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

Forward secrecy (FS) is a security property that guarantees past session keys remain secure even if the long-term encryption key is compromised. Option B directly defines this principle: short-term session keys protect encrypted data independently, so a future compromise of the long-term key cannot retroactively decrypt past communications. Options A, C, and D describe general key management risks, computational strength, and cryptanalytic advances—none of which define the core concept of forward secrecy.

Step-by-step Derivation:
Forward secrecy works by generating unique, ephemeral session keys for each communication session. These keys are discarded after use and are not derived solely from long-term keys. Example: TLS 1.3 with ECDHE (Elliptic Curve Diffie-Hellman Ephemeral) generates a new key pair for each handshake. If an attacker later obtains the server's long-term private key, they cannot decrypt past sessions because the ephemeral keys used in those sessions are no longer stored. Option B captures this exact guarantee.