forwardsecret

Forward secret, more commonly referred to as perfect forward secrecy (PFS), is a property of secure communication protocols that ensures session keys used to encrypt a conversation are not recoverable from the server’s private keys, even if those private keys are compromised in the future. In effect, PFS protects past communications if a private key is later stolen, assuming the ephemeral keys were used correctly and discarded after the session.

PFS is typically achieved through ephemeral key exchange algorithms such as Diffie-Hellman Ephemeral (DHE) or Elliptic Curve Diffie-Hellman Ephemeral (ECDHE). In these schemes, a fresh key pair is generated for each session, and the session encryption key is derived via a Diffie-Hellman calculation that uses the ephemeral keys. Because the private ephemeral keys are not reused and are destroyed after the session, knowledge of the long-term private key does not reveal previous session keys.

In practice, forward secrecy is widely implemented in TLS for HTTPS, SSH, and various messaging protocols. TLS 1.3 requires forward secrecy by default; TLS 1.2 can provide it when the chosen cipher suites use ephemeral keys. Other protocols may support PFS as an optional or configurable feature.

Benefits of PFS include protection against retrospective key compromise and enhanced privacy against memory-based attacks. Drawbacks can include increased computational overhead and more complex key management. Proper implementation requires genuine generation and secure disposal of ephemeral keys and avoidance of key reuse.

See also: Perfect forward secrecy, forward secrecy, TLS.