__________ is the concept that disclosure of the long-term secret keying material that is used to derive an agreed key does not compromise the secrecy of agreed keys that had previously been generated.

A.
Authentication protocol

B.
Diffie-Hellman

C.
Perfect forward secrecy

D.
Key exchange protocol

Explanation:
Perfect forward secrecy means that if an attacker discovers the material used to derive a key, that does not compromise previously generated keys. This is important as it prevents those keys from having to be replaced.

Answer option A is incorrect. An authentication protocol is any protocol used to verify the identity of a user or machine in network communications.

Answer option B is incorrect. Diffie-Hellman is a protocol for exchanging keys over an insecure medium.

Answer option D is incorrect. Key exchange protocols are only concerned with exchanging a symmetric key, not with the situation that might arise should the key derivation process become compromised.