“cosign” is a tool and framework that enables container signing, verification, and storage within an OCI (Open Container Initiative) registry. It provides a mechanism for securely signing container images and verifying the authenticity and integrity of those images. By leveraging digital signatures, cosign helps establish trust in the container supply chain and ensures that only trusted and tamper-free images are deployed.
Container signing is an essential security practice that allows developers and users to verify the origin and integrity of container images. It ensures that the image has not been tampered with or modified since it was signed by the trusted party. With cosign, developers can sign container images using private keys and attach the resulting digital signature to the image metadata.
When an image is signed with cosign, the digital signature contains information such as the signer’s identity, the signing algorithm used, and a cryptographic hash of the image contents. This information is stored within the container image itself or in an external metadata file. The signed image can then be published to an OCI registry, such as Docker Hub or Google Container Registry.
On the verification side, cosign provides the ability to verify the authenticity and integrity of signed container images. Users can verify the signature against the signer’s public key and ensure that the image has not been tampered with. Verification involves checking the integrity of the image contents and verifying that the signature matches the image’s cryptographic hash.
Cosign supports different signing and verification workflows, including single and multi-party signing scenarios. In multi-party signing, multiple signers can contribute their signatures to an image, building a chain of trust and allowing for collaborative verification. This can be particularly useful in scenarios where multiple entities are involved in the container supply chain.
In addition to signing and verification, cosign also supports the storage of signatures in an OCI registry. This allows users to associate signatures with specific image tags and retrieve them when needed. By storing signatures alongside the images, users can easily access and verify the authenticity of the images without relying on external storage or additional infrastructure.
cosign Command Examples
1. Generate a key-pair:
# cosign generate-key-pair
2. Sign a container and store the signature in the registry:
# cosign sign -key cosign.key image
3. Sign a container image with a key pair stored in a Kubernetes secret:
# cosign sign -key k8s://namespace/key image
4. Sign a blob with a local key pair file:
# cosign sign-blob --key cosign.key /path/to/file
5. Verify a container against a public key:
# cosign verify -key cosign.pub image
6. Verify images with a public key in a Dockerfile:
# cosign dockerfile verify -key cosign.pub path/to/Dockerfile
7. Verify an image with a public key stored in a Kubernetes secret:
# cosign verify -key k8s://namespace/key image
8. Copy a container image and its signatures:
# cosign copy example.com/src:latest example.com/dest:latest
Summary
Overall, cosign provides a comprehensive solution for container signing, verification, and storage within OCI registries. It enhances the security and trustworthiness of container images by enabling digital signatures and ensuring their integrity. By using cosign, developers and users can confidently deploy container images, knowing that they come from trusted sources and have not been compromised in transit or storage.