cipher/affine.py

   1 from support.utilities import *
   2 from support.language_models import *
   3 from logger import logger
   4
   5
   6 modular_division_table = [[0]*26 for _ in range(26)]
   7 for a in range(26):
   8     for b in range(26):
   9         c = (a * b) % 26
  10         modular_division_table[b][c] = a
  11
  12
  13
  14
  15 def affine_encipher_letter(accented_letter, multiplier=1, adder=0, one_based=True):
  16     """Encipher a letter, given a multiplier and adder
  17
  18     >>> cat(affine_encipher_letter(l, 3, 5, True) \
  19             for l in string.ascii_letters)
  20     'hknqtwzcfiloruxadgjmpsvybeHKNQTWZCFILORUXADGJMPSVYBE'
  21     >>> cat(affine_encipher_letter(l, 3, 5, False) \
  22             for l in string.ascii_letters)
  23     'filoruxadgjmpsvybehknqtwzcFILORUXADGJMPSVYBEHKNQTWZC'
  24     """
  25     # letter = unaccent(accented_letter)
  26     # if letter in string.ascii_letters:
  27     #     if letter in string.ascii_uppercase:
  28     #         alphabet_start = ord('A')
  29     #     else:
  30     #         alphabet_start = ord('a')
  31     #     letter_number = ord(letter) - alphabet_start
  32     #     if one_based: letter_number += 1
  33     #     cipher_number = (letter_number * multiplier + adder) % 26
  34     #     if one_based: cipher_number -= 1
  35     #     return chr(cipher_number % 26 + alphabet_start)
  36     # else:
  37     #     return letter
  38     letter = unaccent(accented_letter)
  39     if letter in string.ascii_letters:
  40         letter_number = pos(letter)
  41         if one_based: letter_number += 1
  42         cipher_number = (letter_number * multiplier + adder) % 26
  43         if one_based: cipher_number -= 1
  44         if letter in string.ascii_uppercase:
  45             return unpos(cipher_number).upper()
  46         else:
  47             return unpos(cipher_number)
  48     else:
  49         return letter
  50
  51 def affine_decipher_letter(letter, multiplier=1, adder=0, one_based=True):
  52     """Encipher a letter, given a multiplier and adder
  53
  54     >>> cat(affine_decipher_letter(l, 3, 5, True) \
  55             for l in 'hknqtwzcfiloruxadgjmpsvybeHKNQTWZCFILORUXADGJMPSVYBE')
  56     'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ'
  57     >>> cat(affine_decipher_letter(l, 3, 5, False) \
  58             for l in 'filoruxadgjmpsvybehknqtwzcFILORUXADGJMPSVYBEHKNQTWZC')
  59     'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ'
  60     """
  61     # if letter in string.ascii_letters:
  62     #     if letter in string.ascii_uppercase:
  63     #         alphabet_start = ord('A')
  64     #     else:
  65     #         alphabet_start = ord('a')
  66     #     cipher_number = ord(letter) - alphabet_start
  67     #     if one_based: cipher_number += 1
  68     #     plaintext_number = (
  69     #         modular_division_table[multiplier]
  70     #                               [(cipher_number - adder) % 26])
  71     #     if one_based: plaintext_number -= 1
  72     #     return chr(plaintext_number % 26 + alphabet_start)
  73     # else:
  74     #     return letter
  75     if letter in string.ascii_letters:
  76         cipher_number = pos(letter)
  77         if one_based: cipher_number += 1
  78         plaintext_number = (
  79             modular_division_table[multiplier]
  80                                   [(cipher_number - adder) % 26])
  81         if one_based: plaintext_number -= 1
  82         if letter in string.ascii_uppercase:
  83             return unpos(plaintext_number).upper()
  84         else:
  85             return unpos(plaintext_number)
  86     else:
  87         return letter
  88
  89 def affine_encipher(message, multiplier=1, adder=0, one_based=True):
  90     """Encipher a message
  91
  92     >>> affine_encipher('hours passed during which jerico tried every ' \
  93            'trick he could think of', 15, 22, True)
  94     'lmyfu bkuusd dyfaxw claol psfaom jfasd snsfg jfaoe ls omytd jlaxe mh'
  95     """
  96     enciphered = [affine_encipher_letter(l, multiplier, adder, one_based)
  97                   for l in message]
  98     return cat(enciphered)
  99
 100 def affine_decipher(message, multiplier=1, adder=0, one_based=True):
 101     """Decipher a message
 102
 103     >>> affine_decipher('lmyfu bkuusd dyfaxw claol psfaom jfasd snsfg ' \
 104            'jfaoe ls omytd jlaxe mh', 15, 22, True)
 105     'hours passed during which jerico tried every trick he could think of'
 106     """
 107     enciphered = [affine_decipher_letter(l, multiplier, adder, one_based)
 108                   for l in message]
 109     return cat(enciphered)
 110
 111
 112
 113 def affine_break(message, fitness=Pletters):
 114     """Breaks an affine cipher using frequency analysis
 115
 116     >>> affine_break('lmyfu bkuusd dyfaxw claol psfaom jfasd snsfg jfaoe ls ' \
 117           'omytd jlaxe mh jm bfmibj umis hfsul axubafkjamx. ls kffkxwsd jls ' \
 118           'ofgbjmwfkiu olfmxmtmwaokttg jlsx ls kffkxwsd jlsi zg tsxwjl. jlsx ' \
 119           'ls umfjsd jlsi zg hfsqysxog. ls dmmdtsd mx jls bats mh bkbsf. ls ' \
 120           'bfmctsd kfmyxd jls lyj, mztanamyu xmc jm clm cku tmmeaxw kj lai ' \
 121           'kxd clm ckuxj.') # doctest: +ELLIPSIS
 122     ((15, 22, True), -340.601181913...)
 123     """
 124     sanitised_message = sanitise(message)
 125     best_multiplier = 0
 126     best_adder = 0
 127     best_one_based = True
 128     best_fit = float("-inf")
 129     for one_based in [True, False]:
 130         for multiplier in [x for x in range(1, 26, 2) if x != 13]:
 131             for adder in range(26):
 132                 plaintext = affine_decipher(sanitised_message,
 133                                             multiplier, adder, one_based)
 134                 fit = fitness(plaintext)
 135                 logger.debug('Affine break attempt using key {0}x+{1} ({2}) '
 136                              'gives fit of {3} and decrypt starting: {4}'.
 137                              format(multiplier, adder, one_based, fit,
 138                                     plaintext[:50]))
 139                 if fit > best_fit:
 140                     best_fit = fit
 141                     best_multiplier = multiplier
 142                     best_adder = adder
 143                     best_one_based = one_based
 144     logger.info('Affine break best fit with key {0}x+{1} ({2}) gives fit of '
 145                 '{3} and decrypt starting: {4}'.format(
 146                     best_multiplier, best_adder, best_one_based, best_fit,
 147                     affine_decipher(sanitised_message, best_multiplier,
 148                                     best_adder, best_one_based)[:50]))
 149     return (best_multiplier, best_adder, best_one_based), best_fit