Lab ready

ciphers/caesar.py

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+from easybits import Bits
+
+class CaesarCipher:
+    """Implements the caeser cipher. 
+    In the caeser cipher, each character is converted into a number, 
+    then a secret number is added to each (if the result is too large, we 
+    loop it back around), and the result is converted back into another 
+    character.
+
+    int_min and int_max represent the lowest and highest allowed int values
+    of characters. They are set to include all the ASCII printable
+    characters (https://en.wikipedia.org/wiki/ASCII#Printable_character_table)
+
+    ASCII values outside this range (for example, '\n', the newline character),
+    just get passed through unencrypted.
+    """
+
+    int_min = 32
+    int_max = 127
+
+    def __init__(self, secret):
+        self.secret = secret
+
+    def encrypt(self, plaintext):
+        "Converts a plaintext message into an encrypted ciphertext"
+        ciphertext = []
+        for char in plaintext:
+            plain_int = Bits(char, encoding='ascii').int
+            if self.int_min <= plain_int and plain_int < self.int_max:
+                cipher_int = self.rotate(self.secret, plain_int)
+                ciphertext.append(Bits(cipher_int, length=8).ascii)
+            else:
+                ciphertext.append(char)
+        return ''.join(ciphertext)
+
+    def decrypt(self, ciphertext):
+        "Converts an encrypted ciphertext into a plaintext message"
+        plaintext = []
+        for char in ciphertext:
+            cipher_int = Bits(char, encoding='ascii').int
+            if self.int_min <= cipher_int and cipher_int < self.int_max:
+                plain_int = self.rotate(-self.secret, cipher_int)
+                plaintext.append(Bits(plain_int, length=8).ascii)
+            else:
+                plaintext.append(char)
+        return ''.join(plaintext)
+
+    def rotate(self, secret, x):
+        """Adds a secret number to x. 
+        The modulo operator (%) is used to ensure that the result
+        is greater than equal to int_min and less than int_max. 
+        """
+        range_size = self.int_max - self.int_min
+        return (x + secret - self.int_min) % range_size + self.int_min
+

ciphers/poly.py

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+from itertools import cycle
+from easybits import Bits
+
+class PolyCipher:
+    """Implements a polyalphabetic cipher.
+    The polyalphabetic cipher is like a Caesar cipher except that 
+    the secret number changes for each character to be encrypted or
+    decrypted. This makes frequency analysis much harder, because
+    each plaintext space can be encrypted as a different character. 
+
+    int_min and int_max represent the lowest and highest allowed int values
+    of characters. They are set to include all the ASCII printable
+    characters (https://en.wikipedia.org/wiki/ASCII#Printable_character_table)
+
+    ASCII values outside this range (for example, '\n', the newline character),
+    just get passed through unencrypted.
+    """
+
+    int_min = 32
+    int_max = 127
+
+    def __init__(self, secret):
+        self.secret = secret
+
+    def encrypt(self, plaintext):
+        "Converts a plaintext message into an encrypted ciphertext"
+        ciphertext = []
+        for char, secret_char in zip(plaintext, cycle(self.secret)):
+            plain_int = Bits(char, encoding='ascii').int
+            if self.int_min <= plain_int and plain_int < self.int_max:
+                secret_int = self.get_int(secret_char)
+                cipher_int = self.rotate(secret_int, plain_int)
+                ciphertext.append(Bits(cipher_int, length=8).ascii)
+            else:
+                ciphertext.append(char)
+        return ''.join(ciphertext)
+
+    def decrypt(self, ciphertext):
+        "Converts an encrypted ciphertext into a plaintext message"
+        plaintext = []
+        for char, secret_char in zip(ciphertext, cycle(self.secret)):
+            cipher_int = Bits(char, encoding='ascii').int
+            if self.int_min <= cipher_int and cipher_int < self.int_max:
+                secret_int = self.get_int(secret_char)
+                plain_int = self.rotate(-secret_int, cipher_int)
+                plaintext.append(Bits(plain_int, length=8).ascii)
+            else:
+                plaintext.append(char)
+        return ''.join(plaintext)
+
+    def rotate(self, secret, x):
+        """Adds a secret number to x. 
+        The modulo operator (%) is used to ensure that the result
+        is greater than equal to int_min and less than int_max. 
+        """
+        range_size = self.int_max - self.int_min
+        return (x + secret - self.int_min) % range_size + self.int_min
+
+    def get_int(self, secret_char):
+        """Converts an int or a single-character string into an int.
+        When `secret_char` is an int, we just return it. Otherwise we
+        return the character's ASCII value.
+        """
+        if isinstance(secret_char, int):
+            return secret_char
+        else:
+            return Bits(secret_char, encoding='ascii').int
+
+