Dream (crypto)

I hear python MT can be broken with 624 outputs, but I only really need 8 random numbers. Surely you can't break it... right?

982B

chall.py

225B

Dockerfile

97B

docker-compose.yml

We are given the following code:

#!/usr/local/bin/python
if __name__ != "__main__":
    raise Exception("not a lib?")

from os import urandom
# check if seed.txt exists
try:
    seed = open("seed.txt", "rb").read()
except:
    seed = urandom(8)
    # seed is 8 bytes
    open("seed.txt", "wb").write(seed)

seed = int.from_bytes(seed, "big")
import random
random.seed(seed)
from ast import literal_eval
idxs = literal_eval(input(">>> "))
if len(idxs) > 8:
    print("Ha thats funny")
    exit()
for idx in range(624):
    rand_out = random.getrandbits(32)
    if idx in idxs:
        print(rand_out)


key = random.getrandbits(256)
nonce = random.getrandbits(256)
flag = open("flag.txt").read()
from Crypto.Cipher import AES
from Crypto.Util.Padding import pad, unpad
from hashlib import sha256
aes_key = sha256(str(key).encode()).digest()[:16]
aes_nonce = sha256(str(nonce).encode()).digest()[:16]
cipher = AES.new(aes_key, AES.MODE_GCM, nonce=aes_nonce)
ct = cipher.encrypt(pad(flag.encode(), 16))
print(ct.hex())

The seed is hardcoded in seed.txt. It is read and used. Then a list is accepted as input. The given list must be at most of length 8. Then, 624 random 32-bit numbers are generated. If their index is present in the given list, they will be printed out.

Then, the key and nonce are generated (again using random.getrandbits), and the flag is encrypted.

In order to retrieve the seed, we need to retrieve all 624 numbers. Since the seed is hardcoded, we can repeatedly connect to the server to retrieve all of them. Each time we would retrieve a different idx from the server. Then, using randcrack, we can work out the Mersenne Twister seed, generate the key and nonce, and decrypt the flag.

from pwn import *
from randcrack import RandCrack
from Crypto.Cipher import AES
from Crypto.Util.Padding import pad, unpad
from hashlib import sha256

context.log_level = 'debug'
rc = RandCrack()
curr_idx = 0

for i in range(78):
    conn = remote("vsc.tf", 5001)
    payload = b"["
    for j in range(8):
        payload += str(curr_idx).encode()

        payload += b","
        curr_idx += 1
    payload += b"]"
    
    conn.sendlineafter(b">>> ", payload)
    for j in range(8):
        x = conn.recvline()
        info(x)
        x = int(x[:-1].decode())
        rc.submit(x)
    conn.close()

conn = remote("vsc.tf", 5001)
conn.sendlineafter(b">>> ", b"[0]")
conn.recvline()
ct = bytes.fromhex(conn.recvline()[:-1].decode()) # receive ciphertext in hex, convert to number in bytes
conn.close()
key = rc.predict_getrandbits(256)
nonce = rc.predict_getrandbits(256)
aes_key = sha256(str(key).encode()).digest()[:16]
aes_nonce = sha256(str(nonce).encode()).digest()[:16]
cipher = AES.new(aes_key, AES.MODE_GCM, nonce=aes_nonce)
pt = cipher.decrypt(ct)
print(unpad(pt, 16))
# b'vsctf{dream_luck???_5e3ec2f2d338fc9f}'