backend/app/auth.py

"""HTTP Basic auth for the API.

Gate is a single ASGI middleware that protects `/api/*` (the data + functionality).
`/health`, the static SPA shell, and `/portal` stay open so the login screen can
load and Vercel health checks pass. Credentials come from APERTURE_USER /
APERTURE_PASS (defaults: demo / aperture2026 for the demo deployment).

Stateless: the SPA stores base64(user:pass) and sends it on every request; there
is no server-side session to manage on serverless.
"""
from __future__ import annotations

import base64
import secrets

from starlette.requests import Request
from starlette.responses import JSONResponse


def _check(authorization: str | None, user: str, pwd: str) -> bool:
    if not authorization or not authorization.lower().startswith("basic "):
        return False
    try:
        decoded = base64.b64decode(authorization.split(" ", 1)[1]).decode("utf-8")
        u, _, p = decoded.partition(":")
    except Exception:
        return False
    return secrets.compare_digest(u, user) and secrets.compare_digest(p, pwd)


def make_auth_middleware(user: str, pwd: str):
    async def auth_middleware(request: Request, call_next):
        path = request.url.path
        # Only guard the API; allow preflight, health, SPA shell, and portal.
        if request.method == "OPTIONS" or not path.startswith("/api/"):
            return await call_next(request)
        if _check(request.headers.get("authorization"), user, pwd):
            return await call_next(request)
        # NOTE: deliberately NO `WWW-Authenticate: Basic` header — that triggers the
        # browser's native credential popup. The React SPA handles 401 itself and
        # shows its own login screen, so we return a plain JSON 401.
        return JSONResponse({"detail": "Authentication required"}, status_code=401)

    return auth_middleware