PfS

Perfect Forward Secrecy (PFS) is a security property of cryptographic protocols that ensures session keys used to encrypt communications are not recoverable from a server’s private key, even if the private key is later compromised. In practice, PFS protects past conversations from retrospective decryption.

It is most commonly associated with TLS/SSL connections. PFS is achieved through ephemeral key exchanges, typically Diffie-Hellman Ephemeral (DHE) or Elliptic Curve Diffie-Hellman Ephemeral (ECDHE). In these schemes, each new connection generates fresh, temporary key material that is discarded after the session ends. Because the session key is not derived from a long-term secret, compromising long-term keys does not reveal prior session keys.

Adoption and configuration: Software and servers that support TLS should prefer PFS-enabled cipher suites, such as those using DHE/ECDHE rather than static RSA key exchange. Enabling PFS has trade-offs, including computational overhead and the need for proper randomness. Some environments may fall back to non-PFS configurations if not properly configured.

Limitations: PFS protects the confidentiality of past sessions if the ephemeral keys are handled securely and not compromised. It does not protect against endpoint compromise, traffic analysis, or metadata exposure. It also requires correct implementation; incomplete negotiations can undermine protection.

Other uses: In other areas, PFS can refer to different concepts, but in cryptography, it most commonly stands for Perfect Forward Secrecy.