Simpler sender authentication

[nikolamilekic]

In order to authenticate the sender, the recipient currently needs to parse the file's header to get the sender's public key so that he can compare it to the expected one. This basically means the onus is on the recipient to conduct authentication which is easy to forget, and cumbersome because he needs to know how to parse the header. It would be nice if the Decrypt methods accepted the expected sender's public key, and threw if it does not match.