# Privacy design This is the engineering note. The user-facing privacy policy is drafted before release and must stay consistent with what is written here. ## Principle The backend holds no user data of any kind. There is no account, no device registry, no user table. This is not a policy promise — it is a property of the architecture, and it is what makes the Data safety declaration simple and honest. With advertising out of scope, there is currently **no third party that receives anything about the user at all**. The only outbound requests are for map tiles and radar frames, neither of which carries a user identity. ## Location The device position is used to centre the map and, once the worker publishes station data, to pick the nearest measuring station. It is optional throughout. ### Coarse only, and enforced The app declares **`ACCESS_COARSE_LOCATION` and nothing else**. Stations are kilometres apart and the map is looked at, not navigated by, so street-level precision would buy nothing and cost a more invasive permission. This takes active enforcement. The `geolocator` plugin declares `ACCESS_FINE_LOCATION` in its own manifest, and the merger pulls it into ours. Left alone the system dialog offers "Precise", Play Services nags about Location Accuracy, and the Data safety form has to declare precise location — all for accuracy the app never asks for. The app manifest therefore removes it explicitly: ```xml ``` Verified on a device: the system dialog reads "access this device's **approximate** location" and offers no Precise option. **Do not add the fine permission back** without a feature that genuinely needs it and a Data safety update to match. ### No Play Services in the path Location requests use the platform `LocationManager` (`AndroidSettings(forceLocationManager: true)`) rather than the Google Play Services fused provider. The fused provider prompts "your device will need to use Location Accuracy", and declining it yields **no fix at all** — an absurd outcome for an app that only ever wanted an approximate one. It also means location works on devices without Play Services. ### The permission flow - A **prominent disclosure** is shown before the system dialog, as Google Play requires, stating what is used, why, and that it stays on the device. The same text is repeated in Settings so someone who already answered can still read it. - Declining leaves the app fully usable: the map opens on the region centre and places are chosen by hand. - The coordinates never leave the device. ## Saved places A saved place — a name and a point — is the closest thing this app has to a home address. It is stored in `SharedPreferences` **on the device only**, under a versioned key, and is never uploaded, backed up to our servers or shared. There is no account to attach it to and no server that would accept it. Places are what the rain notifications will eventually key off, and that must not change where the data lives: the device will subscribe to the topic for the cell containing the place, so the correspondence between a person and a place stays on their phone. ## Rain notifications without a server-side location The worker computes rain per geographic cell and publishes per-cell state. The device decides which cells it cares about and subscribes to the matching FCM topics **itself**. The consequence is that the server never learns which cell any user is in — it publishes to topics, not to devices. There is nothing to correlate, nothing to subpoena, and nothing to breach. Cell size is coarse enough that a cell identifies an area, not a household. **Never** replace this with device tokens registered against coordinates. It would be simpler and it would destroy the property. ## Caching on the device Radar frames are cached in memory while the app runs. The cache holds published weather imagery only — no personal data — and does not outlive the process. Saved places are the only thing written to persistent storage, and only locally. ## What each third party receives | Party | What it receives | Why | |---|---|---| | OpenFreeMap | tile requests for the area being viewed | to draw the base map | | Our CDN | frame and manifest requests | radar imagery | | Firebase Cloud Messaging | topic subscriptions, no coordinates | rain notifications | Map tile requests inevitably reveal roughly where the map is looking, to whoever serves the tiles. That is inherent to any hosted base map; the alternative is self-hosting, which is noted in [stack-decisions.md](stack-decisions.md) as the escape hatch if it ever matters enough. Our own backend receives nothing that identifies a user, by construction. ## Out of scope, and therefore absent No advertising, no consent management platform, no IAB TCF, no ad identifiers, no forecast provider. If advertising returns, this document and the privacy policy must be revised **before** the SDK is added, not after.