Chapter ? - Code Cracker

secret3.py
  1. import secret2 as cipher
  2. #import secret3 as cipher
  3. import cryptoLib
  4. import string, time
  6. def main():
  7.     msg = cryptoLib.getMessage()
  8.     crack(msg)
  10. def getLikelihood(message, key, words):
  11.     testPlaintext = cipher.decrypt(message, key)
  12.         
  13.     realWord = 0
  14.     notRealWord = 0
  15.     for word in testPlaintext.split():
  16.         if word in words:
  17.             realWord += 1
  18.         else:
  19.             notRealWord += 1
  20.         
  21.     return (realWord / float(realWord + notRealWord)) * 100.0
  23. def getWordsFromDictionaryFile():
  24.     # Load the words from the dictionary file.
  25.     dictionaryFile = open('dictionary.txt')
  26.     words = dictionaryFile.readlines()
  27.     dictionaryFile.close()
  28.     
  29.     # Remove the newline character at the end of each word.
  30.     for i in range(len(words)):
  31.         words[i] = words[i].strip()
  32.     
  33.     return words
  34.         
  35. def crack(message):
  36.     maxLikelihood = 0.0
  37.     maxTestKey = 1
  38.     
  39.     words = getWordsFromDictionaryFile()
  40.     
  41.     # Replace any non-letter and non-apostrophe characters with spaces, so
  42.     # that we can match words in the dictionary. Also make all letters
  43.     # lowercase.
  44.     testMessage = ''
  45.     for letter in message:
  46.         if letter not in string.letters and letter != "'":
  47.             testMessage += ' '
  48.         else:
  49.             testMessage += letter.lower()
  50.     
  51.     # Do a test with a few keys to determine how long it will take to
  52.     # go through all of the keys.
  53.     startTime = time.time()
  54.     for i in range(3):
  55.         getLikelihood(testMessage, 1, words)
  56.     endTime = time.time()
  57.     averageTime = (endTime - startTime) / 3.0
  58.     totalTime = averageTime * cipher.MAX_KEY_SIZE
  59.     probableTime = totalTime / 2
  60.     
  61.     print
  62.     print 'Average time to attempt one key: %.1f seconds' % (averageTime)
  63.     print 'Total time to attempt all keys: %.1f seconds' % (totalTime)
  64.     print 'Most likely time to find the key: %.1f seconds' % (probableTime)
  65.     print
  66.         
  67.     
  68.     for testKey in range(1, cipher.MAX_KEY_SIZE + 1):
  69.         print 'Testing key: %s...' % (testKey),
  70.         
  71.         likelihood = getLikelihood(testMessage, testKey, words)
  72.         
  73.         if likelihood > maxLikelihood:
  74.             maxLikelihood = likelihood
  75.             maxTestKey = testKey
  76.             
  77.         print '%.1f%%' % (likelihood)
  78.         
  79.         if likelihood > 95.0:
  80.             break
  81.     
  82.     print 'Most probable key: %s (%.1f%%)' % (maxTestKey, maxLikelihood)
  83.     print 'Decrypted text with this key:'
  84.     print cipher.decrypt(message, maxTestKey)
  86. # TODO: This is an additional function to crack the vigenere cipher
  87. def getKeyFromKeyWord(keyWord):
  88.     key = 0
  89.     for i in range(len(keyWord)):
  90.         #print keyWord[i],
  91.         if keyWord[i] == keyWord[i].upper():
  92.             key += (ord(keyWord[i]) - ord('A')+1) * (52 ** i)
  93.             #print (ord(keyWord[i]) - ord('A')+1) * (52 ** i)
  94.         else:
  95.             key += (ord(keyWord[i]) - ord('a')+1 + 26) * (52 ** i)
  96.             #print (ord(keyWord[i]) - ord('a')+1 + 26) * (52 ** i)
  97.     return key
  99. def getKeyWordFromKey(key):
  100.     pass
  101.         
  103. import sys
  104. if __name__ == '__main__':
  105.     main()
cryptoLib.py
  1. def getMode():
  2.     mode = ''
  3.     while (mode not in 'encrypt e decrypt d'.split()):
  4.         print 'Do you wish to encrypt or decrypt a message?'
  5.         mode = raw_input()
  6.         mode = mode.lower()
  8.         if (mode not in 'encrypt e decrypt d'.split()):
  9.             print 'Enter either "encrypt" or "e" or "decrypt" or "d".'
  10.     return mode
  12. def getMessage():
  13.     print 'Enter your message:'
  14.     message = raw_input()
  15.     
  16.     return message
  18. def getKey(maxKeySize):
  19.     key = 0
  20.     while (not (key >= 1 and key <= maxKeySize)):
  21.         print 'Enter the key number (1-%s)' % (maxKeySize)
  22.         key = int(raw_input())
  23.     return key

Things Covered In This Chapter: