cipher/caesar.py

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