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 enum import Enum
   8 from itertools import zip_longest, cycle, chain
   9 from language_models import unaccent, sanitise
  10
  11
  12 modular_division_table = [[0]*26 for _ in range(26)]
  13 for a in range(26):
  14     for b in range(26):
  15         c = (a * b) % 26
  16         modular_division_table[b][c] = a
  17
  18
  19 def deduplicate(text):
  20     """If a string contains duplicate letters, remove all but the first. Retain
  21     the order of the letters.
  22
  23     >>> deduplicate('cat')
  24     ['c', 'a', 't']
  25     >>> deduplicate('happy')
  26     ['h', 'a', 'p', 'y']
  27     >>> deduplicate('cattca')
  28     ['c', 'a', 't']
  29     """
  30     return list(collections.OrderedDict.fromkeys(text))
  31
  32
  33 def caesar_encipher_letter(accented_letter, shift):
  34     """Encipher a letter, given a shift amount
  35
  36     >>> caesar_encipher_letter('a', 1)
  37     'b'
  38     >>> caesar_encipher_letter('a', 2)
  39     'c'
  40     >>> caesar_encipher_letter('b', 2)
  41     'd'
  42     >>> caesar_encipher_letter('x', 2)
  43     'z'
  44     >>> caesar_encipher_letter('y', 2)
  45     'a'
  46     >>> caesar_encipher_letter('z', 2)
  47     'b'
  48     >>> caesar_encipher_letter('z', -1)
  49     'y'
  50     >>> caesar_encipher_letter('a', -1)
  51     'z'
  52     >>> caesar_encipher_letter('A', 1)
  53     'B'
  54     >>> caesar_encipher_letter('é', 1)
  55     'f'
  56     """
  57     letter = unaccent(accented_letter)
  58     if letter in string.ascii_letters:
  59         if letter in string.ascii_uppercase:
  60             alphabet_start = ord('A')
  61         else:
  62             alphabet_start = ord('a')
  63         return chr(((ord(letter) - alphabet_start + shift) % 26) +
  64                    alphabet_start)
  65     else:
  66         return letter
  67
  68 def caesar_decipher_letter(letter, shift):
  69     """Decipher a letter, given a shift amount
  70
  71     >>> caesar_decipher_letter('b', 1)
  72     'a'
  73     >>> caesar_decipher_letter('b', 2)
  74     'z'
  75     """
  76     return caesar_encipher_letter(letter, -shift)
  77
  78 def caesar_encipher(message, shift):
  79     """Encipher a message with the Caesar cipher of given shift
  80
  81     >>> caesar_encipher('abc', 1)
  82     'bcd'
  83     >>> caesar_encipher('abc', 2)
  84     'cde'
  85     >>> caesar_encipher('abcxyz', 2)
  86     'cdezab'
  87     >>> caesar_encipher('ab cx yz', 2)
  88     'cd ez ab'
  89     >>> caesar_encipher('Héllo World!', 2)
  90     'Jgnnq Yqtnf!'
  91     """
  92     enciphered = [caesar_encipher_letter(l, shift) for l in message]
  93     return ''.join(enciphered)
  94
  95 def caesar_decipher(message, shift):
  96     """Decipher a message with the Caesar cipher of given shift
  97
  98     >>> caesar_decipher('bcd', 1)
  99     'abc'
 100     >>> caesar_decipher('cde', 2)
 101     'abc'
 102     >>> caesar_decipher('cd ez ab', 2)
 103     'ab cx yz'
 104     >>> caesar_decipher('Jgnnq Yqtnf!', 2)
 105     'Hello World!'
