What is forward secrecy?

Forward secrecy means each session uses a fresh, temporary key that is thrown away afterwards, so stealing a server's long-term private key later does not decrypt sessions recorded earlier. It comes from ephemeral key exchanges such as ECDHE or X25519: the long-term key only proves the server's identity, while the session key comes from throwaway values chosen by both sides. Older TLSTLS: Transport Layer Security. The encryption that protects most internet traffic, such as HTTPS. configurations with static RSA key exchange lacked it, so a single stolen key could unlock years of recorded traffic. TLS 1.3 makes forward secrecy mandatory, and well-configured TLS 1.2 servers offer it too. It also explains why harvest now, decrypt later is about the key exchange: forward secrecy protects against a future theft of the key, but not against a future quantum computer that can break the ephemeral elliptic-curve exchange itself. Hybrid post-quantum key exchange keeps forward secrecy and adds the missing protection. Check your servers with a TLS scanner: every suite they offer should have it.

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