Client.dfy Encrypt and Decrypt methods need to be verified

[WouterSchols]

The Encrypt and Decrypt methods from Client.dfy need to be verified. The specification for Encrypt is given here. We want to verify that the resulting encryption contains a header and body which matches the specification and encrypts the plaintext.

The Decrypt specification can be found here. We want to validate that the decryption is performed correctly. Furthermore we want to prove that encrypting plaintext and then decrypting the result gives the same plaintext.

[mattsb42-aws]

This is blocked by #289