In Perfect Forward Secrecy (PFS), the private key is used to generate a temporary key (the session key) that is used for a short time and then discarded. Subsequent keys are independent of any previously created keys. This way, if a key is compromised, it does not affect any further keys, or compromise the security of data protected by other keys.

PFS provides a way to optimize both efficiently and security. Reasonably-sized keys are much more computationally efficient than large keys, but are also less secure. In PFS, you can use reasonably-sized keys and refresh them frequently.