Signing Container Images for Trust

What is Cosign?

Cosign is an open-source tool from the Sigstore project that enables cryptographic signing, verification, and storage of signatures for container images. Cosign simplifies the process of ensuring the integrity and authenticity of container images by allowing developers and system administrators to sign images and verify them before deployment.

Why Sign Container Images?

In containerized environments, it’s essential to verify that the images you deploy are secure and come from trusted sources. By signing container images, you can:

Verify Authenticity: Confirm that the image was created by the expected source.
Ensure Integrity: Detect any unauthorized changes or tampering with the image.
Build Trust: Establish a secure chain of trust for container images across different environments (e.g., development, testing, production).

This is particularly important in DevSecOps pipelines where security is integrated into every stage of development and deployment.

How Cosign Works

Cosign uses cryptographic keys to sign container images, similar to how digital signatures work in other contexts. Here’s how it generally works:

Generate a Key Pair: Cosign requires a public-private key pair to sign and verify images. The private key is used to sign the image, and the public key is shared with others to verify the signature.
Sign the Container Image: Cosign generates a cryptographic signature for the image using the private key. This signature is then attached to the image in the registry.
Store and Verify the Signature: When deploying or sharing the image, others can use the public key to verify the signature and ensure the image hasn’t been altered.

Step-by-Step Process for Signing Container Images with Cosign

Install Cosign: Ensure Cosign is installed on the system you are using to manage and sign images (e.g., a DevSecOps environment with Docker).
Generate a Key Pair:
```
 cosign generate-key-pair
```
- This command creates a private key (cosign.key) and a public key (cosign.pub).
Build and Tag Your Container Image: Use Docker to build and tag the image you want to sign.
```
 docker build -t your-registry/your-image:tag .
```
Sign the Container Image: Use Cosign to sign the image with the private key.
```
 cosign sign --key cosign.key your-registry/your-image:tag
```
- This attaches a signature to the image in the registry. The signature is cryptographically linked to the image and stored in the registry.
Verify the Container Image: Anyone with access to the public key can verify the image.
```
 cosign verify --key cosign.pub your-registry/your-image:tag
```
- This command checks the image against the signature to ensure it hasn’t been modified.

Benefits of Using Cosign for Signing Images

Automated Verification: Integrate Cosign into CI/CD pipelines to automatically verify signatures before deploying images to production.
Tamper Detection: Detect if an image has been altered since it was signed, preventing malicious images from being deployed.
Supply Chain Security: Cosign helps secure the software supply chain by ensuring only signed, trusted images enter the deployment pipeline.

Example Use Case

Suppose you are deploying a critical web application in a Kubernetes cluster. By signing your container images with Cosign and configuring the cluster to verify image signatures:

You can prevent untrusted or malicious images from running in the cluster.
Any tampering with the images in your registry would be detected before deployment.

In summary, signing container images with Cosign is a security best practice that helps you establish a trusted and verified deployment pipeline, reducing risks and increasing confidence in the software you deploy.