Docker Content Trust

• 08/28/2019
• 2 minutes to read
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Azure Pipelines

Docker Content Trust (DCT) provides the ability to use digital signatures for data sent to and received from remote Docker registries. These signatures allow client-side or runtime verification of the integrity and publisher of specific image tags.

Signing images in Azure Pipelines

Prerequisites on development machine

1. Use Docker trust's built in generator or manually generate delegation key pair. If the built-in generator is used, the delegation private key is imported into the local Docker trust store. Else, the private key will need to be manually imported into the local Docker trust store
2. Using the public key generated from the step above, upload the first key to a delegation and initiate the repository

Set up pipeline for signing images

1. Fetch the delegation private key, which is present in the local Docker trust store of your development machine used earlier, and add the same as a secure file in Pipelines

2. Authorize this secure file for use in all pipelines

3. Create a pipeline based on the following YAML snippet -

   pool:
     vmImage: 'Ubuntu 16.04'
   
   variables:
     system.debug: true
     containerRegistryServiceConnection: serviceConnectionName
     imageRepository: foobar/content-trust
     tag: test
   
   steps:
   - task: Docker@2
     inputs:
       command: login
       containerRegistry: $(containerRegistryServiceConnection)
   
   - task: DownloadSecureFile@1
     name: privateKey
     inputs:
       secureFile: cc8f3c6f998bee63fefaaabc5a2202eab06867b83f491813326481f56a95466f.key
   - script: |
       mkdir -p $(DOCKER_CONFIG)/trust/private
       cp $(privateKey.secureFilePath) $(DOCKER_CONFIG)/trust/private
   
   - task: Docker@2
     inputs:
       command: build
       Dockerfile: '**/Dockerfile'
       containerRegistry: $(containerRegistryServiceConnection)
       repository: $(imageRepository)
       tags: |
         $(tag)
       arguments: '--disable-content-trust=false'
   
   - task: Docker@2
     inputs: 
       command: push
       containerRegistry: $(containerRegistryServiceConnection)
       repository: $(imageRepository)
       tags: |
         $(tag)
       arguments: '--disable-content-trust=false'
     env:
       DOCKER_CONTENT_TRUST_REPOSITORY_PASSPHRASE: $(DOCKER_CONTENT_TRUST_REPOSITORY_PASSPHRASE)
   

   Note

   In the above snippet, the variable DOCKER_CONFIG is set by the login action done by Docker task. It is recommended to setup DOCKER_CONTENT_TRUST_REPOSITORY_PASSPHRASE as a secret variable for the pipeline as the alternative approach of using a pipeline variable in YAML would expose the passphrase in plaintext form.