ex_7.py

import os
import random
from Crypto.Cipher import AES
from AES_CBC import *

prepend_string = "comment1=cooking%20MCs;userdata="
append_string = ";comment2=%20like%20a%20pound%20of%20bacon"
parameter = b";admin=true;"

keysize = 16
random_key = os.urandom(keysize)
IV = os.urandom(keysize)


def encryptor(text: bytes, IV: bytes, key: bytes) -> bytes:
	# 将给定的字符串添加到自定义文本中，并通过AES_CBC模式进行加密

	plaintext = (prepend_string.encode() + text + append_string.encode()).replace(b';', b'";"').replace(b'=', b'"="')
	ciphertext = AES_CBC_encrypt(PKCS7_pad(plaintext, len(key)), IV, key)
	return ciphertext


def decryptor(byte_string: bytes, IV: bytes, key: bytes) -> bool:

	decrypted_string = PKCS7_unpad(AES_CBC_decrypt(byte_string, IV, key))
	if b";admin=true;" in decrypted_string:
		return True
	else:
		return False


def CBC_bit_flipping(parameter: bytes, keysize: int, encryptor: callable) -> bytes:

	padding = 0
	random_blocks = 0
	cipher_length = len(encryptor(b'', IV, random_key))
	prefix_length = len(os.path.commonprefix([encryptor(b'AAAA', IV, random_key), encryptor(b'', IV, random_key)]))
	print("Prefix length: ", prefix_length)

	for i in range(int(cipher_length / keysize)):
		if prefix_length < i * keysize:
			random_blocks = i
			break
	print("Random blocks: ", random_blocks)

	base_cipher = encryptor(b'', IV, random_key)
	for i in range(1, keysize):
		new_cipher = encryptor(b'A' * i, IV, random_key)
		new_prefix_length = len(os.path.commonprefix([base_cipher, new_cipher]))
		if new_prefix_length > prefix_length:
			padding = i - 1
			break
		base_cipher = new_cipher
	print("Number of bytes of padding required: ", padding)

	input_text = b'A' * padding + b"heytheremama"
	string = parameter
	modified_string = b""
	ciphertext = encryptor(input_text, IV, random_key)
	for i in range(len(string)):
		modified_string += (
					ciphertext[i + (random_blocks - 1) * keysize] ^ (input_text[i + padding] ^ string[i])).to_bytes(1,
		                                                                                                            "big")

	modified_ciphertext = ciphertext[:(random_blocks - 1) * keysize] + modified_string + ciphertext[(
				                                                                                                random_blocks - 1) * keysize + len(
		modified_string):]

	return modified_ciphertext


modified_ciphertext = CBC_bit_flipping(parameter, keysize, encryptor)
print(AES_CBC_decrypt(modified_ciphertext, IV, random_key))