Codebusters Test Codebusters Frequency Analysis Techniques 2

Question 1

When using frequency analysis on a Caesar cipher ciphertext, what is the quickest way to find the shift value?

Accepted Answer

Find the most frequent ciphertext letter and assume it represents E

Answer

The most frequent letter in a Caesar cipher almost always represents E; the numerical difference between its position and E's position gives the shift key.

Question 2

In frequency analysis, which set of letters is known as ETAOIN SHRDLU and why is it useful?

Accepted Answer

The 12 most common English letters in order of frequency

Answer

ETAOIN SHRDLU lists the 12 most frequently occurring letters in English text in approximate descending order, serving as a quick reference for frequency analysis.

Question 3

Which letter is the most common final letter in English words, making it a useful anchor point in monoalphabetic cipher analysis?

Accepted Answer

E

Answer

The letter E is the most common word-final letter in English, often appearing as a silent terminal vowel, making it a strong anchor in frequency analysis.

Question 4

What is a 'criб' in the context of frequency analysis and cipher-breaking?

Accepted Answer

A known or guessed plaintext segment used to attack the cipher

Answer

A crib is a known or suspected plaintext word or phrase that a cryptanalyst uses to test against the ciphertext to find the encryption key.

Question 5

If a ciphertext shows very uneven letter frequency distribution (some letters very common, some absent), this suggests it was encrypted with which type of cipher?

Accepted Answer

Monoalphabetic substitution

Answer

Monoalphabetic ciphers preserve the frequency distribution of the original language, so common plaintext letters map to common ciphertext letters, creating an uneven distribution.

Codebusters Test Practice Test

Codebusters Test Practice Test

Codebusters Test Codebusters Frequency Analysis Techniques 2