Codebusters Test Codebusters Polyalphabetic Ciphers 2

Question 1

The autokey cipher improves on the Vigenère cipher by doing what after the initial keyword is used?

Accepted Answer

Using the plaintext itself as the continuing key

Answer

Repeating the keyword indefinitely

Answer

Switching to a transposition cipher

Answer

Reversing the keyword

Question 2

In the Vigenère tableau, to encipher plaintext letter 'M' with key letter 'D', which row and column are used?

Accepted Answer

Row D, column M (or equivalently row M, column D)

Answer

Row A, column M+D

Answer

Row M only

Answer

Column D only, counting 13 positions

Question 3

Which 19th-century cryptanalyst is credited with publishing the method for breaking the Vigenère cipher using repeated sequence analysis?

Accepted Answer

Friedrich Kasiski

Answer

Charles Babbage

Answer

William Friedman

Answer

Blaise de Vigenère

Question 4

The Gronsfeld cipher is a simplified variant of the Vigenère cipher where the key is restricted to which type of characters?

Accepted Answer

Digits 0–9 only

Answer

Letters A–Z only

Answer

Symbols and punctuation

Answer

Binary (0 and 1) only

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?

Accepted Answer

The correct key length, so each subset contains letters enciphered by the same shift

Answer

The most common letter in the full ciphertext

Answer

The cipher type (transposition vs. substitution)

Answer

The starting position of the key

Question 6

In Science Olympiad Codebusters, the Xenocrypt is a variant of which cipher type, but with Spanish plaintext?

Accepted Answer

Aristocrat (monoalphabetic substitution)

Answer

Vigenère (polyalphabetic)

Answer

Rail Fence (transposition)

Answer

Playfair (digraphic)