The Three-Eyed Oracle

chall.py

from Crypto.Cipher import AES
from Crypto.Util.Padding import pad
import random
import signal
import subprocess
import socketserver

FLAG = b'HTBREDACTED'
prefix = random.randbytes(12)
key = random.randbytes(16)


def encrypt(key, msg):
    msg = bytes.fromhex(msg)
    crypto = AES.new(key, AES.MODE_ECB)
    padded = pad(prefix + msg + FLAG, 16)
    return crypto.encrypt(padded).hex()


def challenge(req):
    req.sendall(b'Welcome to Klaus\'s crypto lab.\n' +
                b'It seems like there is a prefix appended to the real firmware\n' +
                b'Can you somehow extract the firmware and fix the chip?\n')
    while True:
        req.sendall(b'> ')
        try:
            msg = req.recv(4096).decode()

            ct = encrypt(key, msg)
        except:
            req.sendall(b'An error occurred! Please try again!')

        req.sendall(ct.encode() + b'\n')


class incoming(socketserver.BaseRequestHandler):
    def handle(self):
        signal.alarm(1500)
        req = self.request
        challenge(req)


class ReusableTCPServer(socketserver.ForkingMixIn, socketserver.TCPServer):
    pass


def main():
    socketserver.TCPServer.allow_reuse_address = True
    server = ReusableTCPServer(("0.0.0.0", 1337), incoming)
    server.serve_forever()


if __name__ == "__main__":
    main()

enkripsi menggunakan AES.ECB, memiliki kelemahan pada ECB, contoh

enc = "xxxxxxxxxxxxxxxx"
cipher = "<block_enkripsi>"

enc = "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx"
cipher = "<block_enkripsi><block_enkripsi>"

# dengan nilai block_enkripsi yg sama 

ide dekripsi

padded = prefix + msg + flag

padded = ???????????? + xxxx + yyyyyyyyyyyyyyyy yyyyyyyyyyyyyyyy yyyyyyyyyyyyyyyy yyyyyyyyyyyyyyyy + HTB{selamat_kamu_berhasil}

output = <block_enkripsi_1> <block_enkripsi_2> <block_enkripsi_3> <block_enkripsi_2> <block_enkripsi_3> <block_enkripsi_4>

yyyyyyyyyyyyyyyy yyyyyyyyyyyyyyyy yyyyyyyyyyyyyyyy yyyyyyyyyyyyyyyy HTB{...
yyyyyyyyyyyyyyyy yyyyyyyyyyyyyyy(0..}) yyyyyyyyyyyyyyyy yyyyyyyyyyyyyyyH TB{...
yyyyyyyyyyyyyyyy yyyyyyyyyyyyyyH(0..}) yyyyyyyyyyyyyyyy yyyyyyyyyyyyyyHT B{...
yyyyyyyyyyyyyyyy yyyyyyyyyyyyyHT(0..}) yyyyyyyyyyyyyyyy yyyyyyyyyyyyyHTB {...

flag.py

from pwn import *
import string

url = "138.68.161.126"
port = 31192
target = remote(url, port)

p = b'78787878'
y = b'7979797979797979797979797979797979797979797979797979797979797979'
st = string.printable
f = b''

print(target.recvuntil(b'> ').decode(),end='')
inp = p+y+y
target.send(inp)
print(inp.decode())
output = target.recv().decode()
print(output,end='')

for i in range(len(inp)):
    y = y[:-2]
    for j in st:
        f += hex(ord(j))[2:].encode()
        s = p+y+f+y
        tmp = target.recvuntil(b'> ').decode()
        print(tmp,end='')
        target.send(s)
        print(bytes.fromhex(s.decode()))
        resp = target.recv().decode()
        print(resp,end='')
        if resp[32:96] == resp[96:160]:
            flag = bytes.fromhex(f.decode())
            print(flag)
            break
        else:
            f = f[:-2]

# ^C manually

flag