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 def sanitise(text):
  11     sanitised = [c.lower() for c in text if c in string.ascii_letters]
  12     return ''.join(sanitised)
  13
  14 def letter_frequencies(message):
  15     frequencies = collections.defaultdict(int)
  16     for letter in sanitise(message):
  17         frequencies[letter]+=1
  18     return frequencies
  19
  20 def scale_freq(frequencies):
  21     total= sum(frequencies.values())
  22     scaled_frequencies = collections.defaultdict(int)
  23     for letter in frequencies.keys():
  24         scaled_frequencies[letter] = frequencies[letter] / total
  25     return scaled_frequencies
  26
  27 def value_diff(frequencies1, frequencies2):
  28     total= 0
  29     for letter in frequencies1.keys():
  30         total += abs(frequencies1[letter]-frequencies2[letter])
  31     return total
  32
  33
  34
  35 def caesar_cipher_letter(letter, shift):
  36     if letter in string.ascii_letters:
  37         if letter in string.ascii_lowercase:
  38             return chr((ord(letter) - ord('a') + shift) % 26 + ord('a'))
  39         else:
  40             new_letter = letter.lower()
  41             yolo = chr((ord(new_letter) - ord('a') + shift) % 26 + ord('a'))
  42             return yolo.upper()
  43     else:
  44         return letter
  45
  46 def caesar_decipher_letter(letter, shift):
  47     return caesar_cipher_letter(letter, -shift)
  48
  49 def caesar_cipher_message(message, shift):
  50     big_cipher = [caesar_cipher_letter(l, shift) for l in message]
  51     return ''.join(big_cipher)
  52
  53 def caesar_decipher_message(message, shift):
  54     return caesar_cipher_message(message, -shift)
  55
  56 def affine_cipher_letter(letter, multiplier, shift, one_based=True):
  57     if letter in string.ascii_letters:
  58         if letter in string.ascii_lowercase:
  59             alphastart = ord('a')
  60         else:
  61             alphastart = ord('A')
  62         letter_number = ord(letter) - alphastart
  63         if one_based: letter_number += 1
  64         enciphered_letter_number = letter_number * multiplier + shift
  65         if one_based: enciphered_letter_number -=1
  66         enciphered_letter = chr(enciphered_letter_number % 26 + alphastart)
  67         return enciphered_letter
  68     else:
  69         return letter
  70
  71
  72
  73
  74 def caesar_break(message):
  75     best_key = 0
  76     best_fit = float("inf")
  77     for shift in range(26):
  78         plaintxt = caesar_decipher_message(message, shift)
  79         lettertxt = letter_frequencies(plaintxt)
  80         total1 = scale_freq(lettertxt)
  81         total2 = scale_freq(english_counts)
  82         fit = value_diff(total2, total1)
  83         if fit < best_fit:
  84             best_key = shift
  85             best_fit = fit
  86     return best_key
  87