Codebusters Test Codebusters Cipher Decryption 3

Question 1

Using the Vigenère cipher with key 'DOG', encrypting plaintext 'CAT' gives which ciphertext?

Accepted Answer

FOZ

Answer

CAT

Answer

DPH

Answer

GAW

Question 2

Which method finds repeated trigrams in Vigenère ciphertext and uses their spacing to estimate the key length?

Accepted Answer

Kasiski examination

Answer

Index of coincidence

Answer

Frequency analysis

Answer

Chi-square test

Question 3

In a Vigenère cipher, ciphertext letter 'T' was encrypted with key letter 'F' (index 5). What is the plaintext letter?

Accepted Answer

O

Answer

Y

Answer

N

Answer

P

Question 4

The index of coincidence (IC) for typical English plaintext is approximately:

Accepted Answer

0.065

Answer

0.038

Answer

0.100

Answer

0.025

Question 5

Once the key length of a Vigenère cipher is known, each column of letters extracted at that interval can be solved as a separate:

Accepted Answer

Caesar cipher

Answer

Vigenère cipher

Answer

Transposition cipher

Answer

Hill cipher

Question 6

In the Vigenère tableau, encrypting plaintext letter 'P' (row 15) with key letter 'M' (column 12) yields which ciphertext letter?

Accepted Answer

B

Answer

C

Answer

X

Answer

Y

Question 7

A Vigenère cipher whose key is as long as the message and is never reused is equivalent to which theoretically unbreakable cipher?

Accepted Answer

One-time pad

Answer

Running key cipher

Answer

Perfect forward secrecy cipher

Answer

Infinite Beaufort cipher