What is the Vigenere Cipher?
Vigenere is a symmetric, polyalphabetic substituion cipher. The “substitution” part of that label means that it encrypts by replacing characters. The “polyalphabetic” part of that label means that it uses a different ciphertext alphabet for each character it encrypts – until it runs out of alphabets & re-uses the first one again. The “symmetric” part means that it uses the same key for encryption & decryption. Here’s an example: • Let the plaintext be “hello world”. (The plaintext alphabet is all twenty-six case-insensitive letters written as lower case. The ciphertext alphabet is all twenty-six case-insensitive letters written as upper case.) • Let the encryption key be “vig” (as in Vigenere). • The first character from the plaintext, “h”, is encrypted with the first character from the key, “v”. H is letter number seven from the alphabet, V is the 22nd letter in the alphabet, so we add the 21 from V to the seven from H, giving us 28. We map 28 back into the alphabet with
