Frequency Analysis and Differential Cryptanalysis ECES Practice Questions (Page 1)

Questions 1–5

1. 1expert · medium

   An analyst is constructing a differential characteristic for a 4-round Feistel cipher. The analyst has identified a 1-round characteristic with probability p1 = 2^-4. Assuming the rounds are independent, what is the approximate probability of the 4-round characteristic?

   • A2^-20
   • B2^-16
   • C2^-8
   • D2^-4
   Reveal Solution Discussion Select an answer first
2. 2foundation · easy

   Why are digraph (two-letter) frequencies, such as 'TH' and 'HE', useful in breaking a simple substitution cipher?

   • AThey are the only way to distinguish between 'E' and 'T'.
   • BThey help identify common letter pairs, which can confirm or refine single-letter mappings.
   • CThey eliminate the need for single-letter frequency analysis.
   • DThey reveal the key length of the cipher.
   Reveal Solution Discussion Select an answer first
4. 3expert · hard

   A cryptanalyst is designing a differential attack on a 4-round block cipher. The analyst has a 3-round differential characteristic with probability 1/64. To recover the last round subkey, the analyst needs to count how many plaintext pairs produce the predicted output difference before the last round. The analyst has a budget of 1,000,000 chosen plaintext pairs. What is the most important consideration when deciding whether this budget is sufficient?

   • AThe key schedule must be reversible to perform the attack
   • BThe probability of the 3-round characteristic must be high enough that the correct subkey stands out from noise
   • CThe block size of the cipher determines the number of pairs needed
   • DThe S-box must be bijective for the attack to work
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5. 4application · medium

   A cryptanalyst is trying to break a cipher and has collected a large amount of ciphertext. The ciphertext letter frequencies are nearly uniform, and the index of coincidence is close to 0.038. Which type of cipher is most likely in use?

   • ATransposition cipher
   • BCaesar cipher
   • CPolyalphabetic cipher
   • DMonoalphabetic substitution cipher
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6. 5foundation · easy

   In differential cryptanalysis, what is typically meant by a 'difference' between two plaintexts?

   • AThe difference in length between the two plaintexts.
   • BThe Hamming distance between the two plaintexts.
   • CThe arithmetic subtraction of the two plaintexts as integers.
   • DThe bitwise XOR of the two plaintext blocks.
   Reveal Solution Discussion Select an answer first