"Eavesdroppers wishing to decrypt past communication which has used PFS face a daunting task: each previous session needs to be attacked independently. Even knowing the long-term private key does not help as the session key is not available by simple decryption. Conversely, when SSL connections do not use PFS, the secret key used to encrypt the rest of the session is generated by the SSL site and sent encrypted with the long-term private-public key pair. If this long-term private key is ever compromised all previous encrypted sessions are easily decrypted."