Chapter 21 – Hacking the Vigenère Cipher 331

188. for indexes in itertools.product(range(NUM_MOST_FREQ_LETTERS),
     repeat=mostLikelyKeyLength):


189. 
     

     Create a possible key from the letters in allFreqScores

     
     


190. possibleKey = ''


191. for i in range(mostLikelyKeyLength):


192. possibleKey += allFreqScores[i][indexes[i]][0]


193. 
     


194. if not SILENT_MODE:


195. print('Attempting with key: %s' % (possibleKey))


196. 
     


197. decryptedText = vigenereCipher.decryptMessage(possibleKey,
     ciphertextUp)


198. 
     


199. if detectEnglish.isEnglish(decryptedText):


200. 
     

     Set the hacked ciphertext to the original casing.

     
     


201. origCase = []


202. for i in range(len(ciphertext)):


203. if ciphertext[i].isupper():


204. origCase.append(decryptedText[i].upper())


205. else:


206. origCase.append(decryptedText[i].lower())


207. decryptedText = ''.join(origCase)


208. 
     


209. 
     

     Check with user to see if the key has been found.

     
     


210. print('Possible encryption hack with key %s:' % (possibleKey))


211. print(decryptedText[:200]) # only show first 200 characters


212. print()


213. print('Enter D for done, or just press Enter to continue
     hacking:')


214. response = input('> ')


215. 
     


216. if response.strip().upper().startswith('D'):


217. return decryptedText


218. 
     


219. 
     

     No English-looking decryption found, so return None.

     
     


220. return None


221. 
     


222. 
     


223. def hackVigenere(ciphertext):


224. 
     

     First, we need to do Kasiski Examination to figure out what the

     
     


225. 
     

     length of the ciphertext's encryption key is.

     
     


226. allLikelyKeyLengths = kasiskiExamination(ciphertext)


227. if not SILENT_MODE:


228. keyLengthStr = ''


229. for keyLength in allLikelyKeyLengths:


230. keyLengthStr += '%s ' % (keyLength)