In this lesson, you will learn how to ensure the authenticity and integrity of container images by implementing cryptographic signatures.
The Problem of Trust
When you pull a container image from a registry, how do you verify that it was actually built by your organization and hasn't been tampered with? Standard transport mechanisms like TLS protect the transit, but they don't confirm that the image contents are identical to what the developer originally signed. To close this gap, we use cryptographic signing, which binds a binary blob to a specific digital identity.
Cryptographic Verification
The core idea is to treat the image manifest as a document that can be signed. A developer (or a CI/CD system) uses a private key to generate a signature for the image digest. When your runtime environment pulls the image, it uses the corresponding public key to verify that the signature is valid. If even a single byte of the image has changed, the hash will mismatch, the signature check will fail, and the container will refuse to start.
Tools like Cosign (from the Sigstore project) simplify this process by embedding these signatures directly into the OCI registry alongside the image.
Practical Implementation
To verify an image, you define a policy that the runtime must check before execution. For example, using a tool integrated with your container runtime, you might run:
# Verify the image against a known public key
cosign verify --key cosign.pub my-registry.com/my-app:latest
If the verification fails, the runtime prevents the container from ever entering your namespace. By combining this with Seccomp (lesson 20) and User Namespaces (lesson 7), you ensure that only verified, trusted code is allowed to execute within your isolated environments. This creates a "secure supply chain," where identity is cryptographically anchored rather than just assumed based on the registry URL.
This process ensures that your container runtime is running exactly what the builder intended, preventing "man-in-the-middle" attacks or unauthorized image swaps from compromising your system.
Key Takeaway: Cryptographic signing provides a verifiable chain of custody for container images, ensuring that only trusted, untampered binaries can be deployed into your isolated environments.