introduced shuffled_shift_cipher.py in /ciphers (#1424)

* introduced shuffled_shift_cipher.py in /ciphers

* made requested changes

* introduced doctests, type hints
removed __make_one_digit()

* test_end_to_end() inserted

* Make test_end_to_end() a test ;-)

ciphers/shuffled_shift_cipher.py

@@ -0,0 +1,177 @@
+import random
+import string
+
+
+class ShuffledShiftCipher(object):
+    """
+    This algorithm uses the Caesar Cipher algorithm but removes the option to
+    use brute force to decrypt the message.
+
+    The passcode is a a random password from the selection buffer of
+    1. uppercase letters of the English alphabet
+    2. lowercase letters of the English alphabet
+    3. digits from 0 to 9
+
+    Using unique characters from the passcode, the normal list of characters,
+    that can be allowed in the plaintext, is pivoted and shuffled. Refer to docstring
+    of __make_key_list() to learn more about the shuffling.
+
+    Then, using the passcode, a number is calculated which is used to encrypt the
+    plaintext message with the normal shift cipher method, only in this case, the
+    reference, to look back at while decrypting, is shuffled.
+
+    Each cipher object can possess an optional argument as passcode, without which a
+    new passcode is generated for that object automatically.
+    cip1 = ShuffledShiftCipher('d4usr9TWxw9wMD')
+    cip2 = ShuffledShiftCipher()
+    """
+
+    def __init__(self, passcode: str = None):
+        """
+        Initializes a cipher object with a passcode as it's entity
+        Note: No new passcode is generated if user provides a passcode
+        while creating the object
+        """
+        self.__passcode = passcode or self.__passcode_creator()
+        self.__key_list = self.__make_key_list()
+        self.__shift_key = self.__make_shift_key()
+
+    def __str__(self):
+        """
+        :return: passcode of the cipher object
+        """
+        return "Passcode is: " + "".join(self.__passcode)
+
+    def __neg_pos(self, iterlist: list) -> list:
+        """
+        Mutates the list by changing the sign of each alternate element
+
+        :param iterlist: takes a list iterable
+        :return: the mutated list
+
+        """
+        for i in range(1, len(iterlist), 2):
+            iterlist[i] *= -1
+        return iterlist
+
+    def __passcode_creator(self) -> list:
+        """
+        Creates a random password from the selection buffer of
+        1. uppercase letters of the English alphabet
+        2. lowercase letters of the English alphabet
+        3. digits from 0 to 9
+
+        :rtype: list
+        :return: a password of a random length between 10 to 20
+        """
+        choices = string.ascii_letters + string.digits
+        password = [random.choice(choices) for i in range(random.randint(10, 20))]
+        return password
+
+    def __make_key_list(self) -> list:
+        """
+        Shuffles the ordered character choices by pivoting at breakpoints
+        Breakpoints are the set of characters in the passcode
+
+        eg:
+            if, ABCDEFGHIJKLMNOPQRSTUVWXYZ are the possible characters
+            and CAMERA is the passcode
+            then, breakpoints = [A,C,E,M,R] # sorted set of characters from passcode
+            shuffled parts: [A,CB,ED,MLKJIHGF,RQPON,ZYXWVUTS]
+            shuffled __key_list : ACBEDMLKJIHGFRQPONZYXWVUTS
+
+        Shuffling only 26 letters of the english alphabet can generate 26!
+        combinations for the shuffled list. In the program we consider, a set of
+        97 characters (including letters, digits, punctuation and whitespaces),
+        thereby creating a possibility of 97! combinations (which is a 152 digit number in itself),
+        thus diminishing the possibility of a brute force approach. Moreover,
+        shift keys even introduce a multiple of 26 for a brute force approach
+        for each of the already 97! combinations.
+        """
+        # key_list_options contain nearly all printable except few elements from string.whitespace
+        key_list_options = (
+            string.ascii_letters + string.digits + string.punctuation + " \t\n"
+        )
+
+        keys_l = []
+
+        # creates points known as breakpoints to break the key_list_options at those points and pivot each substring
+        breakpoints = sorted(set(self.__passcode))
+        temp_list = []
+
+        # algorithm for creating a new shuffled list, keys_l, out of key_list_options
+        for i in key_list_options:
+            temp_list.extend(i)
+
+            # checking breakpoints at which to pivot temporary sublist and add it into keys_l
+            if i in breakpoints or i == key_list_options[-1]:
+                keys_l.extend(temp_list[::-1])
+                temp_list = []
+
+        # returning a shuffled keys_l to prevent brute force guessing of shift key
+        return keys_l
+
+    def __make_shift_key(self) -> int:
+        """
+        sum() of the mutated list of ascii values of all characters where the
+        mutated list is the one returned by __neg_pos()
+        """
+        num = sum(self.__neg_pos([ord(x) for x in self.__passcode]))
+        return num if num > 0 else len(self.__passcode)
+
+    def decrypt(self, encoded_message: str) -> str:
+        """
+        Performs shifting of the encoded_message w.r.t. the shuffled __key_list
+        to create the decoded_message
+
+        >>> ssc = ShuffledShiftCipher('4PYIXyqeQZr44')
+        >>> ssc.decrypt("d>**-1z6&'5z'5z:z+-='$'>=zp:>5:#z<'.&>#")
+        'Hello, this is a modified Caesar cipher'
+
+        """
+        decoded_message = ""
+
+        # decoding shift like Caesar cipher algorithm implementing negative shift or reverse shift or left shift
+        for i in encoded_message:
+            position = self.__key_list.index(i)
+            decoded_message += self.__key_list[
+                (position - self.__shift_key) % -len(self.__key_list)
+            ]
+
+        return decoded_message
+
+    def encrypt(self, plaintext: str) -> str:
+        """
+        Performs shifting of the plaintext w.r.t. the shuffled __key_list
+        to create the encoded_message
+
+        >>> ssc = ShuffledShiftCipher('4PYIXyqeQZr44')
+        >>> ssc.encrypt('Hello, this is a modified Caesar cipher')
+        "d>**-1z6&'5z'5z:z+-='$'>=zp:>5:#z<'.&>#"
+
+        """
+        encoded_message = ""
+
+        # encoding shift like Caesar cipher algorithm implementing positive shift or forward shift or right shift
+        for i in plaintext:
+            position = self.__key_list.index(i)
+            encoded_message += self.__key_list[
+                (position + self.__shift_key) % len(self.__key_list)
+            ]
+
+        return encoded_message
+
+
+def test_end_to_end(msg: str = "Hello, this is a modified Caesar cipher"):
+    """
+    >>> test_end_to_end()
+    'Hello, this is a modified Caesar cipher'
+    """
+    cip1 = ShuffledShiftCipher()
+    return cip1.decrypt(cip1.encrypt(msg))
+
+
+if __name__ == "__main__":
+    import doctest
+
+    doctest.testmod()