caesar.py

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