Frequency Analysis and Differential Cryptanalysis ECES Practice Questions (Page 2)

Questions 6–10

1. 6application · easy

   A cryptanalyst has a long ciphertext that was produced by a monoalphabetic substitution cipher. The analyst computes the frequency of each ciphertext letter and finds that the letter 'Q' appears 12.7% of the time, 'X' appears 9.1%, and 'Z' appears 8.2%. The analyst also notes that the digraph 'QX' appears very frequently. Which plaintext letter is most likely represented by 'Q'?

   • AO
   • BA
   • CE
   • DT
   Reveal Solution Discussion Select an answer first
2. 7expert · hard

   A cryptanalyst is comparing two block ciphers for resistance to differential cryptanalysis. Cipher X has a maximum differential characteristic probability of 2^-30 for a 64-bit block size. Cipher Y has a maximum differential characteristic probability of 2^-40 for the same block size. Which cipher is more resistant to differential cryptanalysis?

   • ACipher Y, because its maximum characteristic probability is lower, making the bias harder to exploit.
   • BBoth are equally resistant because they have the same block size.
   • CNeither is resistant because both have probabilities above 2^-64.
   • DCipher X, because its maximum characteristic probability is higher, making it more secure.
   Reveal Solution Discussion Select an answer first
4. 8foundation · easy

   In constructing a differential characteristic for a simple block cipher, what does the characteristic specify?

   • AThe exact key values used in each round.
   • BThe sequence of input and output differences for each round of the cipher.
   • CThe S-box lookup table of the cipher.
   • DThe plaintext and ciphertext pairs used in the attack.
   Reveal Solution Discussion Select an answer first
5. 9foundation · easy

   What is the core idea of differential cryptanalysis?

   • AIt compares the ciphertext with the plaintext to find the key directly.
   • BIt tries all possible keys until the plaintext is found.
   • CIt uses the frequency of letters in the ciphertext to guess the key.
   • DIt examines how differences between two plaintexts propagate through the cipher to reveal information about the key.
   Reveal Solution Discussion Select an answer first
6. 10application · medium

   An attacker is performing a differential attack on a block cipher. The attacker has a differential characteristic with probability p = 2^-20 for a 64-bit block cipher. The attacker collects N plaintext-ciphertext pairs with the required input difference. How many pairs are approximately needed to reliably identify the correct key candidate?

   • AAbout 2^20 pairs
   • BAbout 2^32 pairs
   • CAbout 2^64 pairs
   • DAbout 2^10 pairs
   Reveal Solution Discussion Select an answer first