Codebusters Test Data-Driven Instruction 2 — Questions and Answers
Question 1: In English plaintext, which letter has the highest frequency of occurrence, making it the primary target in frequency analysis attacks?
- A
- E (Correct answer)
- I
- O
Correct answer: E
The letter E appears most frequently in English text, comprising roughly 13% of all letters.
Question 2: A Codebusters team notices that in a ciphertext, the digraph 'XQ' appears 8 times out of 200 digraph occurrences. What does this high frequency most likely suggest?
- XQ represents a rare English consonant cluster
- XQ likely represents the plaintext digraph 'TH' (Correct answer)
- XQ is a random coincidence with no significance
- XQ represents a number rather than letters
Correct answer: XQ likely represents the plaintext digraph 'TH'
The digraph 'TH' is the most common two-letter combination in English, so a high-frequency ciphertext digraph likely maps to it.
Question 3: When analyzing a substitution cipher, a student counts letter frequencies and finds three letters tied near 8% each. What does this pattern most likely indicate?
- The cipher is a Vigenère cipher
- The plaintext letters A, O, and I are likely involved (Correct answer)
- The ciphertext has been tampered with
- The cipher uses a keyword of length 3
Correct answer: The plaintext letters A, O, and I are likely involved
In English, the vowels A, O, and I cluster near 8% frequency, so tied letters near that rate suggest these vowels.
Question 4: A teacher uses a data-driven approach to identify that students consistently miss questions about the Index of Coincidence. What does the Index of Coincidence measure?
- The total number of repeated letters in a message
- The probability that two randomly chosen letters from a text are identical (Correct answer)
- The ratio of vowels to consonants in a cipher
- The key length of a transposition cipher
Correct answer: The probability that two randomly chosen letters from a text are identical
The Index of Coincidence (IC) measures the probability that two randomly selected letters from the text are the same, helping distinguish monoalphabetic from polyalphabetic ciphers.
Question 5: A Codebusters coach reviews test data showing students score poorly on aristocrat ciphers with apostrophes. What unique information do apostrophes provide?
- They always precede the letter E
- They indicate a word boundary before a possessive or contraction, narrowing possible plaintext letters (Correct answer)
- They mark the start of a new sentence
- They indicate a number follows
Correct answer: They indicate a word boundary before a possessive or contraction, narrowing possible plaintext letters
Apostrophes in aristocrats signal contractions or possessives, constraining which letters can follow (e.g., 'T in can't, 'S in possessives).
Question 6: After tracking student errors, a coach finds that students misidentify monoalphabetic vs. polyalphabetic ciphers. Which IC value range is consistent with a monoalphabetic substitution cipher of English text?
- Around 0.038 (near random)
- Around 0.065 (near natural English) (Correct answer)
- Around 0.500 (near maximum)
- Around 0.010 (near minimum)
Correct answer: Around 0.065 (near natural English)
English monoalphabetic ciphers preserve the original letter frequency distribution, yielding an IC near 0.065, similar to normal English text.
Question 7: A data analysis of past Codebusters tests shows that the letter pair 'NN' almost never appears in valid English plaintext. How should solvers use this insight when testing a proposed decryption?
- Accept the decryption as correct if NN appears
- Reject or revise the key mapping if NN appears in the result (Correct answer)
- Ignore doubled letters entirely during analysis
- Treat NN as a space character
Correct answer: Reject or revise the key mapping if NN appears in the result
Since 'NN' is extremely rare in English, its presence in a trial decryption is a strong signal that the current key mapping is incorrect.
Codebusters Test Data-Driven Instruction 2
In English plaintext, which letter has the highest frequency of occurrence, making it the primary target in frequency analysis attacks?