README.md

TokyoWesterns - easy_hash (75pt / 226 solves)

We are provided with the source code:

import struct
import os

MSG = b'twctf: please give me the flag of 2020'

assert os.environ['FLAG']

def easy_hash(x):
    m = 0
    for i in range(len(x) - 3):
        m += struct.unpack('<I', x[i:i + 4])[0]
        m = m & 0xffffffff
    return m

def index(request):
    message = request.get_data()
    if message[0:7] != b'twctf: ' or message[-4:] != b'2020':
        return b'invalid message format: ' + message

    if message == MSG:
        return b'dont cheet'

    msg_hash = easy_hash(message)
    expected_hash = easy_hash(MSG)
    if msg_hash == expected_hash:
        return 'Congrats! The flag is ' + os.environ['FLAG']
    return 'Failed: easy_hash({}) is {}. but expected value is {}.'.format(message, msg_hash, expected_hash)

The hashing function adds 4-byte windows of the input string, mod 2³². Thus, the hash won't change if we increment one of the bytes while decrementing another:

$ curl https://crypto01.chal.ctf.westerns.tokyo -d 'twctf: qleare give me the flag of 2020'
Congrats! The flag is TWCTF{colorfully_decorated_dream}