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Container Image Sign and Verify with cosign tool

cosign is tool that allows you to sign and verify container images with the public/private key pairs or without them by providing a Keyless support.

Keyless uses ephemeral keys and certificates, which are signed automatically by the fulcio root CA. Signatures are stored in the rekor transparency log, which automatically provides an attestation as to when the signature was created.

You can enable container signing and verifying features with push and pull commands of nerdctl by using cosign under the hood with make use of flags --sign while pushing the container image, and --verify while pulling the container image.

Prepare your environment:

# Create a sample Dockerfile
$ cat <<EOF | tee Dockerfile.dummy
FROM alpine:latest
CMD [ "echo", "Hello World" ]
EOF

Please do not forget, we won't be validating the base images, which is alpine:latest in this case, of the container image that was built on, we'll only verify the container image itself once we sign it.

# Build the image
$ nerdctl build -t devopps/hello-world -f Dockerfile.dummy .

# Generate a key-pair: cosign.key and cosign.pub
$ cosign generate-key-pair

# Export your COSIGN_PASSWORD to prevent CLI prompting
$ export COSIGN_PASSWORD=$COSIGN_PASSWORD

Sign the container image while pushing:

# Sign the image with Keyless mode
$ nerdctl push --sign=cosign devopps/hello-world

# Sign the image and store the signature in the registry
$ nerdctl push --sign=cosign --cosign-key cosign.key devopps/hello-world

Verify the container image while pulling:

REMINDER: Image won't be pulled if there are no matching signatures in case you passed --verify flag.

# Verify the image with Keyless mode
$ nerdctl pull --verify=cosign devopps/hello-world
INFO[0004] cosign:
INFO[0004] cosign: [{"critical":{"identity":...}]
docker.io/devopps/nginx-new:latest:                                               resolved       |++++++++++++++++++++++++++++++++++++++|
manifest-sha256:0910d404e58dd320c3c0c7ea31bf5fbfe7544b26905c5eccaf87c3af7bcf9b88: done           |++++++++++++++++++++++++++++++++++++++|
config-sha256:1de1c4fb5122ac8650e349e018fba189c51300cf8800d619e92e595d6ddda40e:   done           |++++++++++++++++++++++++++++++++++++++|
elapsed: 1.4 s                                                                    total:  1.3 Ki (928.0 B/s)

# You can not verify the image if it is not signed
$ nerdctl pull --verify=cosign --cosign-key cosign.pub devopps/hello-world-bad
INFO[0003] cosign: Error: no matching signatures:
INFO[0003] cosign: failed to verify signature
INFO[0003] cosign: main.go:46: error during command execution: no matching signatures:
INFO[0003] cosign: failed to verify signature