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Supply chain security extends beyond internal threat modeling—it ensures every external dependency (libraries, container images, third-party APIs) is verified, tamper-free, and compliant. In this guide, we’ll cover the four core areas of supply chain security, show you practical commands, and point to best-in-class tools and standards.

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Core Areas of Supply Chain Security

Core AreaDescriptionTool / Standard
ArtifactsBuild outputs: images, binariesSigstore Cosign
MetadataSoftware Bill of Materials (SBOM)SPDX
AttestationsSigned provenance statementsin-toto
PoliciesAutomated compliance enforcementSigstore Policy Controller

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1. Artifacts: Signing and Verification

Artifacts—your container images, binaries, and libraries—must be signed to prove integrity and origin.
Sigstore’s Cosign offers a simple, keyless workflow for signing container images.
To sign an image: 
cosign sign $IMAGE
Sample output: 
Generating ephemeral keys...
Retrieving signed certificate...
By typing 'y', you attest that you have permission to grant signing.
Are you sure you would like to continue? [y/N] y
Successfully verified SCT...
tlog entry created with index: 12086900
Pushing signature to: $IMAGE
To verify a binary or image: 
cosign verify-blob "$BINARY" \
  --signature "$BINARY.sig" \
  --certificate "$BINARY.cert" \
  --certificate-identity [email protected] \
  --certificate-oidc-issuer https://accounts.google.com

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2. Metadata: Generating and Validating SBOMs

A Software Bill of Materials (SBOM) is an “ingredients list” for your application, detailing file checksums, licenses, and origins.
An SBOM (Software Bill of Materials) is often authored in SPDX format. It tracks every component and its license.
Example SPDX excerpt: 
FileName: bin/linux/amd64/kube-controller-manager
SPDXID: SPDXRef-File-kube-controller-manager-v1.31.2
FileChecksum: SHA1: c5e8da214abd18e96aabe7d1bab6addf76455
FileChecksum: SHA256: b16b6becee2bc76af97384ca611d8e972aa7ed213ea75255
LicenseConcluded: Apache-2.0
Retrieve and verify the Kubernetes SBOM: 
# 1. Get the latest stable release
VERSION=$(curl -Ls https://dl.k8s.io/release/stable.txt)

# 2. Download SBOM and checksum
curl -Ls "https://sbom.k8s.io/$VERSION/release" -o "$VERSION.spdx"
echo "$(curl -Ls "https://sbom.k8s.io/$VERSION/release.sha512")  $VERSION.spdx" | sha512sum --check

# 3. Fetch Sigstore signature and certificate
curl -Ls "https://sbom.k8s.io/$VERSION/release.sig"  -o "$VERSION.spdx.sig"
curl -Ls "https://sbom.k8s.io/$VERSION/release.cert" -o "$VERSION.spdx.cert"

# 4. Verify SBOM integrity
cosign verify-blob \
  --certificate "$VERSION.spdx.cert" \
  --signature   "$VERSION.spdx.sig" \
  --certificate-identity [email protected] \
  --certificate-oidc-issuer  https://accounts.google.com \
  "$VERSION.spdx"

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3. Attestations: Building a Chain of Trust

Attestations are cryptographic statements that vouch for metadata such as provenance, SBOM authenticity, or vulnerability scans. Sign an SBOM attestation: 
cosign sign --key <PRIVATE_KEY> sbom.k8s.io/v1.27.4/release.spdx > sbom.attestation
Verify the attestation: 
cosign verify-attestation \
  --key <PUBLIC_KEY> \
  --certificate-identity [email protected] \
  --certificate-oidc-issuer https://accounts.google.com \
  sbom.k8s.io/v1.27.4/release.spdx
in-toto defines and verifies attestations across your entire pipeline. Sample step definition:
- name: build
  expected_command:
    - "make"
  pubkeys: ["developer"]
  expected_materials:
    - "MATCH repo/* WITH REPO"
  expected_products:
    - "CREATE binary"

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4. Policies: Automated Compliance Enforcement

Policies block deployments of unsigned or non-compliant artifacts. A ClusterImagePolicy example: 
apiVersion: policy.sigstore.dev/v1beta1
kind: ClusterImagePolicy
metadata:
  name: secure-image-policy
spec:
  images:
    - glob: "gcr.io/my-organization/*"
  authorities:
    - key:
        data: |
          -----BEGIN PUBLIC KEY-----
          YOUR_PUBLIC_KEY_HERE
          -----END PUBLIC KEY-----
  attestations:
    - name: sbom-check
      predicateType: https://in-toto.io/Statement/v0.1
    - name: vulnerability-check
      predicateType: https://slsa.dev/provenance/v0.2
  policy:
    validate:
      all:
        - name: sbom-validation
          match:
            attestation-name: sbom-check
        - name: vulnerability-validation
          match:
            attestation-name: vulnerability-check
        - name: signing-validation
          match:
            signed: true
Enforce policies at admission time with Sigstore’s Policy Controller.
The image outlines key components of supply chain security, including artifact signing, metadata detailing, attestation signing, and policy enforcement. It emphasizes the use of Cosign for signing binaries and container images, and the role of SBOM in identifying risks and ensuring trustworthiness.

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References

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