Forward Secrecy

Problem

An attacker records your encrypted TLS traffic today. Five years later, they somehow steal the server's long-term private certificate key. Should that let them decrypt everything they recorded?

Solution — Ephemeral Key Exchange

Modern TLS can establish session secrets using temporary ephemeral key pairs. Those ephemeral private keys are discarded. Long-term key compromised later ≠ old session secrets

Core idea: forward secrecy prevents later compromise of a long-term key from revealing past sessions.