affine.py

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