In an Additive cipher (aka Caesar Cipher), each character of the alphabet is assigned a value and a key number within the alphabet is chosen. Each character is added with this key number and the corresponding letter is substituted (mod 26). The values of the alphabet characters start at 0 so A = 0, B = 1, etc. Example the letter M (12th letter) and key 16 would be 12 + 16 = 28. 28 modulo 26 = 2 so the letter C would be chosen. Note: This cipher is closely related to the Affine cipher.