PFS

A property ensuring past sessions stay secure even if a long-term key leaks.

Perfect Forward Secrecy ensures each session negotiates a unique ephemeral key, so compromise of a server's long-term private key cannot retroactively decrypt previously captured traffic. It is achieved with ephemeral Diffie-Hellman exchanges such as DHE or the elliptic curve variant ECDHE. This matters because adversaries often record encrypted sessions hoping to decrypt them later if a key leaks. Without PFS, a single stolen RSA key can unlock every past conversation, which is why modern TLS configurations favor ephemeral key agreement.