Codebusters Test Codebusters Cryptography Theory and History 2 — Questions and Answers
Question 1: The Vigenère cipher is classified as which type of cipher?
- Monoalphabetic substitution
- Polyalphabetic substitution (Correct answer)
- Transposition
- Block cipher
Correct answer: Polyalphabetic substitution
The Vigenère cipher uses a keyword to apply multiple Caesar shifts, making it polyalphabetic.
Question 2: Which cipher replaces each letter with another letter using a fixed, scrambled alphabet?
- Rail Fence cipher
- Playfair cipher
- Simple substitution cipher (Correct answer)
- Running key cipher
Correct answer: Simple substitution cipher
A simple substitution cipher maps each plaintext letter to a unique ciphertext letter using a fixed scrambled alphabet.
Question 3: The Playfair cipher encrypts text using which unit?
- Single letters
- Digraphs (pairs of letters) (Correct answer)
- Trigraphs (triplets of letters)
- Entire words
Correct answer: Digraphs (pairs of letters)
The Playfair cipher encrypts pairs of letters (digraphs) using a 5×5 key square.
Question 4: What is the key space size of a simple substitution cipher using the 26-letter English alphabet?
- 26
- 26²
- 26! (Correct answer)
- 2²⁶
Correct answer: 26!
There are 26! possible permutations of 26 letters, giving a key space of about 4×10²⁶.
Question 5: Which historical figure is credited with inventing the cipher that bears his name and was used widely in the Roman military?
- Augustus Caesar
- Julius Caesar (Correct answer)
- Nero
- Cicero
Correct answer: Julius Caesar
Julius Caesar used a shift cipher (now called the Caesar cipher) to protect military messages.
Question 6: In the context of ciphers, what does 'key length' affect in the Vigenère cipher?
- The number of letters encrypted per second
- The length of the ciphertext
- The period of the repeating shift pattern (Correct answer)
- The size of the alphabet used
Correct answer: The period of the repeating shift pattern
The key length determines the period of the Vigenère cipher; a longer key creates a longer, harder-to-detect repeating pattern.
Question 7: Which attack method uses the index of coincidence to determine the key length of a Vigenère cipher?
- Brute force attack
- Kasiski examination (Correct answer)
- Dictionary attack
- Meet-in-the-middle attack
Correct answer: Kasiski examination
The Kasiski examination finds repeated ciphertext sequences to estimate the Vigenère key length.
Codebusters Test Codebusters Cryptography Theory and History 2
The Vigenère cipher is classified as which type of cipher?