Multiplicative Cipher
In a Multiplicative cipher, each character of the alphabet is assigned a value (starting at a zero index [A=0, B=1, etc]) and a coprime key to the length of the alphabet is chosen. Each character is multiplied with this key and the corresponding letter is substituted.
Example the letter M (12th letter in this zero indexed alphabet) and key 3 would be 12 * 3 = 36. 36 modulo 26 = 10 so the letter K would be chosen. Note: This cipher is closely related to the Affine cipher.