Cipher Application

About

The Cipher Application is a python based project that allows users to encrpyt/decrypt Messages using popular cipher mechanisms used in the past and enables the user to understand the implementation of the encrypting/decrypting process.

Setup process

1.Run the following command to install all packages

pip install -r requirements.txt

Run the App

2.Run the following command to run the app inside the project folder

python3 main.py

Project

Playfair Cipher

This project includes an advanced version of the default Playfair Cipher designed by Charles Wheatstone in 1854. This Cipher was later used for military purposes by Lord Playfair - hence the name. I chose to implement this Cipher because it is far ahead of time in terms of complexity and allows a decent level of security with a relatively simple protocol. You can get more info about the default Cipher on the following link -> Playfair Cipher

The encrypt.py file has a PlayfairEncrypt() function which takes in two arguments and return the encrypted text as per the Playfair encoding procedure. The two arguments passed into the function is the Message itself and the Keyword which will be used to alter the grid, in order to get a unique Grid (Like a shared secret).The MakeGrid(Keyword) function generates this 6x6 grid and is called inside the PlayfairEncrypt() function which is used to encrypt the Message as per the Keyword entered by the user.

$ python3
Python 3.6.9 (default, Nov  7 2019, 10:44:02) 
[GCC 8.3.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> from encrypt import EncryptPlayfair
>>> EncryptPlayfair('Hi I am Kryptos','lizard')
'CDZRNMI?MWKV'
>>> from decrypt import DecryptPlayfair
>>> DecryptPlayfair('CDZRNMI?MWKV','lizard')
'HIIAMKRYPTOS'

Notice the use of special character in the cryptic message this is because the new grid supports 10 characters for encoding however the default playfair implementation uses only Letters A-Z (26 letters). This way we can modify the grid to increase the complexity of encrypted message and make it harder to decrypt.

Hill Cipher

Another Cipher mechanism that I find quite interesting is the simple implementation of the Hill Cipher which was invented by Lester S. Hill in 1929 using principles of linear algebra. In the encryption process we use a 2x2 matrix with very unique properties which is used as a secret. The inverse modulo 26 of this matrix is used as the key to decrypt the cipher. The way this encryption process works is, we divide the message in blocks of 2s, convert it into ASCII code and multiply with the 'secret matrix'. We decrypt the message using the inverse of this matrix and multiply it with encrypted message the same way(in blocks of 2 letters at a time) and we get back our original message. The only drawback in this method is to find a key matrix that has a inverse modulo of 26.

The hill.py file uses the key matrix from utils.py and the encrypt() function takes in the message and the key Matrix and gives the encrypted message as the output. However, this method uses Block encryption without shuffling the encrypted blocks which is a vulnerability. The decrypt() function works in a similar way as well, except it takes the encrypted message and converts the Key Matrix to the inverse modulo 26 of itself for decryption.

$ python3
Python 3.6.9 (default, Nov  7 2019, 10:44:02) 
[GCC 8.3.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> from hill import encrypt,decrypt
>>> from utils import KeyMatrix
>>> encrypt('I am Kryptos',KeyMatrix)
'DWTCYEDBXU'
>>> decrypt('DWTCYEDBXU',KeyMatrix)
'IAMKRYPTOS'

Feistel Cipher

The Feistel network is the base framework for many Cipher mechanisms like the DES and 3-DES which allows us to select a typical function called the round function and plug it in the framework to result in a completely new/distinct encryption method depending on the number of rounds of encryption and the function selected. A unique property of the Feistel network is that the decryption mechanism is not the reversal of the encryption method, but it is exactly the same, only the keys are used in the reversed order to decrypt the message. This network can effectively reverse any function used as the round function in the encryption process - which even includes all hash functions! The principle behind this is quite basic - if a number is xor'ed with itself odd number of times it will result in the number. This video explains the concept perfectly.

The Encrypt() and Descrypt() functions in the feistel.py file work exactly the same way (apart from some preprocessing). However, while using these functions the order of the keys entered in the latter is reversed so as to reverse the whole encryption process.