Q6 /20 Which is the cryptographic algorithm of forward secrecy (FS)?

Forward Secrecy (FS) is a cryptographic property that protects past encrypted communications even if long-term keys are compromised later. It uses ephemeral (short-lived) session keys that are discarded after use, ensuring historical data remains secure. Option B precisely defines this concept—compromising the long-term key does not allow decryption of past sessions encrypted with short-term keys. The other options describe key management risks (A), computational approaches (C), and cryptanalysis threats (D), none of which define forward secrecy.

Step-by-step Derivation:
Forward Secrecy definition: FS is a key exchange protocol property where session keys are not compromised even if the server's long-term private key is compromised. Each session generates unique, ephemeral keys that are deleted after the session ends. Analysis:

• Option A: Describes general key management failure, not FS.
• Option B: Directly states the core principle of FS—protects past data from future key compromise.
• Option C: Describes computational brute-force approaches, unrelated to FS.
• Option D: Describes advances in cryptanalysis/computing power, a threat FS helps mitigate but not its definition.
  Conclusion: B is correct.