Codebusters Test Codebusters Cryptography Theory and History 3

Question 1

The ENIGMA machine used by Nazi Germany in WWII is best described as which type of cipher device?

Accepted Answer

Electro-mechanical polyalphabetic rotor cipher machine

Answer

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?

Accepted Answer

Estimating the number of alphabets used in a polyalphabetic cipher

Answer

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?

Accepted Answer

The letter 'E' is the most frequent letter in English text

Answer

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:

Accepted Answer

Rearrange the positions of letters without changing them

Answer

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?

Accepted Answer

The key is truly random, used only once, and is as long as the message

Answer

When a truly random key of equal length to the plaintext is used exactly once, every possible plaintext is equally likely, providing perfect secrecy.

Codebusters Test Practice Test

Codebusters Test Practice Test

Codebusters Test Codebusters Cryptography Theory and History 3