Classical Ciphers (Caesar, Vigenere, Playfair, ADFGVX) ECES Practice Questions (Page 7)

Questions 31–35

1. 31expert · hard

   A security researcher is analyzing a set of classical ciphertexts. One ciphertext has a very flat frequency distribution, while another has a frequency distribution that closely matches English. The researcher suspects the first is Vigenere and the second is Playfair. Which cryptanalytic method is most appropriate for the first ciphertext?

   • AFrequency analysis on single letters
   • BDigraph frequency analysis
   • CKasiski examination to determine the key length
   • DBrute force all possible keywords
   Reveal Solution Discussion Select an answer first
2. 32application · medium

   A security trainer is demonstrating why monoalphabetic ciphers are weak. She encrypts the phrase 'ATTACK AT DAWN' with a Caesar shift of 3, producing 'DWWDFN DW GDZQ'. She then asks the class to propose a stronger classical cipher that still uses a keyword but resists simple frequency analysis. Which cipher should she demonstrate next?

   • AADFGVX cipher with a 6x6 Polybius square
   • BVigenere cipher with keyword 'KEY'
   • CPlayfair cipher with key square built from 'MONARCHY'
   • DCaesar cipher with shift 5
   Reveal Solution Discussion Select an answer first
4. 33expert · hard

   A cryptanalyst is trying to break a Vigenere cipher. She has determined the key length is 5 using Kasiski examination. What is her next step?

   • AApply a Caesar shift of 5 to the entire ciphertext
   • BPerform frequency analysis on each of the 5 Caesar-shifted columns
   • CTry all possible 5-letter keywords
   • DUse the Playfair decryption rules
   Reveal Solution Discussion Select an answer first
5. 34expert · hard

   A cryptographer is comparing the security of Playfair and Vigenere for a historical encryption demonstration. She notes that Playfair encrypts digraphs, which hides single-letter frequencies, but Vigenere is generally considered stronger. Which statement best explains why Vigenere is considered stronger than Playfair?

   • APlayfair is vulnerable to known-plaintext attacks, while Vigenere is not
   • BVigenere uses a 6x6 Polybius square, increasing the keyspace
   • CVigenere uses a longer key, making brute force infeasible
   • DVigenere creates a polyalphabetic substitution that flattens frequency distributions more effectively than Playfair's digraph substitution
   Reveal Solution Discussion Select an answer first
6. 35foundation · easy

   Which cryptanalysis technique is specifically effective against the Vigenere cipher by detecting the length of the keyword?

   • ABrute force attack
   • BKnown-plaintext attack
   • CFrequency analysis
   • DKasiski examination
   Reveal Solution Discussion Select an answer first