Diffie-Hellman Key Exchange and Random Number Generators ECES Practice Questions (Page 6)

Questions 26–30

1. 26expert · hard

   A cryptography consultant is evaluating a legacy system that uses a Mersenne Twister to generate Diffie-Hellman private exponents. The system's performance requirements are modest, and the consultant must recommend a replacement that is both secure and minimally disruptive. Which recommendation is the most appropriate?

   • AKeep the Mersenne Twister but increase the seed size to 256 bits to improve security.
   • BUse a linear congruential generator (LCG) with a larger modulus to replace the Mersenne Twister.
   • CReplace the Mersenne Twister with a cryptographically secure PRNG (CSPRNG) such as Fortuna or Yarrow, using the existing entropy sources for seeding.
   • DUse a hardware TRNG directly for each private exponent without any PRNG.
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2. 27foundation · easy

   What is the main difference between a true random number generator (TRNG) and a pseudo-random number generator (PRNG)?

   • AA TRNG produces numbers that are statistically random, while a PRNG produces numbers that are always predictable.
   • BA TRNG produces the same sequence if seeded the same, while a PRNG does not.
   • CA TRNG is faster than a PRNG.
   • DA TRNG uses a physical entropy source, while a PRNG uses a deterministic algorithm to produce numbers.
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4. 28expert · hard

   A cryptographic library is found to generate Diffie-Hellman private keys using a PRNG that was seeded with a low-entropy source. The library has been in production for two years. Which action is most appropriate?

   • ARotate all keys that were generated with the vulnerable PRNG.
   • BIncrease the key size to compensate.
   • CDisable Diffie-Hellman and use RSA only.
   • DPatch the PRNG and continue using existing keys.
   Reveal Solution Discussion Select an answer first
5. 29application · medium

   A penetration tester is analyzing a Diffie-Hellman implementation that uses a weak random number generator for private keys. The tester observes that the generator produces a predictable sequence. Which impact is most likely?

   • AAn attacker can predict the private key and compute the shared secret.
   • BThe attacker can only perform a denial-of-service attack.
   • CThe Diffie-Hellman exchange will fail due to incorrect math.
   • DThe shared secret will be the same for all sessions.
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6. 30foundation · easy

   Why is a cryptographically secure pseudo-random number generator (CSPRNG) required for generating cryptographic keys?

   • AIt produces keys that are longer than those from a regular PRNG.
   • BIt ensures that the generated keys are unique for each user.
   • CIt is faster than other PRNGs.
   • DIt provides the property that the output is computationally indistinguishable from true randomness.
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