The Big Book of Small Python Projects

#80 Vigenère Cipher

The Vigenère cipher, misattributed to 19th-century cryptographer Blaise de Vigenère (others had independently invented it earlier), was impossible to crack for hundreds of years. It is essentially the Caesar cipher, except it makes use of a multipart key. The so-called Vigenère key is a word, or even a random series of letters. Each letter represents a number by which to shift the letter in the message: A represents shifting a letter in the message by 0, B represents 1, C represents 2, and so on.

For example, if a Vigenère key is the word “CAT,” the C represents a shift of 2, the A represents 0, and the T represents 19. ...