Python Programming in Context, 3rd Edition

3.7 The Vigenère Cipher

The key to cracking a substitution cipher was first discovered by Arab scholars in the ninth century, but it was not widely known in the West until the fifteenth century. Once frequency analysis of ciphers became widespread, the plain substitution cipher became useless. To fix the problems inherent in the simple substitution cipher, Blaise de Vigenère invented the strategy of using multiple letter mappings. Rather than using one key for the whole message, Vigenère’s idea was to use a different key for each letter of the message. Using a different key for each letter makes the frequency analysis much more difficult.

The key to the Vigenère cipher is the Vigenère square, shown in TABLE 3.6. Each row in the table corresponds ...