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

C(2)+D(3)=F, A(0)+O(14)=O, T(19)+G(6)=Z, producing FOZ.

Question 2

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

Accepted Answer

Kasiski examination

Answer

The Kasiski examination locates repeated sequences and factors their distances to identify probable key lengths.

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

T is index 19; decryption subtracts the key: 19 − 5 = 14, which is the letter O.

Question 4

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

Accepted Answer

0.065

Answer

English has an IC of ~0.065 due to its non-uniform letter distribution, compared to ~0.038 for random text.

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

Each column uses a single fixed key letter shift, making it equivalent to a Caesar cipher.

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