In a
cryptographic algorithm, the concept that once a
conversation is
terminated, an
eavesdropper who has captured only the
ciphertext will not be able to gain
further information at a
later date to enable them to
decrypt the message later.
True one-time pads, in which the pad is destroyed upon use, and only existed for the sender and the receiver, meet the criteria for forward secrecy. PGP encryption, on the other hand, does not - the receiver must keep their PGP key around for later use; so its compromise in a year allows an attacker to read all past communication.s