What is forward secrecy, and how do DHE/ECDHE ciphersuites provide it?

• A. Forward secrecy ensures session keys are not derived from server private keys; ephemeral key exchanges (DHE/ECDHE) generate fresh keys for each session.
• B. Forward secrecy means sessions never expire; DHE/ECDHE reuses keys.
• C. Forward secrecy stores session keys long-term; DHE/ECDHE are static.
• D. Forward secrecy uses private key to decrypt all past sessions; DHE/ECDHE do not apply.

Answer: (A)

Forward secrecy means the keys that protect a TLS session are not derived from or dependent on the server’s long-term private key, so past conversations can’t be decrypted if that key is later compromised. When a session uses DHE (diffie-hellman) or ECDHE (elliptic-curve diffie-hellman), each connection performs an ephemeral, per-session key exchange. The client and server generate fresh ephemeral keys and compute a shared secret that becomes the basis for the session encryption keys. Since these session keys are tied to these ephemeral values and not the server’s long-term private key, compromising that private key later doesn’t reveal past session traffic. The server’s long-term key is still used to authenticate the handshake, but it doesn’t give away the actual session keys.

The other options miss the idea that forward secrecy is about protecting past sessions from future key compromises, not about session expiration, key reuse, or storing keys long-term. DHE/ECDHE don’t reuse static keys; they generate new ephemeral keys for each session, enabling forward secrecy.