Frequency Analysis and Differential Cryptanalysis ECES Practice Questions (Page 3)

Questions 11–15

1. 11expert · hard

   A cryptanalyst is evaluating the security of a custom block cipher against differential cryptanalysis. The cipher has a 32-bit block size and uses a substitution-permutation network with 8 rounds. The analyst finds a differential characteristic with probability 2^-16. What is the most significant implication of this finding?

   • AThe cipher is vulnerable only if the characteristic probability is exactly 1.
   • BThe cipher is potentially vulnerable to a differential attack because the characteristic probability is much higher than the 2^-32 random expectation.
   • CThe cipher is secure because the probability is low enough to prevent any practical attack.
   • DThe cipher is vulnerable only if the block size is 64 bits or larger.
   Reveal Solution Discussion Select an answer first
2. 12application · medium

   A security researcher is analyzing a new block cipher and wants to assess its resistance to differential cryptanalysis. The researcher constructs a differential characteristic with probability 1/4 for one round. What does this probability indicate about the cipher's resistance?

   • AThe cipher uses a weak S-box
   • BThe cipher is likely vulnerable to differential cryptanalysis if the probability remains high over many rounds
   • CThe cipher is completely secure against differential cryptanalysis
   • DThe cipher has a strong avalanche effect
   Reveal Solution Discussion Select an answer first
4. 13application · medium

   A cryptanalyst is studying a block cipher and wants to use differential cryptanalysis. The analyst chooses pairs of plaintexts with a fixed XOR difference and observes the corresponding ciphertext differences. What is the primary goal of this analysis?

   • ATo identify weak keys that produce identical ciphertexts for different plaintexts.
   • BTo recover information about the secret key by analyzing how plaintext differences propagate through the cipher rounds.
   • CTo determine the block size of the cipher by measuring the output difference distribution.
   • DTo find collisions in the cipher's output to construct a hash function.
   Reveal Solution Discussion Select an answer first
5. 14foundation · easy

   A monoalphabetic substitution ciphertext contains the following frequency counts: 'X' appears 45 times, 'M' appears 30 times, 'Z' appears 25 times, and all other symbols appear fewer than 10 times. Based on English letter frequencies (E, T, A, O, I, N, S, H, R, D, L, U), which mapping is the most reasonable first guess?

   • AX → E, M → T, Z → A
   • BX → E, M → A, Z → T
   • CX → A, M → E, Z → T
   • DX → T, M → E, Z → A
   Reveal Solution Discussion Select an answer first
6. 15application · medium

   A cryptanalyst is performing differential cryptanalysis on a block cipher with a 4-bit S-box. The analyst has a differential characteristic that predicts an output difference with probability 1/16. The analyst collects 1000 plaintext pairs and counts how often the predicted output difference appears for each candidate subkey. The counts are: candidate A: 70, candidate B: 55, candidate C: 80, candidate D: 45. Which candidate is most likely the correct subkey?

   • ACandidate C
   • BCandidate B
   • CCandidate A
   • DCandidate D
   Reveal Solution Discussion Select an answer first