Codebusters Test Codebusters Cryptography Theory and History 3 — Questions and Answers
Question 1: The ENIGMA machine used by Nazi Germany in WWII is best described as which type of cipher device?
- Simple substitution machine
- Electro-mechanical polyalphabetic rotor cipher machine (Correct answer)
- One-time pad generator
- Transposition rail fence device
Correct answer: Electro-mechanical polyalphabetic rotor cipher machine
ENIGMA used a series of rotating electro-mechanical rotors to perform a complex, ever-changing polyalphabetic substitution.
Question 2: What is the Index of Coincidence (IC) primarily used for in cryptanalysis?
- Counting the total number of letters in a ciphertext
- Estimating the number of alphabets used in a polyalphabetic cipher (Correct answer)
- Measuring the encryption speed of a cipher
- Determining the plaintext language
Correct answer: Estimating the number of alphabets used in a polyalphabetic cipher
The IC measures the probability of two randomly chosen letters being the same, helping analysts distinguish monoalphabetic from polyalphabetic ciphers.
Question 3: Which frequency analysis fact is most useful when attacking English monoalphabetic substitution ciphers?
- The letter 'Z' is always encrypted as 'Q'
- The letter 'E' is the most frequent letter in English text (Correct answer)
- Vowels are never adjacent in English
- Consonants appear more often than vowels
Correct answer: The letter 'E' is the most frequent letter in English text
The letter 'E' accounts for roughly 12.7% of letters in typical English text, making it the most useful starting point for frequency analysis.
Question 4: A transposition cipher differs from a substitution cipher in that transposition ciphers:
- Replace each letter with a number
- Rearrange the positions of letters without changing them (Correct answer)
- Use two alphabets simultaneously
- Always require a numeric key
Correct answer: Rearrange the positions of letters without changing them
Transposition ciphers keep the original letters but change their order, while substitution ciphers change the letters themselves.
Question 5: What property makes a one-time pad theoretically unbreakable?
- It uses very long key words
- The key is truly random, used only once, and is as long as the message (Correct answer)
- It encrypts letters in blocks of 64 bits
- It applies three rounds of substitution
Correct answer: The key is truly random, used only once, and is as long as the message
When a truly random key of equal length to the plaintext is used exactly once, every possible plaintext is equally likely, providing perfect secrecy.
Question 6: The Rail Fence cipher is an example of which cipher category?
- Polyalphabetic substitution
- Monoalphabetic substitution
- Transposition (Correct answer)
- Stream cipher
Correct answer: Transposition
The Rail Fence cipher writes plaintext diagonally across a number of 'rails' and reads off rows, rearranging letter positions without substitution.
Question 7: Which mathematical concept underlies modular arithmetic used in cipher shift calculations?
- Prime factorization
- The remainder after division by the alphabet size (Correct answer)
- Logarithmic scaling
- Matrix inversion
Correct answer: The remainder after division by the alphabet size
Cipher shifts use modular arithmetic where letters wrap around; for a 26-letter alphabet, position 27 becomes position 1 (27 mod 26 = 1).
Codebusters Test Codebusters Cryptography Theory and History 3
The ENIGMA machine used by Nazi Germany in WWII is best described as which type of cipher device?