cipher.py

   1 """A set of ciphers with implementations for both enciphering and deciphering
   2 them. See cipherbreak for automatic breaking of these ciphers
   3 """
   4
   5 import string
   6 import collections
   7 from language_models import unaccent, sanitise
   8
   9
  10 modular_division_table = [[0]*26 for _ in range(26)]
  11 for a in range(26):
  12     for b in range(26):
  13         c = (a * b) % 26
  14         modular_division_table[b][c] = a
  15
  16
  17 def caesar_encipher_letter(accented_letter, shift):
  18     """Encipher a letter, given a shift amount
  19
  20     >>> caesar_encipher_letter('a', 1)
  21     'b'
  22     >>> caesar_encipher_letter('a', 2)
  23     'c'
  24     >>> caesar_encipher_letter('b', 2)
  25     'd'
  26     >>> caesar_encipher_letter('x', 2)
  27     'z'
  28     >>> caesar_encipher_letter('y', 2)
  29     'a'
  30     >>> caesar_encipher_letter('z', 2)
  31     'b'
  32     >>> caesar_encipher_letter('z', -1)
  33     'y'
  34     >>> caesar_encipher_letter('a', -1)
  35     'z'
  36     >>> caesar_encipher_letter('A', 1)
  37     'B'
  38     >>> caesar_encipher_letter('é', 1)
  39     'f'
  40     """
  41     letter = unaccent(accented_letter)
  42     if letter in string.ascii_letters:
  43         if letter in string.ascii_uppercase:
  44             alphabet_start = ord('A')
  45         else:
  46             alphabet_start = ord('a')
  47         return chr(((ord(letter) - alphabet_start + shift) % 26) +
  48                    alphabet_start)
  49     else:
  50         return letter
  51
  52 def caesar_decipher_letter(letter, shift):
  53     """Decipher a letter, given a shift amount
  54
  55     >>> caesar_decipher_letter('b', 1)
  56     'a'
  57     >>> caesar_decipher_letter('b', 2)
  58     'z'
  59     """
  60     return caesar_encipher_letter(letter, -shift)
  61
  62 def caesar_encipher(message, shift):
  63     """Encipher a message with the Caesar cipher of given shift
  64
  65     >>> caesar_encipher('abc', 1)
  66     'bcd'
  67     >>> caesar_encipher('abc', 2)
  68     'cde'
  69     >>> caesar_encipher('abcxyz', 2)
  70     'cdezab'
  71     >>> caesar_encipher('ab cx yz', 2)
  72     'cd ez ab'
  73     >>> caesar_encipher('Héllo World!', 2)
  74     'Jgnnq Yqtnf!'
  75     """
  76     enciphered = [caesar_encipher_letter(l, shift) for l in message]
  77     return ''.join(enciphered)
  78
  79 def caesar_decipher(message, shift):
  80     """Decipher a message with the Caesar cipher of given shift
  81
  82     >>> caesar_decipher('bcd', 1)
  83     'abc'
  84     >>> caesar_decipher('cde', 2)
  85     'abc'
  86     >>> caesar_decipher('cd ez ab', 2)
  87     'ab cx yz'
  88     >>> caesar_decipher('Jgnnq Yqtnf!', 2)
  89     'Hello World!'
  90     """
  91     return caesar_encipher(message, -shift)
  92
  93 def affine_encipher_letter(accented_letter, multiplier=1, adder=0,
  94                            one_based=True):
  95     """Encipher a letter, given a multiplier and adder
  96     >>> ''.join([affine_encipher_letter(l, 3, 5, True) \
  97             for l in string.ascii_uppercase])
  98     'HKNQTWZCFILORUXADGJMPSVYBE'
  99     >>> ''.join([affine_encipher_letter(l, 3, 5, False) \
 100             for l in string.ascii_uppercase])
 101     'FILORUXADGJMPSVYBEHKNQTWZC'
 102     """
 103     letter = unaccent(accented_letter)
 104     if letter in string.ascii_letters:
 105         if letter in string.ascii_uppercase:
 106             alphabet_start = ord('A')
 107         else:
 108             alphabet_start = ord('a')
 109         letter_number = ord(letter) - alphabet_start
 110         if one_based: letter_number += 1
 111         cipher_number = (letter_number * multiplier + adder) % 26
 112         if one_based: cipher_number -= 1
 113         return chr(cipher_number % 26 + alphabet_start)
 114     else:
 115         return letter
 116
 117 def affine_decipher_letter(letter, multiplier=1, adder=0, one_based=True):
 118     """Encipher a letter, given a multiplier and adder
 119
 120     >>> ''.join([affine_decipher_letter(l, 3, 5, True) \
 121             for l in 'HKNQTWZCFILORUXADGJMPSVYBE'])
 122     'ABCDEFGHIJKLMNOPQRSTUVWXYZ'
 123     >>> ''.join([affine_decipher_letter(l, 3, 5, False) \
 124             for l in 'FILORUXADGJMPSVYBEHKNQTWZC'])
 125     'ABCDEFGHIJKLMNOPQRSTUVWXYZ'
 126     """
 127     if letter in string.ascii_letters:
 128         if letter in string.ascii_uppercase:
 129             alphabet_start = ord('A')
 130         else:
 131             alphabet_start = ord('a')
 132         cipher_number = ord(letter) - alphabet_start
 133         if one_based: cipher_number += 1
 134         plaintext_number = (
 135             modular_division_table[multiplier]
 136                                   [(cipher_number - adder) % 26]
 137                             )
 138         if one_based: plaintext_number -= 1
 139         return chr(plaintext_number % 26 + alphabet_start)
 140     else:
 141         return letter
 142
 143 def affine_encipher(message, multiplier=1, adder=0, one_based=True):
 144     """Encipher a message
 145
 146     >>> affine_encipher('hours passed during which jerico tried every ' \
 147            'trick he could think of', 15, 22, True)
 148     'lmyfu bkuusd dyfaxw claol psfaom jfasd snsfg jfaoe ls omytd jlaxe mh'
 149     """
 150     enciphered = [affine_encipher_letter(l, multiplier, adder, one_based)
 151                   for l in message]
 152     return ''.join(enciphered)
 153
 154 def affine_decipher(message, multiplier=1, adder=0, one_based=True):
 155     """Decipher a message
 156
 157     >>> affine_decipher('lmyfu bkuusd dyfaxw claol psfaom jfasd snsfg ' \
 158            'jfaoe ls omytd jlaxe mh', 15, 22, True)
 159     'hours passed during which jerico tried every trick he could think of'
 160     """
 161     enciphered = [affine_decipher_letter(l, multiplier, adder, one_based)
 162                   for l in message]
 163     return ''.join(enciphered)
 164
 165
 166
 167 if __name__ == "__main__":
 168     import doctest
 169     doctest.testmod()