Just Take Your Time

The service asked to decrypt the secret it sent, it would give the flag if the secret can be successfully decrypted. The key for encrypt was made from time(), the secret was a 16 bytes token in hex, encrypted with des3-cfb, iv is fixed, b“00000000”. It allowed to guess for 3 times with timeout. To decrypt it using Crypto.Cipher.DES3, needed to find the same key server was used, so it’s matter of time to create the key.

key = str(int(time())).zfill(16).encode("utf-8")
secret = token_hex(16)
cipher = DES3.new(key, DES3.MODE_CFB, b"00000000")
encrypted = cipher.encrypt(secret.encode("utf-8"))

First of all, tried to get the encrypted secret within the first timeout, that was easy with code to read the two numbers then sent the result of a*b.

a = randint(1000000000000000, 9999999999999999)
b = randint(1000000000000000, 9999999999999999)

print("Show me you are worthy and solve for x! You have one second.")
print("{} * {} = ".format(a, b))

answ, _ = timedInput("> ", timeOut = 1, forcedTimeout = True)

try:
    assert(a*b == int(answ))
except:
    print("You are not worthy!")
    exit(1)

Found out a good moment to create the key after several attempts. With one last try, server replied with the flag.

pr.readline()
line = pr.readline().strip().decode().split()
des3 = DES3.new(str(int(time())).zfill(16).encode("utf-8"),DES3.MODE_CFB, b"00000000")
a, b = int(line[0]), int(line[2])
print(a, b)
pr.sendlineafter('>', str(a*b))
pr.readline()
enc = bytes.fromhex(pr.readline().strip().decode())
dec = des3.decrypt(enc)
pr.sendlineafter('>', dec)
print(dec, len(dec))
print(pr.readline())
print(pr.readall(2))

$ python3 exploit.py 
[+] Opening connection to dctf-chall-just-take-your-time.westeurope.azurecontainer.io on port 9999: Done
3813597240164301 1847525826721293
b'ed715a18e9e8d3faffcf71cba9584777' 32
b' Congratulations! Here is your flag.\n'
[+] Receiving all data: Done (29B)
[*] Closed connection to dctf-chall-just-take-your-time.westeurope.azurecontainer.io port 9999
b'dctf{1t_0n1y_t0Ok_2_d4y5...}\n'

Full exploit goes here.