Testing of Simple Substitution Cipher

In this chapter, we will focus on testing substitution cipher using various methods, which helps to generate random strings as given below −

Example - Testing Substitution Cipher

main.py

import random, string

LETTERS = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ'

def main():
   for i in range(1000):
      key = getRandomKey()
      message = random_string()
      print('Test %s: String: "%s.."' % (i + 1, message[:50]))
      print("Key: " + key)
      encrypted = translateMessage(message, key, 'E')
      decrypted = translateMessage(encrypted, key, 'D')
      
      if decrypted != message:
         print('ERROR: Decrypted: "%s" Key: %s' % (decrypted, key))
         sys.exit()
      print('Substutition test passed!')

def random_string(size = 5000, chars = string.ascii_letters + string.digits):
   return ''.join(random.choice(chars) for _ in range(size))
   
def translateMessage(message, key, mode):
   translated = ''
   charsA = LETTERS
   charsB = key
   
   # If decrypt mode is detected, swap A and B
   if mode == 'D':
      charsA, charsB = charsB, charsA
   for symbol in message:
      if symbol.upper() in charsA:
         symIndex = charsA.find(symbol.upper())
         if symbol.isupper():
            translated += charsB[symIndex].upper()
         else:
            translated += charsB[symIndex].lower()
      else:
         translated += symbol
   return translated

def getRandomKey():
   randomList = list(LETTERS)
   random.shuffle(randomList)
   return ''.join(randomList)

if __name__ == '__main__':
   main()

Output

You can observe the output as randomly generated strings which helps in generating random plain text messages, as shown below −

Test 1: String: "7KdDVIFW16UM8U0lNwn3vcnky4EYCKbFUbxh1hYqQdKcKnq4l9.."
Key: VKAYMQGLZHNOUJDCETFPSBWRXI
Substutition test passed!
Test 2: String: "c78WvP3zLihhCogHNTvhQJ5TsZWaCX71p71xU1UPHSOBmAgLQq.."
Key: ROUXJYHEPLGMVDWCQTIAFKZNBS
Substutition test passed!
...

After the test is successfully completed, we can observe the output message Substitution test passed!.

...
Test 998: String: "P9rpUdfVtHYwH8oCMJssNu2AYl8vVkoJB0BwcXmz1ldXPSNlyy.."
Key: SXKWFYBIJAUTVDGELZHCNOPMQR
Substutition test passed!
Test 999: String: "EWGGt9RSrlZwYo3FUNG4JCtDAytUWF0rihlcJEvQd4nZxo0Qvg.."
Key: VCIGEBQTWHNXRSYUMKOFLJADPZ
Substutition test passed!
Test 1000: String: "GYsSq3xU33VP2f3pnAajgMq1ClUzQPGe72fL9sv9xYV0Bpx04Y.."
Key: REMIUPWXVLDYGHCOJFSKQBTAZN
Substutition test passed!

Thus, you can hack a substitution cipher in the systematic manner.