Classical Ciphers (Caesar, Vigenere, Playfair, ADFGVX) ECES Practice Questions (Page 2)

Questions 6–10

1. 6foundation · easy

   Using the Vigenere cipher with keyword 'KEY' (K=10, E=4, Y=24), what is the ciphertext for the plaintext 'DOG'?

   • ANIC
   • BNSE
   • CNSG
   • DMSE
   Reveal Solution Discussion Select an answer first
2. 7foundation · easy

   In the ADFGVX cipher, what is the output of the Polybius square substitution stage?

   • AA string consisting only of the letters A, D, F, G, V, and X.
   • BA string of digraphs from a 5x5 Playfair square.
   • CA string of letters shifted by a keyword-derived value.
   • DA string of numbers representing the row and column of each plaintext character.
   Reveal Solution Discussion Select an answer first
4. 8expert · hard

   A security analyst is comparing the Playfair cipher and the Vigenere cipher for a training exercise. She notes that both were considered significant improvements over simple substitution ciphers. Which statement correctly contrasts their cryptanalytic resistance?

   • ABoth are equally vulnerable to simple frequency analysis
   • BVigenere is vulnerable to Kasiski examination, while Playfair is vulnerable to digraph frequency analysis
   • CPlayfair is vulnerable to Kasiski examination, while Vigenere is vulnerable to digraph frequency analysis
   • DNeither cipher can be broken without knowing the key
   Reveal Solution Discussion Select an answer first
5. 9foundation · easy

   What is the purpose of the key square in the Playfair cipher?

   • AIt is a list of substitution pairs that map each plaintext letter to a ciphertext letter.
   • BIt is a 6x6 grid of letters and digits used to map each plaintext character to a coordinate pair.
   • CIt is a 5x5 grid of letters used to determine the substitution for each pair of plaintext letters.
   • DIt is a table of shift values derived from a keyword.
   Reveal Solution Discussion Select an answer first
6. 10expert · hard

   A cryptanalyst has a Vigenere ciphertext and has determined the key length to be 4 using Kasiski examination. She now wants to recover the key. Which approach should she take?

   • APerform a brute-force search over all possible 4-letter keys
   • BUse the index of coincidence on the entire ciphertext to confirm the key length
   • CSplit the ciphertext into 4 columns (one for each key position) and perform frequency analysis on each column to find the Caesar shift
   • DApply a known-plaintext attack using the word 'THE'
   Reveal Solution Discussion Select an answer first