szyfrow/caesar.py

   1 """Enciphering and deciphering using the [Caesar cipher](https://en.wikipedia.org/wiki/Caesar_cipher).
   2 Also attempts to break messages that use a Caesar cipher.
   3
   4 The Caesar cipher operates one letter at a time. It converts each letter to a
   5 number, then enciphers that number by adding the key. The result is taken mod
   6 26 and converted back into a letter.
   7
   8 """
   9
  10 from szyfrow.support.utilities import *
  11 from szyfrow.support.language_models import *
  12
  13 def caesar_encipher_letter(accented_letter, shift):
  14     """Encipher a letter, given a shift amount.
  15
  16     Accented version of latin letters (such as é and ö) are converted to their
  17     non-accented versions before encryption.
  18
  19     >>> caesar_encipher_letter('a', 1)
  20     'b'
  21     >>> caesar_encipher_letter('a', 2)
  22     'c'
  23     >>> caesar_encipher_letter('b', 2)
  24     'd'
  25     >>> caesar_encipher_letter('x', 2)
  26     'z'
  27     >>> caesar_encipher_letter('y', 2)
  28     'a'
  29     >>> caesar_encipher_letter('z', 2)
  30     'b'
  31     >>> caesar_encipher_letter('z', -1)
  32     'y'
  33     >>> caesar_encipher_letter('a', -1)
  34     'z'
  35     >>> caesar_encipher_letter('A', 1)
  36     'B'
  37     >>> caesar_encipher_letter('é', 1)
  38     'f'
  39     """
  40     # letter = unaccent(accented_letter)
  41     # if letter in string.ascii_letters:
  42     #     if letter in string.ascii_uppercase:
  43     #         alphabet_start = ord('A')
  44     #     else:
  45     #         alphabet_start = ord('a')
  46     #     return chr(((ord(letter) - alphabet_start + shift) % 26) +
  47     #                alphabet_start)
  48     # else:
  49     #     return letter
  50
  51     letter = unaccent(accented_letter)
  52     if letter in string.ascii_letters:
  53         cipherletter = unpos(pos(letter) + shift)
  54         if letter in string.ascii_uppercase:
  55             return cipherletter.upper()
  56         else:
  57             return cipherletter
  58     else:
  59         return letter
  60
  61 def caesar_decipher_letter(letter, shift):
  62     """Decipher a letter, given a shift amount
  63
  64     >>> caesar_decipher_letter('b', 1)
  65     'a'
  66     >>> caesar_decipher_letter('b', 2)
  67     'z'
  68     """
  69     return caesar_encipher_letter(letter, -shift)
  70
  71 def caesar_encipher(message, shift):
  72     """Encipher a message with the Caesar cipher of given shift
  73
  74     >>> caesar_encipher('abc', 1)
  75     'bcd'
  76     >>> caesar_encipher('abc', 2)
  77     'cde'
  78     >>> caesar_encipher('abcxyz', 2)
  79     'cdezab'
  80     >>> caesar_encipher('ab cx yz', 2)
  81     'cd ez ab'
  82     >>> caesar_encipher('Héllo World!', 2)
  83     'Jgnnq Yqtnf!'
  84     """
  85     enciphered = [caesar_encipher_letter(l, shift) for l in message]
  86     return cat(enciphered)
  87
  88 def caesar_decipher(message, shift):
  89     """Decipher a message with the Caesar cipher of given shift
  90
  91     >>> caesar_decipher('bcd', 1)
  92     'abc'
  93     >>> caesar_decipher('cde', 2)
  94     'abc'
  95     >>> caesar_decipher('cd ez ab', 2)
  96     'ab cx yz'
  97     >>> caesar_decipher('Jgnnq Yqtnf!', 2)
  98     'Hello World!'
  99     """
 100     return caesar_encipher(message, -shift)
 101
 102
 103 def caesar_break(message, fitness=Pletters):
 104     """Breaks a Caesar cipher using frequency analysis
 105
 106     It tries all possible keys, scores the fitness of the text decipherd with
 107     each key, and returns the key that produces the most fit deciphered text.
 108
 109     >>> caesar_break('ibxcsyorsaqcheyklxivoexlevmrimwxsfiqevvmihrsasrxliwyrh' \
 110           'ecjsppsamrkwleppfmergefifvmhixscsymjcsyqeoixlm') # doctest: +ELLIPSIS
 111     (4, -130.849989015...)
 112     >>> caesar_break('wxwmaxdgheetgwuxztgptedbgznitgwwhpguxyhkxbmhvvtlbhgtee' \
 113           'raxlmhiixweblmxgxwmhmaxybkbgztgwztsxwbgmxgmert') # doctest: +ELLIPSIS
 114     (19, -128.82410410...)
 115     >>> caesar_break('yltbbqnqnzvguvaxurorgenafsbezqvagbnornfgsbevpnaabjurer' \
 116           'svaquvzyvxrnznazlybequrvfohgriraabjtbaruraprur') # doctest: +ELLIPSIS
 117     (13, -126.25403935...)
 118     """
 119     sanitised_message = sanitise(message)
 120     best_shift = 0
 121     best_fit = float('-inf')
 122     for shift in range(26):
 123         plaintext = caesar_decipher(sanitised_message, shift)
 124         fit = fitness(plaintext)
 125
 126         if fit > best_fit:
 127             best_fit = fit
 128             best_shift = shift
 129
 130     return best_shift, best_fit