Diffie-Hellman Key Exchange and Random Number Generators ECES Practice Questions (Page 3)

Questions 11–15

1. 11application · medium

   Two developers are implementing Diffie-Hellman key exchange for a chat application. They agree on a large prime p and a primitive root g. Developer A sends g^a mod p, and Developer B sends g^b mod p. They then compute the shared secret. Which computation correctly yields the shared secret for Developer A?

   • A(g^a mod p) * (g^b mod p) mod p
   • B(g^b mod p)^a mod p
   • Cg^(a+b) mod p
   • D(g^a mod p)^b mod p
   Reveal Solution Discussion Select an answer first
2. 12application · medium

   A software developer is implementing a Diffie-Hellman key exchange in a legacy system and needs to generate the private exponent. The system currently uses a linear congruential generator (LCG) for random numbers. A security consultant advises against using the LCG for this purpose. Which reason best explains why an LCG is unsuitable for generating Diffie-Hellman private keys?

   • AAn LCG is too slow for generating the large random numbers required by Diffie-Hellman.
   • BAn LCG is predictable: given a few consecutive outputs, an attacker can recover the internal state and predict future outputs, compromising the private key.
   • CAn LCG requires a hardware entropy source, which is not available in the legacy system.
   • DAn LCG produces only even numbers, which would restrict the private key space and make brute-force attacks easier.
   Reveal Solution Discussion Select an answer first
4. 13expert · hard

   A security architect is designing a Diffie-Hellman key exchange for a high-security environment. The requirement is to prevent man-in-the-middle attacks while also ensuring that the private keys are generated with high entropy. The architect considers using digital signatures for authentication and a hardware TRNG for key generation. Which combination of choices best meets the requirement?

   • AUse a certificate authority to distribute the shared secret after the key exchange.
   • BUse a larger prime and a Mersenne Twister for private exponents, relying on the key size for security.
   • CUse a pre-shared key to encrypt the Diffie-Hellman messages and a software PRNG seeded with system time for private exponents.
   • DUse digital signatures to authenticate the public values and a hardware TRNG to generate private exponents.
   Reveal Solution Discussion Select an answer first
5. 14foundation · easy

   Which of the following is an example of a hardware entropy source used in a TRNG?

   • AThe system clock time
   • BThe process ID of a running program
   • CThermal noise from a semiconductor
   • DThe output of a linear congruential generator
   Reveal Solution Discussion Select an answer first
6. 15application · medium

   A developer is implementing a Diffie-Hellman key exchange in a legacy system and needs to generate ephemeral private keys. The developer proposes using the Mersenne Twister PRNG because it is fast and has a long period. Why is this a poor choice for cryptographic key generation?

   • AMersenne Twister has a short period and repeats quickly.
   • BMersenne Twister is not cryptographically secure; its output is predictable given enough output.
   • CMersenne Twister requires a hardware entropy source to operate.
   • DMersenne Twister is too slow for real-time key generation.
   Reveal Solution Discussion Select an answer first