Question 1

What is the Vigenère cipher?

Accepted Answer

The Vigenère cipher is a polyalphabetic substitution cipher that uses a keyword to shift letters. Unlike the Caesar cipher which uses a single shift, each letter in the keyword determines a different shift for the corresponding plaintext letter. This makes it much harder to break than simple substitution ciphers.

Question 2

What is Kasiski examination?

Accepted Answer

Kasiski examination is a method to determine the key length of a Vigenère cipher. It works by finding repeated sequences in the ciphertext (like XYZ appearing multiple times). The distances between these repetitions are likely multiples of the key length. Finding the GCD of these distances reveals probable key lengths.

Question 3

What is the Index of Coincidence?

Accepted Answer

The Index of Coincidence (IC) measures how likely it is that two randomly selected letters from a text are the same. English text has an IC around 0.067, while random text is around 0.038. By calculating the IC for different assumed key lengths, we can determine the actual key length - the correct length will have IC values closer to English.

Question 4

How do I break a Vigenère cipher?

Accepted Answer

First, determine the key length using Kasiski examination or IC analysis. Then, split the ciphertext into groups by key position. Each group is effectively a Caesar cipher that can be solved with frequency analysis. Finally, combine the individual key letters to reveal the full keyword.

Vigenère Cipher with Cryptanalysis

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