Forward Secrecy

Forward secrecy means that recovering a server's long-term private key does not unlock the traffic it protected in the past. It is achieved by deriving each session's keys from ephemeral values that both parties discard when the session ends, so there is nothing left to seize later. Without it, one key compromise retroactively exposes every recorded session that key ever secured.

It addresses a different half of the problem than post-quantum cryptography does, and the two compose. Forward secrecy limits what a stolen key reveals about the past; post-quantum key exchange limits what a future computer reveals about traffic recorded today. Together they are what makes a captured session stay captured noise, which is the outcome the harvest now, decrypt later threat is aimed at.