Diffie-Hellman Key Exchange and Random Number Generators ECES Practice Questions (Page 2)

Questions 6–10

1. 6expert · hard

   A security analyst is reviewing a Diffie-Hellman implementation that uses a random number generator with a backdoor: the generator's output is derived from a secret constant known to the attacker. Which impact does this have?

   • AThe attacker can only perform a denial-of-service attack.
   • BThe attacker can only impersonate one party.
   • CThe attacker can predict the private keys and decrypt all traffic.
   • DThe attacker can only recover the shared secret if they also know the prime.
   Reveal Solution Discussion Select an answer first
2. 7application · medium

   A company deploys a Diffie-Hellman key exchange for secure communication between two servers. An attacker intercepts the messages, modifies the public values exchanged, and establishes separate shared secrets with each server, allowing the attacker to relay and decrypt traffic. Which countermeasure would directly prevent this attack?

   • AAuthenticate the public values using digital signatures or a pre-shared key before they are used in the key exchange.
   • BIncrease the size of the prime p to 4096 bits.
   • CUse a larger generator g to make the public values harder to guess.
   • DEncrypt the Diffie-Hellman messages with a symmetric cipher before sending them.
   Reveal Solution Discussion Select an answer first
4. 8application · medium

   A security engineer is reviewing the random number generation used for Diffie-Hellman private keys in a hardware security module (HSM). The HSM provides a hardware-based entropy source that samples thermal noise from a dedicated circuit. Which type of random number generator does this represent, and why is it suitable for generating private keys?

   • AA pseudo-random number generator (PRNG) that uses a deterministic algorithm seeded by the thermal noise, which is sufficient for key generation.
   • BA true random number generator (TRNG) that produces non-deterministic output based on physical processes, providing high entropy for unpredictable keys.
   • CA cryptographically secure PRNG (CSPRNG) that expands a small random seed into a long stream of unpredictable numbers.
   • DA deterministic random bit generator (DRBG) that relies on a mathematical formula to produce numbers that appear random.
   Reveal Solution Discussion Select an answer first
5. 9application · medium

   A software team is generating Diffie-Hellman private keys for a VPN appliance. They are considering using a hardware random number generator (HRNG) that samples quantum noise. Which statement best describes the role of this HRNG?

   • AIt provides high-entropy random numbers suitable for generating private keys.
   • BIt is only useful for generating nonces, not for key generation.
   • CIt is a pseudo-random generator that must be seeded with a secret seed.
   • DIt is faster than any PRNG and should be used for all cryptographic operations.
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6. 10application · medium

   In a Diffie-Hellman exchange, Alice and Bob agree on p=23 and g=5. Alice chooses a=6 and Bob chooses b=15. What is the shared secret?

   • A8
   • B2
   • C19
   • D10
   Reveal Solution Discussion Select an answer first