Codebusters Test Curriculum Framework 5

Question 1

In the Codebusters event, which of the following best describes a 'running key cipher'?

Accepted Answer

A polyalphabetic cipher that uses a long meaningful text (like a book passage) as the key

Answer

A running key cipher is like a Vigenère cipher but uses a long, non-repeating text (such as a book excerpt) as the key, making frequency analysis much harder.

Question 2

A Codebusters problem uses a Beaufort cipher. How does the Beaufort cipher differ from the standard Vigenère cipher?

Accepted Answer

The Beaufort cipher subtracts the plaintext from the key (C = K − P mod 26) rather than adding

Answer

The Beaufort cipher uses the formula C = (K − P) mod 26 instead of the Vigenère's C = (P + K) mod 26, making the cipher its own inverse (self-reciprocal).

Question 3

In Codebusters, when performing frequency analysis on a short ciphertext, why might observed frequencies differ significantly from expected English frequencies?

Accepted Answer

Short texts have too few samples for statistical frequencies to match theoretical values reliably

Answer

With few letters, random variation dominates — a short text may not contain all letters proportionally, making frequency analysis less reliable.

Question 4

In the Codebusters curriculum, what is the purpose of an 'autokey cipher'?

Accepted Answer

It extends the key by appending the plaintext itself after the initial keyword

Answer

An autokey cipher starts with a short primer keyword and then uses the plaintext letters themselves as the continuing key, avoiding the short-period weakness of the Vigenère.

Question 5

Which Codebusters cipher is also known as the 'Caesar box' or 'Cardano grille' and involves a cut-out mask placed over ciphertext?

Accepted Answer

Transposition grille cipher

Answer

A grille cipher uses a physical or virtual mask with holes; when placed over the ciphertext grid, only the hidden message letters show through the cutouts.

Codebusters Test Practice Test

Codebusters Test Practice Test

Codebusters Test Curriculum Framework 5