 106     """
 107     return caesar_encipher(message, -shift)
 108
 109 def affine_encipher_letter(accented_letter, multiplier=1, adder=0,
 110                            one_based=True):
 111     """Encipher a letter, given a multiplier and adder
 112     >>> ''.join([affine_encipher_letter(l, 3, 5, True) \
 113             for l in string.ascii_uppercase])
 114     'HKNQTWZCFILORUXADGJMPSVYBE'
 115     >>> ''.join([affine_encipher_letter(l, 3, 5, False) \
 116             for l in string.ascii_uppercase])
 117     'FILORUXADGJMPSVYBEHKNQTWZC'
 118     """
 119     letter = unaccent(accented_letter)
 120     if letter in string.ascii_letters:
 121         if letter in string.ascii_uppercase:
 122             alphabet_start = ord('A')
 123         else:
 124             alphabet_start = ord('a')
 125         letter_number = ord(letter) - alphabet_start
 126         if one_based: letter_number += 1
 127         cipher_number = (letter_number * multiplier + adder) % 26
 128         if one_based: cipher_number -= 1
 129         return chr(cipher_number % 26 + alphabet_start)
 130     else:
 131         return letter
 132
 133 def affine_decipher_letter(letter, multiplier=1, adder=0, one_based=True):
 134     """Encipher a letter, given a multiplier and adder
 135
 136     >>> ''.join([affine_decipher_letter(l, 3, 5, True) \
 137             for l in 'HKNQTWZCFILORUXADGJMPSVYBE'])
 138     'ABCDEFGHIJKLMNOPQRSTUVWXYZ'
 139     >>> ''.join([affine_decipher_letter(l, 3, 5, False) \
 140             for l in 'FILORUXADGJMPSVYBEHKNQTWZC'])
 141     'ABCDEFGHIJKLMNOPQRSTUVWXYZ'
 142     """
 143     if letter in string.ascii_letters:
 144         if letter in string.ascii_uppercase:
 145             alphabet_start = ord('A')
 146         else:
 147             alphabet_start = ord('a')
 148         cipher_number = ord(letter) - alphabet_start
 149         if one_based: cipher_number += 1
 150         plaintext_number = (
 151             modular_division_table[multiplier]
 152                                   [(cipher_number - adder) % 26]
 153                             )
 154         if one_based: plaintext_number -= 1
 155         return chr(plaintext_number % 26 + alphabet_start)
 156     else:
 157         return letter
 158
 159 def affine_encipher(message, multiplier=1, adder=0, one_based=True):
 160     """Encipher a message
 161
 162     >>> affine_encipher('hours passed during which jerico tried every ' \
 163            'trick he could think of', 15, 22, True)
 164     'lmyfu bkuusd dyfaxw claol psfaom jfasd snsfg jfaoe ls omytd jlaxe mh'
 165     """
 166     enciphered = [affine_encipher_letter(l, multiplier, adder, one_based)
 167                   for l in message]
 168     return ''.join(enciphered)
 169
 170 def affine_decipher(message, multiplier=1, adder=0, one_based=True):
 171     """Decipher a message
 172
 173     >>> affine_decipher('lmyfu bkuusd dyfaxw claol psfaom jfasd snsfg ' \
 174            'jfaoe ls omytd jlaxe mh', 15, 22, True)
 175     'hours passed during which jerico tried every trick he could think of'
 176     """
 177     enciphered = [affine_decipher_letter(l, multiplier, adder, one_based)
 178                   for l in message]
 179     return ''.join(enciphered)
 180
 181
 182 class KeywordWrapAlphabet(Enum):
 183     """Ways of wrapping the alphabet for keyword-based substitution ciphers."""
 184     from_a = 1
 185     from_last = 2
 186     from_largest = 3
 187
 188
 189 def keyword_cipher_alphabet_of(keyword,
 190         wrap_alphabet=KeywordWrapAlphabet.from_a):
 191     """Find the cipher alphabet given a keyword.
 192     wrap_alphabet controls how the rest of the alphabet is added
 193     after the keyword.
