tpm.dev.tutorials/TPM-Commands/TPM2_ActivateCredential.md

TPM2_ActivateCredential()

TPM2_ActivateCredential() is the flip side to TPM2_MakeCredential(), decrypting a small ciphertext made by TPM2_MakeCredential().

The intersting things about TPM2_ActivateCredential() are that

• the decryption key used may be a restricted key (which TPM2_RSA_Decrypt() would refuse to use)
• and that TPM2_ActivateCredential() evaluates an authorization policy of the sender's choice.

Together with TPM2_MakeCredential() an TPM2_Quote() this function can be used to implement attestation protocols.

Two of the input parameters of TPM2_ActivateCredential(), keyHandle and activateHandle, correspond to the handle and objectName inputs of TPM2_MakeCredential(), respectively. The other inputs are TPM2_MakeCredential()'s outputs. The output, certInfo is TPM2_MakeCredential()'s credential input.

Authorization

TPM2_ActivateCredential() checks the authorization of the caller to perform this operation by enforcing the keyHandle's policy in the USER role, and the activateHandle's policy in the ADMIN role. See section 19.2 of TCG TPM Library part 1: Architecture.

What this means specifically depends on whether the userWithAuth attribute is set on the keyHandle and whether the adminWithPolicy attribute is set on the activateHandle.

In particular, if adminWithPolicy is set on the activateHandle then the authorization session's policyDigest must match the activateHandle's policy and the authorization session's commandCode must be set to TPM_CC_ActivateCredential, which means that the caller must have called TPM2_PolicyCommandCode() with TPM_CC_ActivateCredential as the command code argument.

Some possible authorization policies to enforce include:

• that some non-resettable PCR has not been extended since boot

  This allows the recipient to extend that PCR immediately after activating the credential to prevent the attestation protocol from being used again without rebooting.

• user authentication / attended boot

  The policy could require physical presence, authentication of a user with biometrics and/or a smartcard and/or a password.

• locality

Inputs

• TPMI_DH_OBJECT keyHandle (e.g., handle for an EK corresponding to the EKpub encrypted to by TPM2_MakeCredential())
• TPMI_DH_OBJECT activateHandle (e.g., handle for an AK)
• TPM2B_ID_OBJECT credentialBlob (output of TPM2_MakeCredential())
• TPM2B_ENCRYPTED_SECRET secret (output of TPM2_MakeCredential())

Outputs (success case)

• TPM2B_DIGEST certInfo (not necessarily a digest, but a small [digest-sized] secret that was input to TPM2_MakeCredential())

References

• TCG TPM Library part 1: Architecture, section 24
• TCG TPM Library part 2: Structures
• TCG TPM Library part 3: Commands, section 12
• TCG TPM Library part 3: Commands Code, section 12