Implement GetResponse
Add decrypt_in_key

Analogous to encrypt_in_key.
No kvno is needed in the encrypted data here, as we already have the
keyblock in question.

Change-Id: I356c5fb9d6a9c04b1b93d0c57104790e977c7035
First crack at CheckResponse

Note that both we and the client are using the master key for
the connection, not a derived transport key.

Change-Id: If4994ccaee0cd60ee0e817ac015f0b01db8ab03c