The Vigenère Cipher

Abstract

We saw earlier that the substitution cipher is not that secure, because of attacks based on frequency analysis. In order to combat this, one can build a stronger cipher, known as a polyalphabetic cipher, in which we change the cipher for every character.That is, if a g gets encoded as an X at one point, it does not necessarily get encoded as an X later on in the message. There are many possible ways of setting up a polyalphabetic cipher, but the most famous is the Vigenère cipher. This cipher was considered so strong that it was known as le chiffre indéchiffrable, or “the undecipherable cipher.” However, in the nineteenth century, after it had been in use for roughly 300 years, attacks against it were discovered.