Codebusters Test Codebusters Frequency Analysis Techniques 2 — Questions and Answers
Question 1: When using frequency analysis on a Caesar cipher ciphertext, what is the quickest way to find the shift value?
- Find the most frequent ciphertext letter and assume it represents E (Correct answer)
- Count all vowels
- Divide the ciphertext length by 26
- Look for double letters
Correct answer: Find the most frequent ciphertext letter and assume it represents E
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?
- The 12 most common English letters in order of frequency (Correct answer)
- A secret military code
- The first two rows of a Dvorak keyboard
- Letters that never appear in ciphertext
Correct answer: The 12 most common English letters in order of frequency
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?
- E (Correct answer)
- S
- T
- D
Correct answer: E
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?
- A known or guessed plaintext segment used to attack the cipher (Correct answer)
- A table of letter frequencies
- A type of polyalphabetic cipher
- The key length of a Vigenère cipher
Correct answer: A known or guessed plaintext segment used to attack the cipher
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?
- Monoalphabetic substitution (Correct answer)
- Polyalphabetic substitution
- Transposition
- One-time pad
Correct answer: Monoalphabetic substitution
Monoalphabetic ciphers preserve the frequency distribution of the original language, so common plaintext letters map to common ciphertext letters, creating an uneven distribution.
Question 6: In the Aristocrat cipher used in Science Olympiad Codebusters, a two-letter ciphertext word ending in a repeated letter (e.g., 'XZ' where X=Z is not the case, but a word like 'AA') most likely represents which English word?
- No two-letter English word has doubled letters; it signals a proper noun or error (Correct answer)
- IT
- OF
- TO
Correct answer: No two-letter English word has doubled letters; it signals a proper noun or error
No standard two-letter English word uses a doubled letter, so such a pattern in an Aristocrat suggests a proper noun, abbreviation, or encipherment error that solvers must flag.
Codebusters Test Codebusters Frequency Analysis Techniques 2
When using frequency analysis on a Caesar cipher ciphertext, what is the quickest way to find the shift value?