Codebusters Test Curriculum Framework 4

Question 1

In Codebusters, the Affine cipher encrypts a letter with value x using the formula E(x) = (ax + b) mod 26. What constraint must 'a' satisfy?

Accepted Answer

'a' must be coprime to 26 (gcd(a,26) = 1)

Answer

The multiplier 'a' in an affine cipher must be coprime to 26 to ensure every letter maps to a unique ciphertext letter and decryption is possible.

Question 2

During a Codebusters competition, a 'xenocrypt' is a cipher where:

Accepted Answer

The plaintext is in a foreign language, typically Spanish

Answer

A xenocrypt in Codebusters is a monoalphabetic substitution cipher whose plaintext is in a non-English language, most commonly Spanish.

Question 3

Which Codebusters cipher type uses a tableau of 26 shifted Caesar alphabets and a keyword to determine which row to use for each letter?

Accepted Answer

Vigenère cipher

Answer

The Vigenère cipher uses a 26x26 tableau where each row is a Caesar shift, and the keyword letter selects which row encrypts each plaintext letter.

Question 4

In a route cipher transposition, what distinguishes it from a standard columnar transposition?

Accepted Answer

Route ciphers read the grid along a specified geometric path (spiral, diagonal, etc.)

Answer

A route cipher fills a grid with plaintext and then reads it out along a specified route such as a spiral, diagonal, or zigzag pattern.

Question 5

What does the term 'digraph frequency analysis' help Codebusters solvers detect?

Accepted Answer

Common two-letter combinations like TH, HE, and IN that persist in substitution ciphers

Answer

Digraph frequency analysis examines pairs of letters; in English substitution ciphers, frequent ciphertext digraphs often correspond to common plaintext pairs like TH, HE, or IN.

Codebusters Test Practice Test

Codebusters Test Practice Test

Codebusters Test Curriculum Framework 4