caesar_frequency_attack.py

#!/usr/bin/env python
# Tries to decrypt a Caesar cipher without the key. 
# Uses brute force and compare the results against the English letter frequency.

# Letters in the English alphabet
LETTERS = 26

# Frequency per letter in the English alphabet
english = {
   'a': 0.082, 'b': 0.015, 'c': 0.028, 'd': 0.043, 'e': 0.127, 'f': 0.022,
   'g': 0.020, 'h': 0.061, 'i': 0.070, 'j': 0.002, 'k': 0.008, 'l': 0.040,
   'm': 0.024, 'n': 0.067, 'o': 0.075, 'p': 0.019, 'q': 0.001, 'r': 0.060,
   's': 0.063, 't': 0.091, 'u': 0.028, 'v': 0.010, 'w': 0.023, 'x': 0.001,
   'y': 0.020, 'z': 0.001
}

# Calculate a alphabet frequency for a string
def frequency(cipher):
   freq = {}
   letter_counter = 0
   for letter in cipher:
      if letter >= 'a' and letter <= 'z':
         letter_counter += 1
         if freq.has_key(letter):
            freq[letter] += 1.0
         else:
            freq[letter] = 1.0
   for letter in freq:
      freq[letter] = freq[letter]/letter_counter
   return freq

# Quantificate the distance between two alphabet frequencies
def distance(frequency_a, frequency_b):
   sum = 0
   for letter in frequency_a:
      if frequency_b.has_key(letter):
         sum += abs(frequency_a[letter]-frequency_b[letter])
   return sum

# Decrypts cipher using key
def caesar_decrypt(cipher, key):
   message = ""
   for letter in cipher:
      original = letter
      if letter >= 'a' and letter <= 'z':
         original = chr(ord('a') + ((ord(letter)+key)%LETTERS))
      message += original
   return message

# Find the key that decryptes the cipher to a message with minimal distance
# to the English frequency
def break_caesar(cipher):
   best_distance = float("inf")
   best_key = 0
   best_message = ""
   for key in xrange(LETTERS):
      message = caesar_decrypt(cipher, key)
      freq = frequency(message)
      current_distance = distance(freq, english)
      if current_distance < best_distance:
         best_distance = current_distance
         best_key = key
         best_message = message
   return best_message, best_key, best_distance

print break_caesar('qefp fp qeb cfopq qbuq')