In this lesson, you will learn how to use cryptographic signatures to verify the provenance and authenticity of your images, ensuring that only trusted code enters your production environment.
The Challenge of Image Trust
When you push an image to a registry, how can you be certain that the image pulled by your Kubernetes cluster is the exact same one you built, and that it wasn't intercepted or replaced by a malicious actor? While using Content Addressable Digests (Concept 21) ensures you are pulling a specific version of an image, it doesn't prove who created it or that the image hasn't been tampered with since the signature was applied.
How Image Signing Works
Image signing acts like a digital wax seal on a document. When your CI/CD pipeline finishes building and auditing an image, it uses a private cryptographic key to "sign" the image manifest. This signature creates a unique mathematical bond between the image and the developer or build system that created it.
Later, when your production cluster attempts to pull the image, a verification tool checks the signature against a corresponding public key. If the signature is valid, it proves two things:
- Provenance: The image truly originated from your trusted build system.
- Integrity: The image contents have not been modified or corrupted since the signature was applied.
Implementing the Workflow
To implement this, your CI/CD pipeline should be the only entity with access to your private signing key. After your pipeline finishes building the image, it performs the signing operation:
# Conceptual signing step in your CI pipeline
cosign sign --key cosign.key my-registry.com/my-app:1.0.0
Once signed, the signature is stored in your registry alongside the image. In your Kubernetes cluster, you can use admission controllers to enforce this policy. If a deployment tries to pull an unsigned image, or an image signed by an unknown key, the controller will reject the pod, preventing the unauthorized code from ever executing.
By integrating signing into your lifecycle, you ensure that every piece of software running in production is cryptographically verified.
Key Takeaway: Image signing uses cryptographic keys to establish a chain of trust, ensuring that only verified, untampered images can be deployed to your production environment.