Codebusters Test Codebusters Polyalphabetic Ciphers 1 — Questions and Answers
Question 1: The Vigenère cipher is described as polyalphabetic because it uses how many different substitution alphabets?
- One per letter of the keyword, cycling through them (Correct answer)
- Exactly 26, one for each letter
- Only 2, alternating
- A single alphabet shifted differently each time by random amounts
Correct answer: One per letter of the keyword, cycling through them
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?
- Repeated ciphertext sequences and measuring distances between them to find key length (Correct answer)
- Single-letter ciphertext words
- Double-letter patterns
- The index of coincidence
Correct answer: Repeated ciphertext sequences and measuring distances between them to find key length
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?
- Frequency analysis on every nth letter (where n is the key length) (Correct answer)
- Brute-force all possible keys
- Columnar transposition
- XOR of adjacent letters
Correct answer: Frequency analysis on every nth letter (where n is the key length)
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?
- The key letter is subtracted from the plaintext (reciprocal/involutory), not added (Correct answer)
- It uses a 25-letter alphabet
- It requires two keywords
- It applies a columnar transposition after substitution
Correct answer: The key letter is subtracted from the plaintext (reciprocal/involutory), not added
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?
- Columnar transposition (Correct answer)
- Vigenère
- Baconian
- Aristocrat
Correct answer: Columnar transposition
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.
Question 6: The running key cipher is similar to the Vigenère cipher but theoretically unbreakable if the key is what?
- As long as the message and chosen from a non-repeating, random source (Correct answer)
- A single letter repeated
- A common English word
- Exactly 13 characters long
Correct answer: As long as the message and chosen from a non-repeating, random source
When the key is as long as the message and truly random (a one-time pad principle), there is no repetition for Kasiski or IC analysis to exploit, making it theoretically unbreakable.
Codebusters Test Codebusters Polyalphabetic Ciphers 1
The Vigenère cipher is described as polyalphabetic because it uses how many different substitution alphabets?