cipher/caesar.py

   1 from utilities import *
   2 from language_models import *
   3 from logger import logger
   4
   5 def caesar_encipher_letter(accented_letter, shift):
   6     """Encipher a letter, given a shift amount
   7
   8     >>> caesar_encipher_letter('a', 1)
   9     'b'
  10     >>> caesar_encipher_letter('a', 2)
  11     'c'
  12     >>> caesar_encipher_letter('b', 2)
  13     'd'
  14     >>> caesar_encipher_letter('x', 2)
  15     'z'
  16     >>> caesar_encipher_letter('y', 2)
  17     'a'
  18     >>> caesar_encipher_letter('z', 2)
  19     'b'
  20     >>> caesar_encipher_letter('z', -1)
  21     'y'
  22     >>> caesar_encipher_letter('a', -1)
  23     'z'
  24     >>> caesar_encipher_letter('A', 1)
  25     'B'
  26     >>> caesar_encipher_letter('é', 1)
  27     'f'
  28     """
  29     # letter = unaccent(accented_letter)
  30     # if letter in string.ascii_letters:
  31     #     if letter in string.ascii_uppercase:
  32     #         alphabet_start = ord('A')
  33     #     else:
  34     #         alphabet_start = ord('a')
  35     #     return chr(((ord(letter) - alphabet_start + shift) % 26) +
  36     #                alphabet_start)
  37     # else:
  38     #     return letter
  39
  40     letter = unaccent(accented_letter)
  41     if letter in string.ascii_letters:
  42         cipherletter = unpos(pos(letter) + shift)
  43         if letter in string.ascii_uppercase:
  44             return cipherletter.upper()
  45         else:
  46             return cipherletter
  47     else:
  48         return letter
  49
  50 def caesar_decipher_letter(letter, shift):
  51     """Decipher a letter, given a shift amount
  52
  53     >>> caesar_decipher_letter('b', 1)
  54     'a'
  55     >>> caesar_decipher_letter('b', 2)
  56     'z'
  57     """
  58     return caesar_encipher_letter(letter, -shift)
  59
  60 def caesar_encipher(message, shift):
  61     """Encipher a message with the Caesar cipher of given shift
  62
  63     >>> caesar_encipher('abc', 1)
  64     'bcd'
  65     >>> caesar_encipher('abc', 2)
  66     'cde'
  67     >>> caesar_encipher('abcxyz', 2)
  68     'cdezab'
  69     >>> caesar_encipher('ab cx yz', 2)
  70     'cd ez ab'
  71     >>> caesar_encipher('Héllo World!', 2)
  72     'Jgnnq Yqtnf!'
  73     """
  74     enciphered = [caesar_encipher_letter(l, shift) for l in message]
  75     return cat(enciphered)
  76
  77 def caesar_decipher(message, shift):
  78     """Decipher a message with the Caesar cipher of given shift
  79
  80     >>> caesar_decipher('bcd', 1)
  81     'abc'
  82     >>> caesar_decipher('cde', 2)
  83     'abc'
  84     >>> caesar_decipher('cd ez ab', 2)
  85     'ab cx yz'
  86     >>> caesar_decipher('Jgnnq Yqtnf!', 2)
  87     'Hello World!'
  88     """
  89     return caesar_encipher(message, -shift)
  90
  91
  92 def caesar_break(message, fitness=Pletters):
  93     """Breaks a Caesar cipher using frequency analysis
  94
  95     >>> caesar_break('ibxcsyorsaqcheyklxivoexlevmrimwxsfiqevvmihrsasrxliwyrh' \
  96           'ecjsppsamrkwleppfmergefifvmhixscsymjcsyqeoixlm') # doctest: +ELLIPSIS
  97     (4, -130.849989015...)
  98     >>> caesar_break('wxwmaxdgheetgwuxztgptedbgznitgwwhpguxyhkxbmhvvtlbhgtee' \
  99           'raxlmhiixweblmxgxwmhmaxybkbgztgwztsxwbgmxgmert') # doctest: +ELLIPSIS
 100     (19, -128.82410410...)
 101     >>> caesar_break('yltbbqnqnzvguvaxurorgenafsbezqvagbnornfgsbevpnaabjurer' \
 102           'svaquvzyvxrnznazlybequrvfohgriraabjtbaruraprur') # doctest: +ELLIPSIS
 103     (13, -126.25403935...)
 104     """
 105     sanitised_message = sanitise(message)
 106     best_shift = 0
 107     best_fit = float('-inf')
 108     for shift in range(26):
 109         plaintext = caesar_decipher(sanitised_message, shift)
 110         fit = fitness(plaintext)
 111         logger.debug('Caesar break attempt using key {0} gives fit of {1} '
 112                      'and decrypt starting: {2}'.format(shift, fit,
 113                                                         plaintext[:50]))
 114         if fit > best_fit:
 115             best_fit = fit
 116             best_shift = shift
 117     logger.info('Caesar break best fit: key {0} gives fit of {1} and '
 118                 'decrypt starting: {2}'.format(best_shift, best_fit,
 119                     caesar_decipher(sanitised_message, best_shift)[:50]))
 120     return best_shift, best_fit