Python Assignment Help - Solution for CBC Encryption Task

• 19th Jan, 2023
• 16:59 PM

Python Assignment Help - Solution for CBC Encryption Task

#, ,
#Python 3.6

import Crypto.Cipher.AES as AES

def xor(a, b):
    my_result = bytearray()
    for a, b in zip(a, b):
        xor = a ^ b
        my_result.append(xor)
    return my_result

def get_xor(a, b):
    xor = ""
    for x,y in zip(a,b):
        xor += str(int(x) ^ int(y))
    return xor

def find_flipped_block(my_blocks, n):
    for i in range(1, n+1):
        for j in range(1, 16):
            right = my_blocks[n-i][0]
            left = my_blocks[n-i][j]
            test = right ^ left
            if test != 0:
                return n-i

def get_common_byte(block):
    for i in range(0, 16):
        if block[i] == block[i+1]:
            return block[i]
        else:
            if block[i] == block[i+2]:
                return block[i]
            else:
                return block[i+1]

def cbc_custom_decrypt(k, n, cipher):
    if n <= 0:
        return
    my_blocks = []
    crypto = AES.new(k, AES.MODE_ECB)
    my_blocks.append(cipher[16:32])
    my_decryption = []
    if n > 1:
        for i in range(2, n + 1):
            index = 16 * i
            my_blocks.append(cipher[index: index + 16])
    blocks_number = len(my_blocks)
    for j in range(1, blocks_number):
        decryption = crypto.decrypt(my_blocks[blocks_number - j])
        one_slice = xor(decryption, my_blocks[blocks_number - j - 1])
        my_decryption.append(one_slice)
    last_decryption = crypto.decrypt(my_blocks[0])
    first_slice = xor(last_decryption, cipher[:16])
    my_decryption.append(first_slice)
    my_length = len(my_decryption)
    for k in range(2, my_length + 1):
        first_slice.extend(my_decryption[my_length - k])
    return bytes(first_slice)

def cbc_flip_fix(k, n, cipher):
    my_flipped_plaintext = cbc_custom_decrypt(k, n, cipher)
    my_blocks = []
    byte_index = 0
    for i in range(0, n):
        index = 16 * i
        my_blocks.append(my_flipped_plaintext[index: index + 16])
    flipped_block_num = find_flipped_block(my_blocks, n)
    common = get_common_byte(my_blocks[flipped_block_num])
    for j in range(1, 16):
        if my_blocks[flipped_block_num][0] == common:
            if my_blocks[flipped_block_num][0] != my_blocks[flipped_block_num][j]:
                byte_index = j
        else:
            byte_index = 0
    err = my_blocks[flipped_block_num][byte_index]
    err = "{:08b}".format(err)
    common = "{:08b}".format(common)
    test_xor = get_xor(common, err)
    cipher_index = 16 * (flipped_block_num-1) + 16
    flipped_cipher_block = cipher[cipher_index:cipher_index+17]
    flipped_cipher_byte = "{:08b}".format(flipped_cipher_block[byte_index])
    new_cipher_byte = bytes([int(get_xor(test_xor, flipped_cipher_byte), 2)])
    new_cipher = cipher[:cipher_index + byte_index] + new_cipher_byte + cipher[cipher_index + byte_index + 1:]
    block_plain_index = 16 * (flipped_block_num - 1)
    my_temp = cbc_custom_decrypt(k, n, new_cipher)
    return my_temp[block_plain_index : block_plain_index + 16]

if __name__ == "__main__":
    k = b'\x81\x0ff\t\x04\xb6\xcf\x1f.\x10\x8frd\xb4E\x19'
    print("type(k) = {}".format(type(k)))
    print("k.hex() = {}".format(k.hex()))
    IV = b'e|\x92\xd0\x8b\xd9\x00\xc8X\xf2Noi\xa1\x155'
    print("len(IV) = {}".format(len(IV)))
    cipher = b'e|\x92\xd0\x8b\xd9\x00\xc8X\xf2Noi\xa1\x155\xb5b\xe7r\xb1\xec\xb5\xed\xca\xca\x1f$\xf8\xe33%'
    print("type(cipher) = {}".format(type(cipher)))
    print("len(cipher) = {}".format(len(cipher)))
    print("Question 1:")
    print("N = 1")
    output = cbc_custom_decrypt(k, 1, cipher)
    print("\ttype(output) = {}".format(type(output)))
    print("\toutput = {}".format(output))
    print("N = 2")
    output = cbc_custom_decrypt(k, 2, cipher)
    print("\ttype(output) = {}".format(type(output)))
    print("\toutput = {}".format(output))
    print("Test more:")
    k = b'\x81\x0ff\t\x04\xb6\xcf\x1f.\x10\x8frd\xb4E\x19'
    n = 1
    cipher = b'e|\x92\xd0\x8b\xd9\x00\xc8X\xf2Noi\xa1\x155\x8b\xa5\xb7\xdcka\xaa\x94=a_!x\x1a\xcf\xf4'
    output = cbc_custom_decrypt(k, n, cipher)
    print("\ttype(output) = {}".format(type(output)))
    print("\toutput = {}".format(output))
    print("Question 2:")
    print("N = 1")
    cipher = b'e|\x92\xd0\x8b\xd9\x00\xc8X\xf2Noi\xa1\x155\xb5b\xe7r\xb1\xec\xb5\xed\xca\xca\x1f$\xf8\xe33%'
    output = cbc_flip_fix(k, 1, cipher)
    print("\ttype(output) = {}".format(type(output)))
    print("\toutput = {}".format(output))