cipher.py

   1 import string
   2 import collections
   3
   4 english_counts = collections.defaultdict(int)
   5 with open('count_1l.txt', 'r') as f:
   6     for line in f:
   7         (letter, count) = line.split("\t")
   8         english_counts[letter] = int(count)
   9
  10 modular_division = [[0]* 26 for i in range(26)]
  11 for i in range(26):
  12     for j in range(26):
  13         t = (i*j) % 26
  14         # therefore,  i = t / j
  15         modular_division[t][j] = i
  16
  17
  18 def sanitise(text):
  19     sanitised = [c.lower() for c in text if c in string.ascii_letters]
  20     return ''.join(sanitised)
  21
  22 def letter_frequencies(message):
  23     frequencies = collections.defaultdict(int)
  24     for letter in sanitise(message):
  25         frequencies[letter]+=1
  26     return frequencies
  27
  28 def scale_freq(frequencies):
  29     total= sum(frequencies.values())
  30     scaled_frequencies = collections.defaultdict(int)
  31     for letter in frequencies.keys():
  32         scaled_frequencies[letter] = frequencies[letter] / total
  33     return scaled_frequencies
  34
  35 def value_diff(frequencies1, frequencies2):
  36     total= 0
  37     for letter in frequencies1.keys():
  38         total += abs(frequencies1[letter]-frequencies2[letter])
  39     return total
  40
  41
  42
  43 def caesar_cipher_letter(letter, shift):
  44     if letter in string.ascii_letters:
  45         if letter in string.ascii_lowercase:
  46             return chr((ord(letter) - ord('a') + shift) % 26 + ord('a'))
  47         else:
  48             new_letter = letter.lower()
  49             yolo = chr((ord(new_letter) - ord('a') + shift) % 26 + ord('a'))
  50             return yolo.upper()
  51     else:
  52         return letter
  53
  54 def caesar_decipher_letter(letter, shift):
  55     return caesar_cipher_letter(letter, -shift)
  56
  57 def caesar_cipher_message(message, shift):
  58     big_cipher = [caesar_cipher_letter(l, shift) for l in message]
  59     return ''.join(big_cipher)
  60
  61 def caesar_decipher_message(message, shift):
  62     return caesar_cipher_message(message, -shift)
  63
  64 def affine_cipher_letter(letter, multiplier, shift, one_based=True):
  65     if letter in string.ascii_letters:
  66         if letter in string.ascii_lowercase:
  67             alphastart = ord('a')
  68         else:
  69             alphastart = ord('A')
  70         letter_number = ord(letter) - alphastart
  71         if one_based: letter_number += 1
  72         enciphered_letter_number = letter_number * multiplier + shift
  73         if one_based: enciphered_letter_number -=1
  74         enciphered_letter = chr(enciphered_letter_number % 26 + alphastart)
  75         return enciphered_letter
  76     else:
  77         return letter
  78
  79 def affine_decipher_letter(letter, multiplier, shift, one_based=True):
  80     if letter in string.ascii_letters:
  81         if letter in string.ascii_lowercase:
  82             alphastart = ord('a')
  83         else:
  84             alphastart = ord('A')
  85         letter_number = ord(letter) - alphastart
  86         if one_based: letter_number +=1
  87         after_unshift = letter_number - shift
  88         deciphered_letter_number = modular_division[after_unshift % 26][multiplier]
  89         if one_based: deciphered_letter_number -=1
  90         deciphered_letter = chr(deciphered_letter_number % 26 + alphastart)
  91         return deciphered_letter
  92     else:
  93         return letter
  94
  95 def affine_cipher_message(message, multiplier, shift, one_based=True):
  96     big_cipher = [affine_cipher_letter(l, multiplier, shift, one_based) for l in message]
  97     return ''.join(big_cipher)
  98
  99 def affine_decipher_message(message, multiplier, shift, one_based=True):
 100     big_decipher = [affine_decipher_letter(l, multiplier, shift, one_based) for l in message]
 101     return ''.join(big_decipher)
 102
 103
 104 def caesar_break(message):
 105     best_key = 0
 106     best_fit = float("inf")
 107     for shift in range(26):
 108         plaintxt = caesar_decipher_message(message, shift)
 109         lettertxt = letter_frequencies(plaintxt)
 110         total1 = scale_freq(lettertxt)
 111         total2 = scale_freq(english_counts)
 112         fit = value_diff(total2, total1)
 113         if fit < best_fit:
 114             best_key = shift
 115             best_fit = fit
 116     return best_key
 117
 118 def affine_break(message):
 119     best_key = (0, 0, 0)
 120     best_fit = float("inf")
 121     for multiplier in range(1, 26, 2):
 122         for shift in range(26):
 123             for one_based in [True, False]:
 124                 plaintxt = affine_decipher_message(message, multiplier, shift, one_based)
 125                 lettertxt = letter_frequencies(plaintxt)
 126                 total1 = scale_freq(lettertxt)
 127                 total2 = scale_freq(english_counts)
 128                 fit = value_diff(total2, total1)
 129                 if fit < best_fit:
 130                     best_key = (multiplier, shift, one_based)
 131                     best_fit = fit
 132     return best_key