 194
 195     >>> keyword_cipher_alphabet_of('bayes')
 196     'bayescdfghijklmnopqrtuvwxz'
 197     >>> keyword_cipher_alphabet_of('bayes', KeywordWrapAlphabet.from_a)
 198     'bayescdfghijklmnopqrtuvwxz'
 199     >>> keyword_cipher_alphabet_of('bayes', KeywordWrapAlphabet.from_last)
 200     'bayestuvwxzcdfghijklmnopqr'
 201     >>> keyword_cipher_alphabet_of('bayes', KeywordWrapAlphabet.from_largest)
 202     'bayeszcdfghijklmnopqrtuvwx'
 203     """
 204     if wrap_alphabet == KeywordWrapAlphabet.from_a:
 205         cipher_alphabet = ''.join(deduplicate(sanitise(keyword) +
 206                                               string.ascii_lowercase))
 207     else:
 208         if wrap_alphabet == KeywordWrapAlphabet.from_last:
 209             last_keyword_letter = deduplicate(sanitise(keyword))[-1]
 210         else:
 211             last_keyword_letter = sorted(sanitise(keyword))[-1]
 212         last_keyword_position = string.ascii_lowercase.find(
 213             last_keyword_letter) + 1
 214         cipher_alphabet = ''.join(
 215             deduplicate(sanitise(keyword) +
 216                         string.ascii_lowercase[last_keyword_position:] +
 217                         string.ascii_lowercase))
 218     return cipher_alphabet
 219
 220
 221 def keyword_encipher(message, keyword,
 222                      wrap_alphabet=KeywordWrapAlphabet.from_a):
 223     """Enciphers a message with a keyword substitution cipher.
 224     wrap_alphabet controls how the rest of the alphabet is added
 225     after the keyword.
 226     0 : from 'a'
 227     1 : from the last letter in the sanitised keyword
 228     2 : from the largest letter in the sanitised keyword
 229
 230     >>> keyword_encipher('test message', 'bayes')
 231     'rsqr ksqqbds'
 232     >>> keyword_encipher('test message', 'bayes', KeywordWrapAlphabet.from_a)
 233     'rsqr ksqqbds'
 234     >>> keyword_encipher('test message', 'bayes', KeywordWrapAlphabet.from_last)
 235     'lskl dskkbus'
 236     >>> keyword_encipher('test message', 'bayes', KeywordWrapAlphabet.from_largest)
 237     'qspq jsppbcs'
 238     """
 239     cipher_alphabet = keyword_cipher_alphabet_of(keyword, wrap_alphabet)
 240     cipher_translation = ''.maketrans(string.ascii_lowercase, cipher_alphabet)
 241     return unaccent(message).lower().translate(cipher_translation)
 242
 243 def keyword_decipher(message, keyword,
 244                      wrap_alphabet=KeywordWrapAlphabet.from_a):
 245     """Deciphers a message with a keyword substitution cipher.
 246     wrap_alphabet controls how the rest of the alphabet is added
 247     after the keyword.
 248     0 : from 'a'
 249     1 : from the last letter in the sanitised keyword
 250     2 : from the largest letter in the sanitised keyword
 251
 252     >>> keyword_decipher('rsqr ksqqbds', 'bayes')
 253     'test message'
 254     >>> keyword_decipher('rsqr ksqqbds', 'bayes', KeywordWrapAlphabet.from_a)
 255     'test message'
 256     >>> keyword_decipher('lskl dskkbus', 'bayes', KeywordWrapAlphabet.from_last)
 257     'test message'
 258     >>> keyword_decipher('qspq jsppbcs', 'bayes', KeywordWrapAlphabet.from_largest)
 259     'test message'
 260     """
 261     cipher_alphabet = keyword_cipher_alphabet_of(keyword, wrap_alphabet)
 262     cipher_translation = ''.maketrans(cipher_alphabet, string.ascii_lowercase)
 263     return message.lower().translate(cipher_translation)
 264
 265 if __name__ == "__main__":
 266     import doctest
 267     doctest.testmod()