Which is the cryptographic algorithm of forward secrecy (FS)?

Forward Secrecy (FS) is a cryptographic property that guarantees session keys remain secure even if long-term keys are compromised. Option B accurately defines this core principle: past encrypted data remains protected because it was encrypted with ephemeral session keys, not the long-term key. Options A, C, and D describe key management practices, computational approaches, and technological concerns respectively, but none capture the essence of what forward secrecy actually ensures.

Step-by-step Derivation:
Forward Secrecy works by deriving temporary session keys (short-term keys) that are independent of the long-term private key. If an attacker compromises the long-term key in the future, they can use it to decrypt new sessions but cannot retroactively decrypt past sessions because the session keys were ephemeral and have been discarded. This is the defining characteristic of FS, which is implemented in protocols like TLS 1.3 with ephemeral Diffie-Hellman (DHE) or elliptic curve DHE (ECDHE) key exchanges.