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

A

Answer

I

Answer

O

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

XQ represents a rare English consonant cluster

Answer

XQ is a random coincidence with no significance

Answer

XQ represents a number rather than letters

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

The cipher is a Vigenère cipher

Answer

The ciphertext has been tampered with

Answer

The cipher uses a keyword of length 3

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 total number of repeated letters in a message

Answer

The ratio of vowels to consonants in a cipher

Answer

The key length of a transposition cipher

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

They always precede the letter E

Answer

They mark the start of a new sentence

Answer

They indicate a number follows

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?

Accepted Answer

Around 0.065 (near natural English)

Answer

Around 0.038 (near random)

Answer

Around 0.500 (near maximum)

Answer

Around 0.010 (near minimum)

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?

Accepted Answer

Reject or revise the key mapping if NN appears in the result

Answer

Accept the decryption as correct if NN appears

Answer

Ignore doubled letters entirely during analysis

Answer

Treat NN as a space character