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

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?

Accepted Answer

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

Answer

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?

Accepted Answer

Friedrich Kasiski

Answer

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?

Accepted Answer

Digits 0–9 only

Answer

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?

Accepted Answer

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

Answer

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.

Codebusters Test Practice Test

Codebusters Test Practice Test

Codebusters Test Codebusters Polyalphabetic Ciphers 2