Docker Content Trust Basics

Lab Meta

Difficulty: Beginner

Time: Approximately 10 minutes

In this lab you’ll learn how to enable Docker Content Trust as well as perform some basic signing and verification operations.

You will complete the following steps as part of this lab.


You will need all of the following to complete this lab:

Step 1: Enable Docker Content Trust

In this step you will enable Docker Content Trust on a single node. You will test it by pulling an unsigned and a signed image.

Execute all of the commands in this section form node1 in your lab.

  1. Enable Docker Content Trust


    Docker Content Trust is now enabled on this host and you will no longer be able to pull unsigned images.

  2. Try pulling an unsigned image (any unsigned image will do, you do not have to use the one in this demo)

    $ docker image pull nigelpoulton/tu-demo
    Using default tag: latest
    Error: remote trust data does not exist for does not have trust data for

    The operation fails because the image is not signed (no trust data for the image).

  3. Try pulling the official alpine:latest image

    $ docker image pull alpine:latest
    Pull (1 of 1): alpine:latest@sha256:58e1a1bb75...3f105138f97eb53149673c4
    sha256:58e1a1bb75...3f105138f97eb53149673c4: Pulling from library/alpine
    627beaf3eaaf: Pull complete
    Digest: sha256:58e1a1bb75...3f105138f97eb53149673c4
    Status: Downloaded newer image for alpine@sha256:58e1a1bb75...3f105138f97eb53149673c4
    Tagging alpine@sha256:58e1a1bb75...3f105138f97eb53149673c4 as alpine:latest

    This time the operation succeeds. This is because the image is signed - all official images are signed.

In this step you have seen how easy it is to enable Docker Content Trust (exporting the DOCKER_CONTENT_TRUST environment variable with a value of 1). You have also proved that it is working by attempting to pull an unsigned image.

Step 2: Push and sign an image

In this step you will tag an image and push it to a new repository within your own namespace on Docker Cloud. You will perform this step from the host that you enabled Docker Content Trust on in the previous step. This will ensure that the image gets signed when you push it.

To complete this step you will need a Docker ID.

Execute all of the following commands from node1 (or whichever node you used for the previous step).

  1. Tag the alpine:latest image so that it can be pushed to a new repository in your namespace on Docker Cloud.

    This command will add the following additional tag to the alpine:latest image: nigelpoulton/sec-test:latest. The format of the tag is docker-id/repo-name/image-tag. Be sure to replace the docker-id in the following command with your own Docker ID.

    $ docker image tag alpine:latest nigelpoulton/sec-test:latest
  2. Verify the tagging operation worked

    $ docker image ls
    REPOSITORY              TAG      IMAGE ID       CREATED        SIZE
    alpine                  latest   4a415e366388   4 weeks ago    3.99MB
    nigelpoulton/sec-test   latest   4a415e366388   4 weeks ago    3.99MB

    Look closely and see that the image with IMAGE ID 4a415e366388 has two REPOSITORY tags.

  3. Login to Docker Cloud with your own Docker ID

    $ docker login
    Login with your Docker ID to push and pull images from Docker Store...
    Username: <your-docker-id>
    Login Succeeded
  4. Push the image to a new repository in your Docker Cloud namespace. Remember to use the image tag you created earlier that includes your own Docker ID.

    NOTE: As part of this push operation you will be asked to enter two new keys:

    • A new root key (this only happens the first time you push an image after enabling DCT)
    • A repository signing key
    $ docker image push nigelpoulton/sec-test:latest
    The push refers to a repository []
    23b9c7b43573: Pushed
    latest: digest: sha256:d0a670140...35edb294e4a7a152a size: 528
    Signing and pushing trust metadata
    You are about to create a new root signing key passphrase...
    Enter passphrase for new root key with ID 66997be: <root key passphrase>
    Repeat passphrase for new root key with ID 66997be: <root key passphrase>
    Enter passphrase for new repository key with ID 7ccd1b4 ( <repo key passphrase>
    Repeat passphrase for new repository key with ID 7ccd1b4 ( <repo key passphrase>
    Finished initializing ""
    Successfully signed "":latest

    The output above shows the image being signed as part of the normal docker image push command - no extra commands or steps are required to sign images with Docker Content Trust enabled.

Congratulations. You have pushed and signed an image.

By default the root and repository keys are stored below ~/.docker/trust.

In the real world you will need to generate strong passphrases for each key and store them in a secure password manager/vault.

Step 3: Clean-up

The following commands will clean-up the artifacts from this lab.

  1. Delete the tagged image you created in Step 2

    $ docker image rm nigelpoulton/sec-test:latest
    Untagged: nigelpoulton/sec-test:latest
    Untagged: nigelpoulton/sec-test@sha256:d0a6701...4e4a7a152a
  2. Delete the alpine:latest image

    $ docker image rm alpine:latest
    Untagged: alpine:latest
    Untagged: alpine@sha256:58e1a...38f97eb53149673c4
    Deleted: sha256:4a415e366...718a4698e91ef2a526
    Deleted: sha256:23b9c7b43...5f22803bcbe9ab24c2
  3. Disable Docker Content Trust.

  4. Login to Docker Cloud > Locate the repository you created with the docker image push command > Click Settings > Delete the repository.