cipher.py

   1 import string
   2
   3 def caesar_encipher_letter(letter, shift):
   4     """Encipher a letter, given a shift amount
   5
   6     >>> caesar_encipher_letter('a', 1)
   7     'b'
   8     >>> caesar_encipher_letter('a', 2)
   9     'c'
  10     >>> caesar_encipher_letter('b', 2)
  11     'd'
  12     >>> caesar_encipher_letter('x', 2)
  13     'z'
  14     >>> caesar_encipher_letter('y', 2)
  15     'a'
  16     >>> caesar_encipher_letter('z', 2)
  17     'b'
  18     >>> caesar_encipher_letter('z', -1)
  19     'y'
  20     >>> caesar_encipher_letter('a', -1)
  21     'z'
  22     """
  23     if letter in string.ascii_letters:
  24         if letter in string.ascii_uppercase:
  25             alphabet_start = ord('A')
  26         else:
  27             alphabet_start = ord('a')
  28         return chr(((ord(letter) - alphabet_start + shift) % 26) +
  29                    alphabet_start)
  30     else:
  31         return letter
  32
  33 def caesar_decipher_letter(letter, shift):
  34     """Decipher a letter, given a shift amount
  35
  36     >>> caesar_decipher_letter('b', 1)
  37     'a'
  38     >>> caesar_decipher_letter('b', 2)
  39     'z'
  40     """
  41     return caesar_encipher_letter(letter, -shift)
  42
  43 def caesar_encipher(message, shift):
  44     """Encipher a message with the Caesar cipher of given shift
  45
  46     >>> caesar_encipher('abc', 1)
  47     'bcd'
  48     >>> caesar_encipher('abc', 2)
  49     'cde'
  50     >>> caesar_encipher('abcxyz', 2)
  51     'cdezab'
  52     >>> caesar_encipher('ab cx yz', 2)
  53     'cd ez ab'
  54     """
  55     enciphered = [caesar_encipher_letter(l, shift) for l in message]
  56     return ''.join(enciphered)
  57
  58 def caesar_decipher(message, shift):
  59     """Decipher a message with the Caesar cipher of given shift
  60
  61     >>> caesar_decipher('bcd', 1)
  62     'abc'
  63     >>> caesar_decipher('cde', 2)
  64     'abc'
  65     >>> caesar_decipher('cd ez ab', 2)
  66     'ab cx yz'
  67     """
  68     return caesar_encipher(message, -shift)
  69
  70
  71 if __name__ == "__main__":
  72     import doctest
  73     doctest.testmod()