Frequency Analysis and Differential Cryptanalysis ECES Practice Questions (Page 6)

Questions 26–29

1. 26expert · hard

   An analyst is constructing a differential characteristic for a block cipher. The analyst has two candidate characteristics: Characteristic A has a probability of 2^-10 over 4 rounds, and Characteristic B has a probability of 2^-12 over 4 rounds. The cipher has a 32-bit block size. Which characteristic is more useful for a differential attack?

   • ANeither is useful because the probabilities are too low for a 32-bit block cipher.
   • BCharacteristic B, because it has a lower probability and is more subtle.
   • CCharacteristic A, because it has a higher probability and thus requires fewer plaintext-ciphertext pairs.
   • DBoth are equally useful because they cover the same number of rounds.
   Reveal Solution Discussion Select an answer first
2. 27foundation · easy

   Why does frequency analysis become ineffective against a polyalphabetic cipher such as the Vigenère cipher?

   • ABecause the same plaintext letter can be encrypted to different ciphertext letters depending on its position.
   • BBecause the ciphertext is always shorter than the plaintext.
   • CBecause the cipher uses a key that is longer than the plaintext.
   • DBecause the ciphertext contains only lowercase letters.
   Reveal Solution Discussion Select an answer first
4. 28application · easy

   An analyst is trying to decrypt a monoalphabetic substitution ciphertext. The analyst has identified that the ciphertext symbol 'M' appears most frequently, and the symbol 'N' appears second most frequently. Based on English letter frequencies, which plaintext letters are most likely represented by 'M' and 'N' respectively?

   • AM → A, N → O
   • BM → T, N → E
   • CM → E, N → T
   • DM → E, N → A
   Reveal Solution Discussion Select an answer first
5. 29foundation · easy

   A cryptanalyst has a ciphertext of only 20 characters encrypted with a simple substitution cipher. Why is frequency analysis unlikely to yield a reliable decryption?

   • AThe ciphertext must be at least 1000 characters for any analysis.
   • BSimple substitution ciphers are not vulnerable to frequency analysis.
   • CFrequency analysis only works on digraphs, not single letters.
   • DThe sample is too small for the observed frequencies to approximate the expected English letter frequencies.
   Reveal Solution Discussion Select an answer first