WebsiteDiscord

One Day One Letter

Writeup for One Day One Letter (Web) - Wani CTF (2024) 💜

Description

Everything comes to those who wait

Solution

Source code

content-server.py

import json
import os
from datetime import datetime
from http import HTTPStatus
from http.server import BaseHTTPRequestHandler, HTTPServer
from urllib.request import Request, urlopen
from urllib.parse import urljoin

from Crypto.Hash import SHA256
from Crypto.PublicKey import ECC
from Crypto.Signature import DSS

FLAG_CONTENT = os.environ.get('FLAG_CONTENT', 'abcdefghijkl')
assert len(FLAG_CONTENT) == 12
assert all(c in 'abcdefghijklmnopqrstuvwxyz' for c in FLAG_CONTENT)

def get_pubkey_of_timeserver(timeserver: str):
    req = Request(urljoin('https://' + timeserver, 'pubkey'))
    with urlopen(req) as res:
        key_text = res.read().decode('utf-8')
        return ECC.import_key(key_text)

def get_flag_hint_from_timestamp(timestamp: int):
    content = ['?'] * 12
    idx = timestamp // (60*60*24) % 12
    content[idx] = FLAG_CONTENT[idx]
    return 'FLAG{' + ''.join(content) + '}'

class HTTPRequestHandler(BaseHTTPRequestHandler):
    def do_OPTIONS(self):
        self.send_response(200, "ok")
        self.send_header('Access-Control-Allow-Origin', '*')
        self.send_header('Access-Control-Allow-Methods', 'POST, OPTIONS')
        self.send_header("Access-Control-Allow-Headers", "X-Requested-With")
        self.send_header("Access-Control-Allow-Headers", "Content-Type")
        self.end_headers()

    def do_POST(self):
        try:
            nbytes = int(self.headers.get('content-length'))
            body = json.loads(self.rfile.read(nbytes).decode('utf-8'))

            timestamp = body['timestamp'].encode('utf-8')
            signature = bytes.fromhex(body['signature'])
            timeserver = body['timeserver']

            pubkey = get_pubkey_of_timeserver(timeserver)
            h = SHA256.new(timestamp)
            verifier = DSS.new(pubkey, 'fips-186-3')
            verifier.verify(h, signature)
            self.send_response(HTTPStatus.OK)
            self.send_header('Content-Type', 'text/plain; charset=utf-8')
            self.send_header('Access-Control-Allow-Origin', '*')
            self.end_headers()
            dt = datetime.fromtimestamp(int(timestamp))
            res_body = f'''<p>Current time is {dt.date()} {dt.time()}.</p>
<p>Flag is {get_flag_hint_from_timestamp(int(timestamp))}.</p>
<p>You can get only one letter of the flag each day.</p>
<p>See you next day.</p>
'''
            self.wfile.write(res_body.encode('utf-8'))
            self.requestline
        except Exception:
            self.send_response(HTTPStatus.UNAUTHORIZED)
            self.end_headers()

handler = HTTPRequestHandler
httpd = HTTPServer(('', 5000), handler)
httpd.serve_forever()

time-server.py

Exploit

When we visit the webpage, we get a single letter from the flag.

Checking HTTP history in burp, there is a POST request.

If we change the timestamp, we get a 401: Unauthorized error because the signature does not match.

So, let's try to provide our own timeserver and see if we get a hit.

We do!

So, if we create our own key pair, we can sign custom forged timestamps with our own private key and then trick the server into verifying the signature against our public key!

Let's generate the keys first - remember that the filename needs to be pubkey.

Next, we'll use a script that will loop through all 12 characters (days) of the flag, starting from yesterday (i - 1).

For each day, the script generates and signs a timestamp (using our private key) and then submits it to the content-server, with our ATTACKER_SERVER provided as the timeserver address.

The response is extracted with BeautifulSoup and then printed.

With a little extra effort, you could automatically extract the flag for each copy/paste, but this will suffice for me! 😁

Flag: FLAG{lyingthetime}

PreviousPoWNextAkasec

Last updated