Forward secrecy and cipher suite choices

Imagine an attacker records all of your organization's encrypted traffic for a year, then later steals the web server's private key. If the session keys were derived in a way that depends only on that long-term key, the attacker could go back and decrypt the whole archive. Forward secrecy, often called perfect forward secrecy (PFS), prevents this. With forward secrecy, each session uses a fresh, temporary key pair that is discarded afterward, so compromising the long-term key later does not reveal past session keys.