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tpm.dev.tutorials/TPM-Commands/TPM2_PolicySigned.md
Nicolas Williams 6413adfe6b Describe TPM2_Policy{Secret,Signed}()
2021-08-28 15:11:18 -05:00

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TPM2_PolicySigned()

TPM2_PolicySigned() allows a caller to provide a signature of some assertion, with the signature made by some authorizing entity identified by its public key.

The semantics of the signature are arbitrary and up to the entity constructing the policies that contain this command. Possible semantics include:

  • biometrics user identification (whereby the biometrics device signs an assertion that a user identified by the opaque value of policyRef has been identified biometrically)

  • smartcard-based user authentication (whereby access to a private signing key on a smartcard is used to sign an assertion that the user has been authenticated by the user's possession of the smartcard and interactive PIN entry to unlock it)

  • assertion of attested state being trusted (whereby an attestation server signs such an assertion)

  • etc.

The signature made by the signed is over the following digest:

aHash := H(nonceTPM || expiration || cpHashA || policyRef)

where H() is the digest algorithm associated with the authorizing entity's public key.

When evaluating this assertion in a policy session, the TPM will check that the signature matches the above hash as constructed by the TPM from the TPM2_PolicySigned() command parameters.

When evaluating this assertion in a trial session, the TPM will ignore the signature and will extend the policySession's policyDigest as if the signature had matched the hash.

The nonceTPM input strongly binds the command to the policySession. If the Empty Buffer is given as the nonceTPM, then the TPM2_PolicySigned() command could be altered to refer to any other policy any TPM. For this reason it is important to use the policySession's nonceTPM in any call to TPM2_PolicySigned().

If a policyTicket is requested and output, that ticket can be used (up to its expiration), via TPM2_PolicyTicket(), to satisfy the same TPM2_PolicySigned() that produced the ticket.

That is, a caller can get a ticket from a TPM2_PolicySigned() invocation that allows it to re-use the auth signature many times prior to the ticket's expiration without having to get the authorizing entity to re-sign. For example, if the authorizing entity is a biometrics identification device, or a smartcard, then the interactive human identification or interactive smartcard PIN entry steps can be eschewed by the caller up to the ticket's expiration, using the ticket (via TPM2_PolicyTicket()) instead to satisfy the same TPM2_PolicySigned() command in any policy on that TPM. This is useful to avoid requiring repeated biometrics or PIN entry in a short time span.

Inputs

  • TPMI_DH_OBJECT authObject (handle to the key object whose public key is the signing entity's)
  • TPMI_SH_POLICY policySession (handle to the session being extended)
  • TPM2B_NONCE nonceTPM (the policy nonce for the policySession)
  • TPM2B_DIGEST cpHashA (the command parameter hash of a single command to be authorized, or Empty Buffer to not so-limit the assertion)
  • TPM2B_NONCE policyRef (an opaque value of the caller's and/or signer's choosing that is used to limit the value of the signature and to extend the policySession's policyDigest along with the authObject's name)
  • INT32 expiration (a positive or negative number of milliseconds which, if non-zero, sets an expiration for this assertion; if zero or positive then a policyTicket will not be output)
  • TPMT_SIGNATURE auth (the signature; ignored if the policySession is a trial session)

Outputs

  • TPM2B_TIMEOUT timeout (implementation-specific indication of actual timeout for the session)
  • TPMT_TK_AUTH policyTicket

References