Codebusters Test Codebusters Frequency Analysis Techniques

Question 1

In English, which letter appears most frequently and is often the first target when performing frequency analysis on a monoalphabetic cipher?

Accepted Answer

E

Answer

T

Answer

A

Answer

O

Question 2

What is the most common two-letter combination (digraph) in English, often used as a frequency analysis clue?

Accepted Answer

TH

Answer

ER

Answer

AN

Answer

IN

Question 3

When analyzing an Aristocrat cipher, a single-letter word in the ciphertext most likely represents which plaintext letter?

Accepted Answer

A or I

Answer

E or O

Answer

T or S

Answer

N or R

Question 4

Which statistic measures the probability that two randomly chosen letters from a ciphertext are the same, helping identify whether a cipher is monoalphabetic or polyalphabetic?

Accepted Answer

Index of Coincidence

Answer

Chi-squared score

Answer

Kasiski distance

Answer

Entropy value

Question 5

In a Patristocrat cipher (no word breaks), which technique helps identify word boundaries after partial decryption?

Accepted Answer

Looking for repeated letter patterns that match common words

Answer

Counting every 5th letter

Answer

Reversing the ciphertext

Answer

Applying a Vigenère key

Question 6

The three-letter sequence 'THE' is the most common trigraph in English; what is the second most common trigraph?

Accepted Answer

ING

Answer

AND

Answer

ION

Answer

ENT