Codebusters Test Codebusters Polyalphabetic Ciphers 2 — Questions and Answers
Question 1: The autokey cipher improves on the Vigenère cipher by doing what after the initial keyword is used?
- Using the plaintext itself as the continuing key (Correct answer)
- Repeating the keyword indefinitely
- Switching to a transposition cipher
- Reversing the keyword
Correct answer: Using the plaintext itself as the continuing key
In the autokey cipher, after the keyword letters are exhausted, the plaintext letters themselves serve as the subsequent key, preventing simple repetition that the Kasiski test exploits.
Question 2: In the Vigenère tableau, to encipher plaintext letter 'M' with key letter 'D', which row and column are used?
- Row D, column M (or equivalently row M, column D) (Correct answer)
- Row A, column M+D
- Row M only
- Column D only, counting 13 positions
Correct answer: Row D, column M (or equivalently row M, column D)
In the Vigenère square, the key letter selects the row (D) and the plaintext letter selects the column (M); their intersection gives the ciphertext letter.
Question 3: Which 19th-century cryptanalyst is credited with publishing the method for breaking the Vigenère cipher using repeated sequence analysis?
- Friedrich Kasiski (Correct answer)
- Charles Babbage
- William Friedman
- Blaise de Vigenère
Correct answer: Friedrich Kasiski
Friedrich Kasiski published the systematic method in 1863, though Charles Babbage had independently discovered it earlier but never published.
Question 4: The Gronsfeld cipher is a simplified variant of the Vigenère cipher where the key is restricted to which type of characters?
- Digits 0–9 only (Correct answer)
- Letters A–Z only
- Symbols and punctuation
- Binary (0 and 1) only
Correct answer: Digits 0–9 only
The Gronsfeld cipher uses a numeric key (digits 0–9) rather than alphabetic letters, limiting each key character to a Caesar shift of 0 through 9.
Question 5: When attacking a polyalphabetic cipher of unknown key length, the index of coincidence (IC) of individual letter sets increases toward the English IC (~0.065) when you have correctly identified what?
- The correct key length, so each subset contains letters enciphered by the same shift (Correct answer)
- The most common letter in the full ciphertext
- The cipher type (transposition vs. substitution)
- The starting position of the key
Correct answer: The correct key length, so each subset contains letters enciphered by the same shift
When the assumed key length is correct, each subset of every nth letter was encrypted by the same Caesar shift, so that subset's frequency distribution resembles English and its IC rises to ~0.065.
Question 6: In Science Olympiad Codebusters, the Xenocrypt is a variant of which cipher type, but with Spanish plaintext?
- Aristocrat (monoalphabetic substitution) (Correct answer)
- Vigenère (polyalphabetic)
- Rail Fence (transposition)
- Playfair (digraphic)
Correct answer: Aristocrat (monoalphabetic substitution)
A Xenocrypt is an Aristocrat cipher—a monoalphabetic substitution—but the plaintext is in Spanish, requiring knowledge of Spanish letter frequencies rather than English ones.
Codebusters Test Codebusters Polyalphabetic Ciphers 2
The autokey cipher improves on the Vigenère cipher by doing what after the initial keyword is used?