What does IPsec Perfect Forwarding Secrecy (PFS) ensure?

IPsec Perfect Forward Secrecy (PFS) ensures that a new common secret key is generated for each session, which adds an additional layer of security by preventing the compromise of one session's keys from affecting the security of future sessions. This mechanism ensures that even if the private key of the server is compromised at some point in time, the previously established sessions remain secure and cannot be decrypted because they use unique session keys.

By recalculating a new common secret key with session key expiration, PFS enhances security because it ensures that each session is independent. Therefore, if an adversary manages to obtain session keys for one connection, they cannot use that information to decrypt the traffic of past or subsequent connections since each will have its own unique key. This is why PFS is a pivotal aspect of maintaining data confidentiality and integrity in secure communications.

The other options do not encapsulate this core function of PFS. While symmetric encryption is a component of IPsec, it does not specifically relate to the concept of PFS. Key-agreement and security-association agreement protocols are broader concepts that do not specifically address the advantages provided by PFS in creating unique session keys.