor-cli get --uuid <entry-uuid>

# Verify entry inclusion
cosign verify ghcr.io/myorg/myapp:v1.0.0 \
  --certificate-identity=builder@example.com \
  --certificate-oidc-issuer=https://accounts.google.com

Best Practices

1. Use keyless signing in CI/CD for automated pipelines
2. Sign by digest not by tag for immutable references
3. Attach SBOM attestations alongside signatures
4. Enforce signatures at admission with policy-controller or Kyverno
5. Use OIDC identity verification instead of just key verification
6. Store keys in KMS (AWS KMS, GCP KMS, HashiCorp Vault) for key-based signing
7. Verify the full chain: signature + certificate + Rekor inclusion
8. Include build metadata as annotations on signatures