Key Attestation Parameters

This page defines the mechanism-specific inputs and outputs of AttestKey. For an in-depth look at the mechanisms and hardware tokens that we've considered, you can read our write-up here.

Each mechanism comes with its own definitions for AttestationMechanismParams and AttestationOutput.

(EXPERIMENTAL) The parameters for key attestation are in an experimental phase. No guarantees are offered around the stability of the interface for any key attestation mechanism.

ActivateCredential (TPM provider)

The TPM 2.0 Commands spec describes the TPM2_ActivateCredential operation as follows:

This command enables the association of a credential with an object in a way that ensures that the TPM has validated the parameters of the credentialed object.

TPM2_ActivateCredential allows a 3rd party to be assured of the protection of a key by means of an encrypted credential. The 3rd party produces a random credential and encrypts it using the algorithm defined in the TPM 2.0 Architecture spec, section B.10.4. The outputs of that algorithm (the encrypted and HMAC-protected credential, and a secret seed encrypted with the public part of the attesting key) are sent to the Parsec service which proceeds to perform the operation and returns the decrypted credential. The 3rd party can then be certain that the key is protected by a TPM by confirming that the credential sent and the one received are identical.

The computation mentioned previously relies on a number of parameters that must be obtained from the Parsec service. As some of these parameters are strictly TPM-specific, they can be retrieved with the PrepareKeyAttestation operation. You can see how to perform the preparation step for ActivateCredential here.

This mechanisms is thus aimed at attesting keys that are configured for decryption (as opposed to signing) and is of particular interest because the Endorsement Keys for which TPM manufacturers produce certificates are overwhelmingly decryption keys.

The parameters and output follow the inputs and outputs of TPM2_ActivateCredential as defined in the TPM 2.0 Structures spec.

AttestationMechanismParams

• credential_blob represents the contents of the credential field within the TPM2B_ID_OBJECT structure.
• secret represents the contents of the secret field within the TPM2B_ENCRYPTED_SECRET structure.

AttestationOutput

• credential represents the contents of the buffer field within the TPM2B_DIGEST structure.

Copyright 2021 Contributors to the Parsec project.