For Vigenere's Cipher the key is a keyword, repeated as shown in the presentation. Each of its letters corresponds to a different Caesar's Cipher (monoalphabetic cipher). In the example above, for the keyword CLOUD we get 5 different alphabets which derive from 2, 11, 14, 20, 3 shifts respectively (C, L, O, U, D).
You may use the following interactive tabula recta for encryption/decryption.
Start counting shifts from 0 ( i.e. 3rd shift should take you to alphabet/raw starting with letter D).
music theme: Orbital - The Saint (via YouTube)
