adds affine cipher

README.md

@@ -22,7 +22,7 @@ Algorithms from the book Cryptography & Network Security ~Behrouz A. Forouzan. S
 1. Monoalphabetic Ciphers
     1. [Caesar Shift Cipher](ciphers/CaesarShiftCipher.py)
     1. [Multiplicative Cipher](ciphers/MultiplicativeCipher.py)
-    1. Affine Cipher
+    1. [Affine Cipher](ciphers/AffineCipher.py)
 1. Polyalphabetic Ciphers
     1. Autokey Cipher
     1. [Playfair Cipher](ciphers/PlayfairCipher.py)

ciphers/AffineCipher.py

@@ -0,0 +1,20 @@
+from mathematics import multiplicative_inverse
+
+class AffineCipher:
+    def __init__(self, k_1, k_2):
+        self.k_1 = k_1
+        self.k_2 = k_2
+
+    def char_to_num(self, letter):
+        letter = letter.lower()
+        return ord(letter) - ord('a')
+
+    def encrypt(self, plaintext):
+        return ''.join(
+            [chr((self.char_to_num(letter) * self.k_1 + self.k_2) % 26 + ord('A')) for letter in plaintext.lower()])
+
+    def decrypt(self, ciphertext):
+        return ''.join(
+            [chr(((self.char_to_num(letter) - self.k_2) * multiplicative_inverse(self.k_1, 26)) % 26 + ord('a'))
+             for letter in ciphertext.lower()]
+        )

ciphers/__init__.py

@@ -3,3 +3,4 @@
 from ciphers.PlayfairCipher import PlayFairCipher
 from ciphers.CaesarShiftCipher import CaesarShiftCipher
 from ciphers.MultiplicativeCipher import MultiplicativeCipher
+from ciphers.AffineCipher import AffineCipher