Exercise 3: The Vigenère Cipher (30%)

  Finally develop a program to implement the somewhat more sophisticated Vigenère cipher, invented in the 16th century by Blaise de Vigenère. This has the same basic idea as the Caesar cipher, but the key no longer stays the same throughout the message: instead it follows some periodic cycle. A Vigenère key is thus not a single number but a sequence of some fixed length - say 10, 3, 25, which would mean the first letter should be shifted by 10, the second by 3, the third by 25, and then back to 10 again etc.