Codebusters Test Codebusters Cipher Decryption 3 — Questions and Answers
Question 1: Using the Vigenère cipher with key 'DOG', encrypting plaintext 'CAT' gives which ciphertext?
- FOZ (Correct answer)
- CAT
- DPH
- GAW
Correct answer: FOZ
C(2)+D(3)=F, A(0)+O(14)=O, T(19)+G(6)=Z, producing FOZ.
Question 2: Which method finds repeated trigrams in Vigenère ciphertext and uses their spacing to estimate the key length?
- Kasiski examination (Correct answer)
- Index of coincidence
- Frequency analysis
- Chi-square test
Correct answer: Kasiski examination
The Kasiski examination locates repeated sequences and factors their distances to identify probable key lengths.
Question 3: In a Vigenère cipher, ciphertext letter 'T' was encrypted with key letter 'F' (index 5). What is the plaintext letter?
- O (Correct answer)
- Y
- N
- P
Correct answer: O
T is index 19; decryption subtracts the key: 19 − 5 = 14, which is the letter O.
Question 4: The index of coincidence (IC) for typical English plaintext is approximately:
- 0.065 (Correct answer)
- 0.038
- 0.100
- 0.025
Correct answer: 0.065
English has an IC of ~0.065 due to its non-uniform letter distribution, compared to ~0.038 for random text.
Question 5: Once the key length of a Vigenère cipher is known, each column of letters extracted at that interval can be solved as a separate:
- Caesar cipher (Correct answer)
- Vigenère cipher
- Transposition cipher
- Hill cipher
Correct answer: Caesar cipher
Each column uses a single fixed key letter shift, making it equivalent to a Caesar cipher.
Question 6: In the Vigenère tableau, encrypting plaintext letter 'P' (row 15) with key letter 'M' (column 12) yields which ciphertext letter?
- B (Correct answer)
- C
- X
- Y
Correct answer: B
(15 + 12) mod 26 = 27 mod 26 = 1, which corresponds to the letter B.
Question 7: A Vigenère cipher whose key is as long as the message and is never reused is equivalent to which theoretically unbreakable cipher?
- One-time pad (Correct answer)
- Running key cipher
- Perfect forward secrecy cipher
- Infinite Beaufort cipher
Correct answer: One-time pad
The one-time pad uses a random key equal in length to the message and, when used correctly, is provably unbreakable.
Codebusters Test Codebusters Cipher Decryption 3
Using the Vigenère cipher with key 'DOG', encrypting plaintext 'CAT' gives which ciphertext?