Codebusters Test Learning Standards 4

Question 1

What does the term 'keyword alphabet' mean in the context of monoalphabetic substitution?

Accepted Answer

A cipher alphabet built by writing a keyword first, then the remaining unused letters

Answer

A keyword alphabet is created by writing the non-repeating letters of a keyword followed by the remaining alphabet letters in order.

Question 2

Which of the following best describes a polyalphabetic cipher?

Accepted Answer

A cipher that uses multiple substitution alphabets, switching between them

Answer

Polyalphabetic ciphers such as Vigenère use different substitution alphabets for different positions in the plaintext.

Question 3

If a ciphertext has an IC close to 0.065, what does this suggest about the cipher?

Accepted Answer

It is likely a monoalphabetic substitution cipher

Answer

English text has an IC near 0.065; a ciphertext with this IC likely uses monoalphabetic substitution since letter frequencies are preserved.

Question 4

In a Caesar cipher, if the letter 'X' is enciphered to 'A', what is the shift value?

Accepted Answer

3

Answer

X is the 24th letter (0-indexed: 23); A is the 1st (0-indexed: 0); shifting 23+3=26≡0, so the shift is 3.

Question 5

What distinguishes a 'K1' alphabet from a 'K2' alphabet in a Codebusters Aristocrat?

Accepted Answer

K1 uses a keyword to build the cipher alphabet; K2 uses a keyword to build the plain alphabet

Answer

In K1 the keyword scrambles the cipher alphabet; in K2 the keyword scrambles the plaintext alphabet, producing different lookup tables.

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