US08-01: Make the Trust Boundary Configurable and Authenticated (#96)

README.md

@@ -69,6 +69,34 @@ not a loopback name (DNS rebinding), when `Origin` is any other origin, when
 thumbnail), or when the body exceeds `PHOTO_PIPELINE_MAX_REQUEST_BYTES`. There is no
 CORS middleware at all, so no other origin can read a response.
 
+### Reaching it through a hostname or proxy (US08-01)
+
+| variable | meaning |
+|---|---|
+| `PHOTO_PIPELINE_ALLOWED_HOSTS` | comma-separated extra names the app answers to; empty means loopback only |
+| `PHOTO_PIPELINE_ACCESS_SECRET` | traded for the session cookie at `GET /api/v1/session` via `X-Access-Secret` |
+| `PHOTO_PIPELINE_TRUSTED_PROXIES` | comma-separated peer addresses whose `X-Forwarded-Proto`/`X-Forwarded-Host` are believed |
+
+Being reachable *was* the authentication: whoever could open `127.0.0.1:8000` owned
+the library. So naming any non-loopback host — or binding to one, `0.0.0.0` included
+— makes the access secret mandatory, and `serve` refuses to start without it rather
+than publishing the library. Loopback-only deployments need no secret and behave
+exactly as before.
+
+```bash
+curl -sc /tmp/pp.jar -H "X-Access-Secret: $PHOTO_PIPELINE_ACCESS_SECRET" \
+  https://photos.example.com/api/v1/session
+```
+
+The browser asks for the secret once per tab and keeps it in `sessionStorage`.
+Wrong secrets are rate-limited (5 per minute) and logged with the caller's address
+only. `Host` and `Origin` are judged against the configured names; the *external*
+scheme and host come from the forwarded headers only when the request arrived from a
+`PHOTO_PIPELINE_TRUSTED_PROXIES` address, so a client cannot declare its own origin,
+and the session cookie is marked `Secure` when that external scheme is HTTPS. Health
+endpoints stay reachable without the secret so an orchestrator can restart the
+container; nothing else does.
+
 ## Testing
 
 One offline command runs the whole suite (unit, integration, and browser