Implementing Supply Chain Security with in-toto
Overview
in-toto is a CNCF graduated project that ensures the integrity of software supply chains from initiation to end-user installation. It creates a verifiable record of the entire software development lifecycle by generating cryptographically signed attestations (called "link metadata") at each step, proving what happened, who performed it, and what artifacts were produced. For container environments, in-toto verifies that images deployed to Kubernetes followed approved build processes and have not been tampered with.
When to Use
- When deploying or configuring implementing supply chain security with in toto 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
- Python 3.8+ or Go runtime for in-toto client libraries
- GPG or Ed25519 keys for signing attestations
- Container build pipeline (Docker, Buildah, or Kaniko)
- Container registry (Docker Hub, ECR, GCR, or Harbor)
- Kubernetes cluster for deployment verification
Core Concepts
Supply Chain Layout
The layout is the central policy document that defines:
- Steps: Ordered operations in the supply chain (clone, build, test, package, push)
- Functionaries: Authorized entities (people or CI systems) that perform each step
- Inspections: Client-side verification checks performed at verification time
- Expected artifacts: Input/output relationships between steps
from in_toto.models.layout import Layout, Step, Inspection
from securesystemslib.interface import import_ed25519_privatekey_from_file
# Create the supply chain layout
layout = Layout()
layout.set_relative_expiration(months=3)
# Define the code clone step
step_clone = Step(name="clone")
step_clone.expected_materials = []
step_clone.expected_products = [["CREATE", "src/*"]]
step_clone.pubkeys = [clone_functionary_keyid]
step_clone.expected_command = ["git", "clone"]
step_clone.threshold = 1
# Define the build step
step_build = Step(name="build")
step_build.expected_materials = [["MATCH", "src/*", "WITH", "PRODUCTS", "FROM", "clone"]]
step_build.expected_products = [["CREATE", "image.tar"]]
step_build.pubkeys = [build_functionary_keyid]
step_build.expected_command = ["docker", "build"]
step_build.threshold = 1
# Define the scan step
step_scan = Step(name="scan")
step_scan.expected_materials = [["MATCH", "image.tar", "WITH", "PRODUCTS", "FROM", "build"]]
step_scan.expected_products = [["CREATE", "scan-report.json"]]
step_scan.pubkeys = [scan_functionary_keyid]
step_scan.threshold = 1
layout.steps = [step_clone, step_build, step_scan]
Link Metadata
Each step execution generates a link file containing:
- Materials consumed (input artifacts with hashes)
- Products created (output artifacts with hashes)
- Command executed
- Cryptographic signature of the functionary
Verification Process
At deployment time, the verifier checks:
- All required steps were performed
- Each step was signed by an authorized functionary
- Artifact hashes chain correctly between steps
- No unauthorized modifications occurred between steps
Implementation
Step 1: Generate Signing Keys
# Generate Ed25519 key pairs for each functionary
mkdir -p keys
# Project owner key (signs the layout)
in-toto-keygen --type ed25519 keys/owner
# CI builder key
in-toto-keygen --type ed25519 keys/builder
# Security scanner key
in-toto-keygen --type ed25519 keys/scanner
Step 2: Create the Supply Chain Layout
#!/usr/bin/env python3
"""Generate in-toto supply chain layout for container builds."""
