develop

Security/caesar_cipher.py

@@ -0,0 +1,54 @@
+"""
+Caesar cipher
+Implement a Caesar cipher, both encoding and decoding. The key is an integer
+from 1 to 25. This cipher rotates the letters of the alphabet (A to Z). The
+encoding replaces each letter with the 1st to 25th next letter in the alphabet
+(wrapping Z to A). So key 2 encrypts "HI" to "JK", but key 20 encrypts "HI" to
+"BC". This simple "monoalphabetic substitution cipher" provides almost no
+security, because an attacker who has the encoded message can either use
+frequency analysis to guess the key, or just try all 25 keys.
+"""
+
+
+class Caesar():
+
+    def __init__(self):
+        self.LETTERS = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ'
+        self.translated = ''
+
+    def __crypt(self, mode):
+        for symbol in self.message.upper():
+            if symbol in self.LETTERS:
+                num = self.LETTERS.find(symbol)
+                if mode == 'encrypt':
+                    num = num + self.key
+                elif mode == 'decrypt':
+                    num = num - self.key
+
+                if num >= len(self.LETTERS):
+                    num = num - len(self.LETTERS)
+                elif num < 0:
+                    num = num + len(self.LETTERS)
+
+                self.translated = self.translated + self.LETTERS[num]
+            else:
+                self.translated = self.translated + symbol
+
+        return self.translated
+
+    def encrypt(self, message, key=0):
+        self.translated = ''
+        self.key = key
+        self.message = message
+        return self.__crypt('encrypt')
+
+    def decrypt(self, message, key=0):
+        self.translated = ''
+        self.key = key
+        self.message = message
+        return self.__crypt('decrypt')
+
+if __name__ == '__main__':
+    cipher = Caesar()
+    print cipher.encrypt(message='Secret message.', key=13)
+    print cipher.decrypt(message='FRPERG ZRFFNTR.', key=13)