Codebusters Test Graduation Requirements 5

Question 1

Which cipher, required for Codebusters national-level graduation, uses a tableau of 26 shifted alphabets selected by a keyword letter?

Accepted Answer

Vigenere cipher

Answer

The Vigenere cipher uses a 26×26 tableau where each row is a Caesar shift, and the keyword letter selects which row to use for each plaintext letter.

Question 2

When a Codebusters competitor analyzes a cipher and finds an IC of approximately 0.038, what type of cipher are they most likely facing?

Accepted Answer

A Vigenere cipher with a long key

Answer

An IC near 0.038 (close to random English text IC) suggests a polyalphabetic cipher like Vigenere with a long key that flattens frequency distributions.

Question 3

The Playfair cipher, a graduation requirement at higher Codebusters levels, encrypts plaintext in units of how many letters?

Accepted Answer

Digrams (pairs)

Answer

Playfair encrypts plaintext two letters at a time (digrams), using a 5×5 keyword square to transform each pair according to their geometric relationship.

Question 4

For Codebusters graduation requirements, what is a 'null' in the context of cipher messages?

Accepted Answer

A meaningless character inserted to disguise message length or structure

Answer

Nulls are meaningless characters (letters, numbers, or symbols) inserted into ciphertext to confuse cryptanalysts about the true message length.

Question 5

In Codebusters competition rules, what does the term 'keyword alphabet' mean for a substitution cipher?

Accepted Answer

An alphabet where a keyword is written first, then unused letters follow in order

Answer

A keyword alphabet writes the keyword (removing duplicate letters) at the start, then appends remaining alphabet letters in order to create the cipher substitution alphabet.

Codebusters Test Practice Test

Codebusters Test Practice Test

Codebusters Test Graduation Requirements 5