ExtractCredential extracts a credential from a credential blob and encrypted
seed created by MakeCredential. This function is primarily for testing and
demonstration purposes, since in practice the private key corresponding to
the TPM endorsement key public area will not be available.
MakeCredential makes a credential for the object with the public area
akPublic, to be activated by the object with the public area ekPublic.
The credential blob and the encrypted seed are returned.