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

Forward Secrecy (FS) is a cryptographic property that guarantees protection of past session data even if long-term keys are later compromised. Option B accurately defines this core principle: short-term session keys remain secure because their compromise does not expose the long-term key, and vice versa. Options A, C, and D describe general key management practices, computational security measures, and technological advances respectively—none define the specific mechanism of forward secrecy.

Step-by-step Derivation:
Forward Secrecy is fundamentally about temporal key isolation. Used in protocols like TLS 1.3 with ephemeral Diffie-Hellman (DHE) or elliptic curve DHE (ECDHE), FS ensures each session generates unique, temporary keys. If a long-term private key is later stolen, attackers cannot retroactively decrypt past sessions because those sessions never relied on that long-term key. Option B encapsulates this guarantee, while A addresses poor key hygiene, C discusses computational strength through hardware, and D addresses quantum computing threats—all orthogonal to the FS definition.