Codebusters Test Codebusters Frequency Analysis Techniques 1 — Questions and Answers
Question 1: In English, which letter appears most frequently and is often the first target when performing frequency analysis on a monoalphabetic cipher?
- E (Correct answer)
- T
- A
- O
Correct answer: E
The letter E is the most frequent letter in English, accounting for roughly 12-13% of all letters, making it the primary target in frequency analysis.
Question 2: What is the most common two-letter combination (digraph) in English, often used as a frequency analysis clue?
- TH (Correct answer)
- ER
- AN
- IN
Correct answer: TH
TH is the most common digraph in English, appearing far more often than any other two-letter pair, and is a key clue in breaking substitution ciphers.
Question 3: When analyzing an Aristocrat cipher, a single-letter word in the ciphertext most likely represents which plaintext letter?
- A or I (Correct answer)
- E or O
- T or S
- N or R
Correct answer: A or I
In English, the only common single-letter words are 'a' and 'I', so a lone ciphertext letter almost always decodes to one of these.
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?
- Index of Coincidence (Correct answer)
- Chi-squared score
- Kasiski distance
- Entropy value
Correct answer: Index of Coincidence
The Index of Coincidence (IC) is approximately 0.065 for English monoalphabetic text and near 0.038 for random/polyalphabetic text, distinguishing the two cipher types.
Question 5: In a Patristocrat cipher (no word breaks), which technique helps identify word boundaries after partial decryption?
- Looking for repeated letter patterns that match common words (Correct answer)
- Counting every 5th letter
- Reversing the ciphertext
- Applying a Vigenère key
Correct answer: Looking for repeated letter patterns that match common words
Since Patristocrats strip spaces, solvers look for recognizable partial plaintext sequences like 'THE', 'AND', or 'ING' to infer word boundaries.
Question 6: The three-letter sequence 'THE' is the most common trigraph in English; what is the second most common trigraph?
- AND
- ING (Correct answer)
- ION
- ENT
Correct answer: ING
After 'THE', the trigraph 'ING' is the next most frequent in English, appearing as the gerund suffix in countless words.
Codebusters Test Codebusters Frequency Analysis Techniques 1
In English, which letter appears most frequently and is often the first target when performing frequency analysis on a monoalphabetic cipher?