Use the verification service

Attestation Verification Service for Pre-Silicon CCA Platforms

Linaro provides an attestation verifier service for pre-silicon CCA platforms, such as the Fixed Virtual Platform (FVP). This service is available publicly and is hosted on Linaro infrastructure. This verification service can be used to verify CCA attestation tokens that come from emulated Arm platforms, including the example token that you have been using in this exercise.

Linaro’s verification service is implemented using components from the open source Veraison project.

The URL for reaching this experimental verifier service is http://veraison.test.linaro.org:8080

To check that you can reach the Linaro attestation verifier service, run the following command:

    

        
        
            curl http://veraison.test.linaro.org:8080/.well-known/veraison/verification
        
    

This is a simple call to query the well-known characteristics of the verification service. If it succeeds, it will return a JSON response that looks something like this:

    

        
        {
  "ear-verification-key": {
    "alg": "ES256",
    "crv": "P-256",
    "kty": "EC",
    "x": "usWxHK2PmfnHKwXPS54m0kTcGJ90UiglWiGahtagnv8",
    "y": "IBOL-C3BttVivg-lSreASjpkttcsz-1rb7btKLv8EX4"
  },
  "media-types": [
    "application/vnd.parallaxsecond.key-attestation.tpm",
    "application/eat-cwt; profile=\http://arm.com/psa/2.0.0\",
    "application/eat+cwt; eat_profile=\"tag:psacertified.org,2023:psa#tfm\"",
    "application/eat-collection; profile=\http://arm.com/CCA-SSD/1.0.0\",
    "application/eat+cwt; eat_profile=\"tag:psacertified.org,2019:psa#legacy\"",
    "application/vnd.enacttrust.tpm-evidence",
    "application/vnd.parallaxsecond.key-attestation.cca",
    "application/psa-attestation-token",
    "application/pem-certificate-chain"
  ],
  "version": "commit-2063e7e",
  "service-state": "READY",
  "api-endpoints": {
    "newChallengeResponseSession": "/challenge-response/v1/newSession"
  }
}

        
    

This JSON response contains all the information that you need to use the verification service. Review the different JSON properties.

• The ear-verification-key is the cryptographic key that you will use later to verify the results that are returned by the service.

• The media-types entry provides the list of the different attestation data formats that the verification service supports. If you look down this list, you will find an entry for the CCA profile of the EAT format. It is the fourth entry in the list. This tells us that the service is capable of processing Arm CCA attestation tokens.

• The api-endpoints entry describes the set of RESTful APIs that are supported by the service. When verifying an attestation token, you will use the challenge-response API.

If you can reach the verification service, you are now ready to use it to evaluate the CCA example token.

Save the Public Key of the Verification Service

One of the properties that was returned in the previous step was the public key of the verification service. This key will be needed later to check the signature on the attestation results. All that is needed in this step is to copy the contents of the ear-verification-key field from the previous step and save it to a separate JSON file.

The easiest way to do this is to use the jq utility. You can save the public key by repeating the curl command from the previous step and use jq to filter the response down to just the public key part. Save it into a file called pkey.json:

    

        
        
            curl -s -N http://veraison.test.linaro.org:8080/.well-known/veraison/verification | jq '."ear-verification-key"' > $HOME/pkey.json
        
    

You have now saved the public key of the verification service. You are now ready to submit the CCA example attestation token to the service and get an attestation result.

Submit the CCA Example Token to the Verification Service

To submit the example CCA attestation token to the verification service, you will need to use the evcli tool once again. First, configure the correct API endpoint for the Linaro verifier service:

    

        
        
            export API_SERVER=http://veraison.test.linaro.org:8080/challenge-response/v1/newSession
        
    

Now submit the token using the following command. The output of this command is an attestation result, which will be saved in a file called attestation_result.jwt:

    

        
        
            ./evcli cca verify-as relying-party --token $HOME/cca_example_token.cbor | tr -d \" > $HOME/attestation_result.jwt
        
    

The verification service has now evaluated the token and returned a result, which you have saved. The last two steps in this learning path will be about understanding the result data that came back from the verification service.