3.5 KiB
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 byTPM2_MakeCredential())TPMI_DH_OBJECT activateHandle(e.g., handle for an AK)TPM2B_ID_OBJECT credentialBlob(output ofTPM2_MakeCredential())TPM2B_ENCRYPTED_SECRET secret(output ofTPM2_MakeCredential())
Outputs (success case)
TPM2B_DIGEST certInfo(not necessarily a digest, but a small [digest-sized] secret that was input toTPM2_MakeCredential())