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Container Image Sign and Verify with cosign tool

⚡ Requirement nerdctl >= 0.15

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

Cosign in Compose

Cosign support in Compose is also experimental and implemented based on Compose's extension capibility.

cosign is supported in nerdctl compose up|run|push|pull. You can use cosign in Compose by adding the following fields in your compose yaml. These fields are per service, and you can enable only verify or only sign (or both).

# only put cosign related fields under the service you want to sign/verify.
services:
  svc0:
    build: .
    image: ${REGISTRY}/svc0_image # replace with your registry
    # `x-nerdctl-verify` and `x-nerdctl-cosign-public-key` are for verify
    # required for `nerdctl compose up|run|pull`
    x-nerdctl-verify: cosign
    x-nerdctl-cosign-public-key: /path/to/cosign.pub
    # `x-nerdctl-sign` and `x-nerdctl-cosign-private-key` are for sign
    # required for `nerdctl compose push`
    x-nerdctl-sign: cosign
    x-nerdctl-cosign-private-key: /path/to/cosign.key
    ports:
    - 8080:80
  svc1:
    build: .
    image: ${REGISTRY}/svc1_image # replace with your registry
    ports:
    - 8081:80

Following the cosign tutorial above, first set up environment and prepare cosign key pair:

# 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

We'll use the following Dockerfile and docker-compose.yaml:

$ cat Dockerfile
FROM nginx:1.19-alpine
RUN uname -m > /usr/share/nginx/html/index.html

$ cat docker-compose.yml
services:
  svc0:
    build: .
    image: ${REGISTRY}/svc1_image # replace with your registry
    x-nerdctl-verify: cosign
    x-nerdctl-cosign-public-key: ./cosign.pub
    x-nerdctl-sign: cosign
    x-nerdctl-cosign-private-key: ./cosign.key
    ports:
    - 8080:80
  svc1:
    build: .
    image: ${REGISTRY}/svc1_image # replace with your registry
    ports:
    - 8081:80

The env "COSIGN_PASSWORD="$COSIGN_PASSWORD"" part in the below commands is a walkaround to use rootful nerdctl and make the env variable visible to root (in sudo). You don't need this part if (1) you're using rootless, or (2) your COSIGN_PASSWORD is visible in root.

First let's build and push the two services:

$ sudo nerdctl compose build
INFO[0000] Building image xxxxx/svc0_image
...
INFO[0000] Building image xxxxx/svc1_image
[+] Building 0.2s (6/6) FINISHED

$ sudo env "COSIGN_PASSWORD="$COSIGN_PASSWORD"" nerdctl compose --experimental=true push
INFO[0000] Pushing image xxxxx/svc1_image
...
INFO[0000] Pushing image xxxxx/svc0_image
INFO[0000] pushing as a reduced-platform image (application/vnd.docker.distribution.manifest.v2+json, sha256:4329abc3143b1545835de17e1302c8313a9417798b836022f4c8c8dc8b10a3e9)
INFO[0000] cosign: WARNING: Image reference xxxxx/svc0_image uses a tag, not a digest, to identify the image to sign.
INFO[0000] cosign:
INFO[0000] cosign: This can lead you to sign a different image than the intended one. Please use a
INFO[0000] cosign: digest (example.com/ubuntu@sha256:abc123...) rather than tag
INFO[0000] cosign: (example.com/ubuntu:latest) for the input to cosign. The ability to refer to
INFO[0000] cosign: images by tag will be removed in a future release.
INFO[0000] cosign: Pushing signature to: xxxxx/svc0_image

Then we can pull and up services (run is similar to up):

# ensure built images are removed and pull is performed.
$ sudo nerdctl compose down
$ sudo env "COSIGN_PASSWORD="$COSIGN_PASSWORD"" nerdctl compose --experimental=true pull
$ sudo env "COSIGN_PASSWORD="$COSIGN_PASSWORD"" nerdctl compose --experimental=true up
$ sudo env "COSIGN_PASSWORD="$COSIGN_PASSWORD"" nerdctl compose --experimental=true run svc0 -- echo "hello"
# clean up compose resources.
$ sudo nerdctl compose down

Check your logs to confirm that svc0 is verified by cosign (have cosign logs) and svc1 is not. You can also change the public key in docker-compose.yaml to a random value to see verify failure will stop the container being pull|up|run.