Codebusters Test Codebusters Polyalphabetic Ciphers

Question 1

The Vigenère cipher is described as polyalphabetic because it uses how many different substitution alphabets?

Accepted Answer

One per letter of the keyword, cycling through them

Answer

The Vigenère cipher cycles through as many Caesar alphabets as there are letters in the keyword, so a 5-letter keyword uses 5 different shifted alphabets in rotation.

Question 2

The Kasiski test helps break a Vigenère cipher by looking for what?

Accepted Answer

Repeated ciphertext sequences and measuring distances between them to find key length

Answer

The Kasiski test identifies repeated trigrams or longer sequences in the ciphertext; the distances between repetitions tend to be multiples of the key length.

Question 3

Once the key length of a Vigenère cipher is determined, each position's letters can be solved using which technique?

Accepted Answer

Frequency analysis on every nth letter (where n is the key length)

Answer

With key length n known, every nth letter was enciphered with the same Caesar shift, so frequency analysis on that subset reveals the shift for each key position.

Question 4

In the Beaufort cipher, how does encipherment differ from the standard Vigenère cipher?

Accepted Answer

The key letter is subtracted from the plaintext (reciprocal/involutory), not added

Answer

The Beaufort cipher computes ciphertext as (key − plaintext) mod 26, making it self-reciprocal: the same operation both enciphers and deciphers.

Question 5

Which Science Olympiad Codebusters cipher type involves a keyword-based columnar arrangement to encipher text rather than substituting individual letters?

Accepted Answer

Columnar transposition

Answer

Columnar transposition rearranges plaintext letters by writing them in rows and reading off columns in a keyword-determined order, changing letter positions rather than identities.

Codebusters Test Practice Test

Codebusters Test Practice Test

Codebusters Test Codebusters Polyalphabetic Ciphers