from in_toto.models.layout import Layout, Step, Inspection
from in_toto.models.metadata import Envelope
from securesystemslib.signer import CryptoSigner
from securesystemslib.interface import import_ed25519_publickey_from_file
def create_container_build_layout():
layout = Layout()
layout.set_relative_expiration(months=6)
# Load functionary public keys
builder_key = import_ed25519_publickey_from_file("keys/builder.pub")
scanner_key = import_ed25519_publickey_from_file("keys/scanner.pub")
layout.keys = {
builder_key["keyid"]: builder_key,
scanner_key["keyid"]: scanner_key,
}
# Step 1: Source code checkout
checkout = Step(name="checkout")
checkout.expected_materials = []
checkout.expected_products = [
["CREATE", "Dockerfile"],
["CREATE", "src/*"],
["CREATE", "requirements.txt"],
]
checkout.pubkeys = [builder_key["keyid"]]
checkout.threshold = 1
# Step 2: Build container image
build = Step(name="build")
build.expected_materials = [
["MATCH", "Dockerfile", "WITH", "PRODUCTS", "FROM", "checkout"],
["MATCH", "src/*", "WITH", "PRODUCTS", "FROM", "checkout"],
]
build.expected_products = [["CREATE", "image-digest.txt"]]
build.pubkeys = [builder_key["keyid"]]
build.threshold = 1
# Step 3: Security scan
scan = Step(name="scan")
scan.expected_materials = [
["MATCH", "image-digest.txt", "WITH", "PRODUCTS", "FROM", "build"]
]
scan.expected_products = [
["CREATE", "vulnerability-report.json"],
["CREATE", "sbom.json"],
]
scan.pubkeys = [scanner_key["keyid"]]
scan.threshold = 1
# Inspection: Verify no critical vulnerabilities
inspect_vulns = Inspection(name="verify-no-critical-vulns")
inspect_vulns.expected_materials = [
["MATCH", "vulnerability-report.json", "WITH", "PRODUCTS", "FROM", "scan"]
]
inspect_vulns.run = [
"python", "-c",
"import json,sys; r=json.load(open('vulnerability-report.json')); "
"sys.exit(1) if any(v['severity']=='CRITICAL' for v in r.get('vulnerabilities',[])) else sys.exit(0)"
]
layout.steps = [checkout, build, scan]
layout.inspect = [inspect_vulns]
return layout
if __name__ == "__main__":
layout = create_container_build_layout()
# Sign with owner key and save
owner_signer = CryptoSigner.from_priv_key_uri("file:keys/owner")
envelope = Envelope.from_signable(layout)
envelope.create_signature(owner_signer)
envelope.dump("root.layout")
print("Layout created and signed: root.layout")
Step 3: Record Pipeline Steps
# In CI/CD pipeline - record each step
# Step 1: Checkout
in-toto-run --step-name checkout \
--key keys/builder \
--products Dockerfile src/* requirements.txt \
-- git clone https://github.com/org/app.git .
# Step 2: Build
in-toto-run --step-name build \
--key keys/builder \
--materials Dockerfile src/* \
--products image-digest.txt \
-- bash -c "docker build -t app:latest . && docker inspect --format='{{.Id}}' app:latest > image-digest.txt"
# Step 3: Scan
in-toto-run --step-name scan \
--key keys/scanner \
--materials image-digest.txt \
--products vulnerability-report.json sbom.json \
-- bash -c "trivy image --format json app:latest > vulnerability-report.json && syft app:latest -o json > sbom.json"
Step 4: Verify Before Deployment
# Verify the entire supply chain
in-toto-verify --layout root.layout \
--layout-key keys/owner.pub \
--link-dir ./link-metadata/
# If verification passes, proceed with deployment
if [ $? -eq 0 ]; then
kubectl apply -f deployment.yaml
echo "Supply chain verification passed - deploying"
else
echo "SUPPLY CHAIN VERIFICATION FAILED - blocking deployment"
exit 1
fi
Step 5: Kubernetes Admission Control
Integrate with a policy engine to verify attestations at admission:
apiVersion: admissionregistration.k8s.io/v1
kind: ValidatingWebhookConfiguration
metadata:
name: in-toto-verifier
webhooks:
- name: verify.in-toto.io
rules:
- apiGroups: ["apps"]
resources: ["deployments"]
operations: ["CREATE", "UPDATE"]
clientConfig:
service:
name: in-toto-webhook
namespace: security
path: /verify
failurePolicy: Fail
sideEffects: None
admissionReviewVersions: ["v1"]
SLSA Integration
in-toto attestations map directly to SLSA (Supply chain Levels for Software Artifacts) requirements:
| SLSA Level | in-toto Requirement |
|---|---|
| Level 1 | Build process documented (layout exists) |
| Level 2 | Signed attestations from hosted build service |
| Level 3 | Hardened build platform, non-falsifiable provenance |
| Level 4 | Two-party review, hermetic builds |