"""简易密封：无需 cryptography 依赖。"""

from __future__ import annotations

import base64
import hashlib
import hmac
import os


def _keystream(key: bytes, n: int) -> bytes:
    out = bytearray()
    counter = 0
    while len(out) < n:
        block = hashlib.sha256(key + counter.to_bytes(8, "big")).digest()
        out.extend(block)
        counter += 1
    return bytes(out[:n])


def seal(plaintext: str, secret: str) -> str:
    raw = plaintext.encode("utf-8")
    key = hashlib.sha256(secret.encode("utf-8")).digest()
    iv = os.urandom(16)
    stream = _keystream(key + iv, len(raw))
    cipher = bytes(a ^ b for a, b in zip(raw, stream))
    mac = hmac.new(key, iv + cipher, hashlib.sha256).digest()
    return base64.urlsafe_b64encode(iv + mac + cipher).decode("ascii")


def unseal(blob: str, secret: str) -> str:
    data = base64.urlsafe_b64decode(blob.encode("ascii"))
    if len(data) < 16 + 32:
        raise ValueError("invalid sealed blob")
    iv, mac, cipher = data[:16], data[16:48], data[48:]
    key = hashlib.sha256(secret.encode("utf-8")).digest()
    expect = hmac.new(key, iv + cipher, hashlib.sha256).digest()
    if not hmac.compare_digest(expect, mac):
        raise ValueError("sealed blob mac mismatch")
    stream = _keystream(key + iv, len(cipher))
    raw = bytes(a ^ b for a, b in zip(cipher, stream))
    return raw.decode("utf-8")