What provides forward secrecy?

• A. Public key pinning
• B. RSA key exchange
• C. Ephemeral key exchange
• D. Certificate signing

Answer: (C)

Forward secrecy is achieved when the keys used to encrypt a session are created for that session and can’t be derived from the server’s long-term private key. Using ephemeral keys for the key exchange does this because each connection gets a fresh, temporary key pair that is discarded after the session ends. In TLS, using ephemeral Diffie-Hellman (DHE) or ephemeral Elliptic Curve Diffie-Hellman (ECDHE) ensures the session keys are produced independently for every session, so even if the server’s private key is compromised later, past sessions remain secure.

Public key pinning focuses on binding a host to a specific certificate to guard against MITM attacks, not on how session keys are generated, so it doesn’t provide forward secrecy. RSA key exchange relies on the server’s long-term private key during the handshake; if that key is compromised, past session keys could be exposed, so it does not provide forward secrecy. Certificate signing validates identity but does not affect the forward-security properties of the session keys.