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