In this chapter, you will learn about monoalphabetic cipher and its hacking using Python.

Monoalphabetic Cipher

A Monoalphabetic cipher uses a fixed substitution for encrypting the entire message. A monoalphabetic cipher using a Python dictionary with JSON objects is shown here −

monoalpha_cipher = {
   ''a'': ''m'',
   ''b'': ''n'',
   ''c'': ''b'',
   ''d'': ''v'',
   ''e'': ''c'',
   ''f'': ''x'',
   ''g'': ''z'',
   ''h'': ''a'',
   ''i'': ''s'',
   ''j'': ''d'',
   ''k'': ''f'',
   ''l'': ''g'',
   ''m'': ''h'',
   ''n'': ''j'',
   ''o'': ''k'',
   ''p'': ''l'',
   ''q'': ''p'',
   ''r'': ''o'',
   ''s'': ''i'',
   ''t'': ''u'',
   ''u'': ''y'',
   ''v'': ''t'',
   ''w'': ''r'',
   ''x'': ''e'',
   ''y'': ''w'',
   ''z'': ''q'',
	'' '': '' '',
}

With help of this dictionary, we can encrypt the letters with the associated letters as values in JSON object. The following program creates a monoalphabetic program as a class representation which includes all the functions of encryption and decryption.

from string import letters, digits
from random import shuffle

def random_monoalpha_cipher(pool = None):
   if pool is None:
      pool = letters + digits
   original_pool = list(pool)
   shuffled_pool = list(pool)
   shuffle(shuffled_pool)
   return dict(zip(original_pool, shuffled_pool))

def inverse_monoalpha_cipher(monoalpha_cipher):
   inverse_monoalpha = {}
   for key, value in monoalpha_cipher.iteritems():
      inverse_monoalpha[value] = key
   return inverse_monoalpha

def encrypt_with_monoalpha(message, monoalpha_cipher):
   encrypted_message = []
   for letter in message:
      encrypted_message.append(monoalpha_cipher.get(letter, letter))
   return ''''.join(encrypted_message)

def decrypt_with_monoalpha(encrypted_message, monoalpha_cipher):
   return encrypt_with_monoalpha(
      encrypted_message,
      inverse_monoalpha_cipher(monoalpha_cipher)
   )

This file is called later to implement the encryption and decryption process of Monoalphabetic cipher which is mentioned as below −

import monoalphabeticCipher as mc

cipher = mc.random_monoalpha_cipher()
print(cipher)
encrypted = mc.encrypt_with_monoalpha(''Hello all you hackers out there!'', cipher)
decrypted = mc.decrypt_with_monoalpha(''sXGGt SGG Nt0 HSrLXFC t0U UHXFX!'', cipher)

print(encrypted)
print(decrypted)

Output

You can observe the following output when you implement the code given above −

Thus, you can hack a monoalphabetic cipher with specified key value pair which cracks the cipher text to actual plain text.