Cosign & Sigstore

Cosign and Sigstore Integration
Cosign is a toolchain designed to integrate seamlessly with the Sigstore project, a collection of open-source tools focused on securing container images and other artifacts. This integration leverages Sigstore's core components—such as the Sign client, Rekor transparency log, and Cosign itself—to provide cryptographic signing, verification, and transparency for container images. By combining these tools, Cosign enables developers and operators to ensure the integrity and authenticity of container images across multi-cloud environments.

Sigstore Components in Cosign¶

Sigstore's architecture is divided into two primary components:
1. Sign: A client tool for signing artifacts using OpenPGP standards. Cosign acts as the Sigstore implementation of this tool.
2. Rekor: A transparency log that records cryptographic signatures of artifacts, ensuring immutability and auditability.

Cosign integrates with Rekor to store and verify signatures, ensuring that every signed image has a verifiable record in the log. This combination provides end-to-end security for container images, preventing tampering and ensuring trust in the supply chain.

Signing Workflow with Cosign and Sigstore¶

The signing process involves the following steps:
1. Sign the image: Cosign uses the Sigstore Sign client to generate a cryptographic signature for the container image.
2. Store the signature: The signature is recorded in Rekor, a public transparency log, ensuring it cannot be altered.
3. Verify the signature: During runtime or deployment, Cosign checks the signature against the Rekor log to confirm authenticity.

Example:

# Sign a container image with Cosign (integrates with Sigstore)
cosign sign --output cosign.sig my-registry/my-image:tag

This command generates a signature file (cosign.sig) and records the signature in Rekor. The transparency log entry is immutable, providing a tamper-proof record of the signing event.

Verification Process¶

Verification ensures that an image has not been tampered with and was signed by a trusted entity. Cosign checks:
1. Signature validity: The cryptographic signature matches the image.
2. Log entry: The signature exists in Rekor and has not been altered.

Example:

# Verify a container image with Cosign
cosign verify --signature cosign.sig my-registry/my-image:tag

If the signature is valid and the log entry is intact, Cosign confirms the image's authenticity. This process is critical for enforcing security policies in production environments.

Diagram of Integration¶

[Container Image]  
        |  
        v  
[cosign sign] --> [Cryptographic Signature]  
        |  
        v  
[Rekor Transparency Log]  
        |  
        v  
[cosign verify] --> [Signature Validity Check]

This flow illustrates how Cosign leverages Sigstore's components to secure and verify container images.

Key takeaways¶

Cosign integrates with Sigstore to provide cryptographic signing and verification for container images.
Sigstore's Sign client (via Cosign) generates OpenPGP signatures, while Rekor stores these signatures in a transparent, immutable log.
The signing workflow ensures signatures are recorded in Rekor, enabling auditability and trust.
Verification checks both the signature's validity and its presence in the Rekor log, ensuring image integrity.
This integration is essential for securing containerized applications in multi-cloud environments.