Prepare AWS KMS for attestation

Note

If you are attesting to a third-party service, you must build your own custom mechanisms for receiving, parsing, and validating Attestation Documents. For more information, see Validate a NitroTPM Attestation Document.

After you have created your Attestable AMI, you should have reference measurements that you can use to validate requests from an Amazon EC2 instance. AWS KMS provides built-in support for attestation with NitroTPM.

For the AWS KMS key that you used to encrypt your secret data, add a key policy that allows key access only if API requests include an Attestation Document with PCR4 or PCR7 measurements that match the reference measurements you generated during the Attestable AMI creation process. This ensures that only requests from instances launched using the Attestable AMI can perform cryptographic operations using the AWS KMS key.

AWS KMS provides kms:RecipientAttestation:PCR4 and kms:RecipientAttestation:PCR7 condition keys that enable you to create attestation-based conditions for KMS key policies. For more information, see AWS KMS condition keys for AWS Nitro Enclaves and NitroTPM.

For example, the following AWS KMS key policy allows key access only if the request originates from an instance with the MyEC2InstanceRole instance profile attached, and if the request includes an Attestation Document with specific PCR 4 and PCR 7 values.

{
  "Version": "2012-10-17",		 	 	 
  "Statement": [
    {
      "Sid": "Allow requests from instances with attested AMI only",
      "Effect": "Allow",
      "Principal": {
        "AWS": "arn:aws:iam::111122223333:role/MyEC2InstanceRole"
      },
      "Action": [
        "kms:Decrypt",
        "kms:GenerateDataKey",
        "kms:GenerateRandom"
      ],
      "Resource": "*",
      "Condition": {
        "StringEqualsIgnoreCase": {
          "kms:RecipientAttestation:PCR4":"EXAMPLE6b9b3d89a53b13f5dfd14a1049ec0b80a9ae4b159adde479e9f7f512f33e835a0b9023ca51ada02160EXAMPLE",
          "kms:RecipientAttestation:PCR7":"EXAMPLE34a884328944cd806127c7784677ab60a154249fd21546a217299ccfa1ebfe4fa96a163bf41d3bcfaeEXAMPLE"
        }
      }
    }
  ]
}

For more information, see AWS KMS condition keys for NitroTPM.