Since the disk came from directly from Bill, Al could be sure the key on it really belonged to Bill. Similarly, Bill can trust the key on the disk Al gave him. Now, Al and Bill can generate signed, encrypted messages to each other. Since the signature is generated using his secret key, PGP asks for Bill's pass phrase. Now, when Al decrypts it, he sees File has signature.
Public key is required to check signature.
Bill Since PGP reported a good signature by Bill, Al knows the message is authentic, and avoids embarrassing himself at the party. Unencrypted Signatures Digital signatures can also be used without encryption. For example, Bill might want to sent out a press release via e-mail. If they're cautious, they'll check the PGP signature before accepting that the message really is from Bill Clinton. Either way, the signature offers Bill some protection.