Introduction to Cryptography with Coding Theory, 3rd Edition

2.3 The Vigenère Cipher

A variation of the shift cipher was invented back in the sixteenth century. It is often attributed to Vigenère, though Vigenère’s encryption methods were more sophisticated. Well into the twentieth century, this cryptosystem was thought by many to be secure, though Babbage and Kasiski had shown how to attack it during the nineteenth century. In the 1920s, Friedman developed additional methods for breaking this and related ciphers.

The key for the encryption is a vector, chosen as follows. First choose a key length, for example, 6. Then choose a vector of this size whose entries are integers from 0 to 25, for example $k=(21\text{,}4\text{,}2\text{,}19\text{,}14\text{,}17)$. Often the key corresponds to a word that is easily remembered. In our case, the ...