Codebusters Test Data-Driven Instruction 2

Question 1

In English plaintext, which letter has the highest frequency of occurrence, making it the primary target in frequency analysis attacks?

Accepted Answer

E

Answer

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?

Accepted Answer

XQ likely represents the plaintext digraph 'TH'

Answer

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?

Accepted Answer

The plaintext letters A, O, and I are likely involved

Answer

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?

Accepted Answer

The probability that two randomly chosen letters from a text are identical

Answer

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?

Accepted Answer

They indicate a word boundary before a possessive or contraction, narrowing possible plaintext letters

Answer

Apostrophes in aristocrats signal contractions or possessives, constraining which letters can follow (e.g., 'T in can't, 'S in possessives).

Codebusters Test Practice Test

Codebusters Test Practice Test

Codebusters Test Data-Driven Instruction 2