cipher.py

   1 """A set of ciphers with implementations for both enciphering and deciphering
   2 them. See cipherbreak for automatic breaking of these ciphers
   3 """
   4
   5 import string
   6 import collections
   7 from language_models import unaccent, sanitise
   8
   9
  10 def caesar_encipher_letter(accented_letter, shift):
  11     """Encipher a letter, given a shift amount
  12
  13     >>> caesar_encipher_letter('a', 1)
  14     'b'
  15     >>> caesar_encipher_letter('a', 2)
  16     'c'
  17     >>> caesar_encipher_letter('b', 2)
  18     'd'
  19     >>> caesar_encipher_letter('x', 2)
  20     'z'
  21     >>> caesar_encipher_letter('y', 2)
  22     'a'
  23     >>> caesar_encipher_letter('z', 2)
  24     'b'
  25     >>> caesar_encipher_letter('z', -1)
  26     'y'
  27     >>> caesar_encipher_letter('a', -1)
  28     'z'
  29     >>> caesar_encipher_letter('A', 1)
  30     'B'
  31     >>> caesar_encipher_letter('é', 1)
  32     'f'
  33     """
  34     letter = unaccent(accented_letter)
  35     if letter in string.ascii_letters:
  36         if letter in string.ascii_uppercase:
  37             alphabet_start = ord('A')
  38         else:
  39             alphabet_start = ord('a')
  40         return chr(((ord(letter) - alphabet_start + shift) % 26) +
  41                    alphabet_start)
  42     else:
  43         return letter
  44
  45 def caesar_decipher_letter(letter, shift):
  46     """Decipher a letter, given a shift amount
  47
  48     >>> caesar_decipher_letter('b', 1)
  49     'a'
  50     >>> caesar_decipher_letter('b', 2)
  51     'z'
  52     """
  53     return caesar_encipher_letter(letter, -shift)
  54
  55 def caesar_encipher(message, shift):
  56     """Encipher a message with the Caesar cipher of given shift
  57
  58     >>> caesar_encipher('abc', 1)
  59     'bcd'
  60     >>> caesar_encipher('abc', 2)
  61     'cde'
  62     >>> caesar_encipher('abcxyz', 2)
  63     'cdezab'
  64     >>> caesar_encipher('ab cx yz', 2)
  65     'cd ez ab'
  66     >>> caesar_encipher('Héllo World!', 2)
  67     'Jgnnq Yqtnf!'
  68     """
  69     enciphered = [caesar_encipher_letter(l, shift) for l in message]
  70     return ''.join(enciphered)
  71
  72 def caesar_decipher(message, shift):
  73     """Decipher a message with the Caesar cipher of given shift
  74
  75     >>> caesar_decipher('bcd', 1)
  76     'abc'
  77     >>> caesar_decipher('cde', 2)
  78     'abc'
  79     >>> caesar_decipher('cd ez ab', 2)
  80     'ab cx yz'
  81     >>> caesar_decipher('Jgnnq Yqtnf!', 2)
  82     'Hello World!'
  83     """
  84     return caesar_encipher(message, -shift)
  85
  86
  87 if __name__ == "__main__":
  88     import doctest
  89     doctest.testmod()