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

Forward Secrecy (FS) is a cryptographic property that guarantees the confidentiality of past encrypted sessions even if long-term keys are compromised in the future. It achieves this by using ephemeral (short-term) session keys that are discarded after use, so compromising the long-term key does not expose previously encrypted data. Option B correctly defines this core principle, while A describes key management risks (unrelated to FS), C discusses computational strength (not the defining feature of FS), and D refers to cryptanalysis threats (a different concern from FS's guarantee).

Step-by-step Derivation:
Forward Secrecy definition: A cryptographic system has forward secrecy if compromise of long-term keys does not compromise the confidentiality of past sessions. Analysis: (A) is about key management failures, not a defining feature of FS. (C) is about computational hardness, not session isolation. (D) is about Moore's Law and brute-force feasibility, not FS's core property. (B) directly states the essence: even if long-term keys leak later, short-term session keys used earlier remain protected. This is achieved via protocols like TLS 1.3 with ephemeral Diffie-Hellman, where each session generates unique keys that are not derivable from static keys.