Affine Cipher is a type of monoalphabetic substitution cipher, where each letter in an alphabet is mapped to its numeric equivalent, encrypted using a simple mathematical function, and converted back to a letter. The formula used means that each letter encrypts to one other letter, and back again, meaning the cipher is essentially a standard substitution cipher with a rule governing which letter goes to which. As such, it has the weaknesses of all substitution ciphers. It encrypts a text using an affine function (f(x) = ax + b). [Wikipedia]

How it works?

Firstly, each character of the initial text (message to encrypt) is converted in a number from 0 to 25, corresponding to its position in the Latin alphabet which contains 26 letters --> (a = 0, b = 1 ... z = 25 ).

Then, each number obtained is transformed by an affine function (f(x) = ax + b). "x" is representing the number while "a" and "b" are defined during the encryption. "a" and "b" are the keys required to decrypt the final message.

If we take all the images and put them in a list, we obtain n numbers corresponding to n characters of the initial text. The next step consists in finding the values of modulo 26 of each number. (Modulo means remainder)

Example : Modulo 4 of 19 is 3 because 15 = 4 * 4 + 3 In the other hand, modulo 26 of 26 is 0 because 26 = 26 * 1 + 0

Therefore, we obtain a new list with n element, each between 0 and 25 both included. All these numbers are converted in letters of the Latin Alphabet using the tables below.

We finally create the final message by putting all the letters side by side.

In this example showing encryption and decryption, the alphabet is going to be the letters A through Z, and will have the corresponding values found in the following table.