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flip.py
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flip.py
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#!/usr/bin/env python3
##########################################################
# THM - Flip (https://tryhackme.com/room/flip)
#
# A CTF Challenge involving an attack on AES-CBC
# encryption by intelligently using a bit flip.
#
##########################################################
from Crypto.Cipher import AES
from Crypto.Util.Padding import pad, unpad
from Crypto.Random import get_random_bytes
from binascii import unhexlify
from pwn import *
import re
# EDIT this to point at your target address once you pressed "Start Machine"
target_ip: str = '10.10.96.249'
# I copied encrypt_data() and decrypt_data() from the "leaked" app sourcecode
# in the downloadable challenge files:
def encrypt_data(unencrypted_params: str, enc_key: bytes, i_vector: bytes) -> str:
padded = pad(unencrypted_params.encode(), 16, style='pkcs7')
cipher = AES.new(enc_key, AES.MODE_CBC, i_vector)
enc = cipher.encrypt(padded)
return enc.hex()
def decrypt_data(encrypted_params: str, enc_key: bytes, i_vector: bytes) -> int:
cipher = AES.new(enc_key, AES.MODE_CBC, i_vector)
padded_params = cipher.decrypt(unhexlify(encrypted_params))
print(padded_params)
if b'admin&password=sUp3rPaSs1' in unpad(padded_params, 16, style='pkcs7'):
return 1
else:
return 0
def main() -> None:
# Generate a key and an initialization vector for AES
key: bytes = get_random_bytes(16)
iv: bytes = get_random_bytes(16)
# Let's do a PoC locally...
# Flip one "s" in password to "r"
user: str = 'admin&parsword=sUp3rPaSs1'
password: str = 'sUp3rPaSs1'
msg: str = 'logged_username=' + user + '&password=' + password
print(msg, len(msg))
xor: int = ord('r') ^ ord('s')
cipher = encrypt_data(msg, key, iv)
# Correct the cipher on the correct position:
cipher = cipher[:16] + hex(int(cipher[16:18], 16) ^ xor)[2:] + cipher[18:]
# Now this should return "1":
print(decrypt_data(cipher, key, iv))
# Let's connect to the THM remote machine
# and weaponize our PoC code with pwntools:
conn = remote(target_ip, 1337)
print(conn.recv().decode())
print(conn.recv().decode())
# Send the tampered payload:
conn.send(b'admin&parsword=sUp3rPaSs1\r\n')
print(conn.recv().decode())
conn.send(b'\r\n')
match = re.match(r'Leaked ciphertext: (.+)\n', conn.recv().decode())
print('[*] Ciphertext:', match[1])
# Correct the leaked cipher to read "admin&password=sUp3rPaSs1" again...
cipher = match[1]
cipher = cipher[:16] + hex(int(cipher[16:18], 16) ^ xor)[2:] + cipher[18:]
print('[*] Modified Ciphertext:', cipher)
# ... and send it back to the server.
conn.send(cipher.encode() + b'\r\n')
# Hopefully collect our flag from the server, then close the connection:
print(conn.recv().decode())
conn.close()
if __name__ == '__main__':
main()