S-AES

Repository for cryptography homework, teamed up with Yuyang Hu @Gracivio, a S-AES instance.

Procedure

1. Encryption

   • According to S-AES tutorial, the encryption procedure follows first XOR, nibblebyte substitution, shift rows, confuse columns, second XOR, nibblebyte substitution, shift rows and third XOR. For each XOR, a different key is used so that we need three keys. To generate three keys from the original one, a key extend function is used.
   • The key extend function first divide the key into two 8-bits parts, the right of which goes into a function that follows a bunch of steps for better performance, which is also called as the g-function. The g-function first swaps the two parts, then do a substitution, after that do a XOR process with initial vector(settled).

   def function_g(w, round):
       left = w[:4]
       right = w[4:]
       temp = left
       left = right
       right = temp
       left_x = int(left[:2], 2)
       left_y = int(left[2:], 2)
       right_x = int(right[:2], 2)
       right_y = int(right[2:], 2)
       left = bin(S_Box[left_x][left_y])[2:].zfill(4)
       right = bin(S_Box[right_x][right_y])[2:].zfill(4)
       res = left + right
       if round == 1:
         res = XOR(res, RCON[0])
       elif round == 2:
         res = XOR(res, RCON[1])
       return res
   

   • The nibblebyte substituion divides the plain text into four 4-bits and turn into a state matrix. And for each part, we do a substituion.
   • The row shift only swaps the second row
   • The column confusion goes a GF(2^4) with mod x^4+x+1 calculation.

2. Decryption

   • According to S-AES tutorial, the decryption procedure is alike encryption but reversed. Following first XOR(use Key-2), reversed shift rows, reversed nibblebyte substitution, second XOR(use Key-1), reversed column confusion, reversed shift rows, reversed nibblebyte substitution, third XOR(use Key-0). For nibblebyte substituion, row shifts and column confusion, a reversed S-Box and matrix is used, which under the calculation of math.

   S_Box = [(9, 4, 10, 11), (13, 1, 8, 5), (6, 2, 0, 3), (12, 14, 15, 7)]
   S_Box_Verse = [(10, 5, 9, 11), (1, 7, 8, 15), (6, 0, 2, 3), (12, 4, 13, 14)]
   

Feature

1. 2-AES
   • 2-AES has 16-bit plain text or cipher text input, and 32-bit key. In 2-AES, we use two S-AES to encrypt the plaintext via two keys, and each key is seperated of 16-bit from the 32-bit key.
   • Also, we add a crack feature in 2-AES, given plain text and corresponding cipher text, we use a key to encrypt the plaintext and use the other to decrypt the cipher text, and compare the middle text to perform the attack.
2. 3-AES
   • Like 2-AES, we use three S-AES to encrypt the plaintext, the process is encrypt,encrypt,encrypt.
3. CBC Mode
   • Also known as Cipher Block Chaining, which be able to encrypt long bits plaintext.

GUI

• We use tkinter to create the user interface. Relative layout is used for better demonstration, but some particular devices still see wrong layout.

How to use

• Download S-AES.py and utility.py, and you need to download some packages to meet the environment it runs on.

Results

• Go to Test results.md in this repository for further information :)