What best describes perfect forward secrecy in TLS?

• A. PFS guarantees long-lived encryption keys.
• B. PFS eliminates the need for TLS.
• C. PFS ensures session keys are ephemeral and not recoverable; achieved via ephemeral Diffie-Hellman; enhances forward secrecy.
• D. PFS uses static RSA keys for all sessions.

Answer: (C)

Perfect forward secrecy means that the keys used to encrypt a TLS session are created for that session alone and cannot be recovered later, even if the server’s private key is compromised. In TLS this is achieved by using an ephemeral key exchange, such as ephemeral Diffie-Hellman (DHE) or Elliptic-Curve Diffie-Hellman (ECDHE). Each connection gets its own temporary key material, so the actual session keys aren’t derived from long‑term keys and aren’t stored in a way that can be reconstructed later. If someone later steals the server’s private key, they still can’t decrypt past conversations because those past session keys never depended on that private key.

That’s why the best description is: session keys are ephemeral and not recoverable; achieved via ephemeral Diffie-Hellman; enhances forward secrecy. The other ideas don’t fit: using long-lived keys would undermine forward secrecy, TLS is still needed for secure transport, and relying on static RSA keys for all sessions does not provide forward secrecy because compromise of the private key could reveal past traffic.