Pure Python implementation for HashCash proof-of work, friendly for CTF.

hashcash_pow.py

from typing import Optional
import re
from pwn import *


def mint(
    resource: str,
    bits: int = 20,
    now: Optional[float] = None,
    ext: str = "",
    salt_length: int = 8,
    stamp_seconds: bool = False,
):
    from secrets import token_urlsafe
    import hashlib
    from time import strftime, localtime, time
    from itertools import count

    def _mint(challenge: str, bits: int) -> str:  # type: ignore
        counter = 0
        hex_zeros = "0" * (bits // 4)
        remain_zeros = {0: 0b0000, 1: 0b0111, 2: 0b0011, 3: 0b0001}.get(bits % 4, 0)
        counter_hex = "0"
        for counter in count():
            counter_hex = f"{counter:x}"
            hash = hashlib.sha1(f"{challenge}{counter_hex}".encode()).hexdigest()
            if not hash.startswith(hex_zeros):
                continue
            if not remain_zeros:
                break
            if int(hash[bits // 4], 16) <= remain_zeros:
                break
        return counter_hex

    ver = "1"
    now = now or time()
    if stamp_seconds:
        ts = strftime("%y%m%d%H%M%S", localtime(now))
    else:
        ts = strftime("%y%m%d", localtime(now))
    challenge = "%s:" * 6 % (ver, bits, ts, resource, ext, token_urlsafe(salt_length))
    return challenge + _mint(challenge, bits)


HASHCASH_RE = re.compile(r"`hashcash -mb(\d+) (\S+)`")

r = remote("120.46.65.156", 32106)

matched = None
while matched is None:
    line = r.recvline().decode()
    print(line)
    matched = HASHCASH_RE.search(line)

pow_bits, pow_resource = matched.groups()
mint_result = mint(pow_resource, bits=int(pow_bits))

r.sendline(mint_result.encode())
r.interactive()