SignalLock — Windows System Tray App

Windows 10/11 system-tray utility that automatically locks the workstation when a trusted Bluetooth Low Energy device (e.g. your phone, watch, AirPods, BLE keychain tag) moves out of range. Functional and architectural mirror of the macOS app — same algorithm, same proximity logic, same safety defaults.

Requirements

• Windows 10 1809 (build 17763) or newer (Windows 11 supported)
• Bluetooth Low Energy capable hardware
• .NET 8 SDK to build (the published binary needs only the .NET 8 runtime, or none if published self-contained)

Build & Run

# From the repository root
cd windows

# Restore + build (debug)
dotnet build

# Run the tray app
dotnet run

# Release build, framework-dependent
dotnet publish -c Release -r win-x64 --no-self-contained -o publish

# Release build, single-file self-contained (no .NET install required)
dotnet publish -c Release -r win-x64 --self-contained -p:PublishSingleFile=true -o publish

The output executable is publish\SignalLock.exe. Double-click to launch — the icon appears in the system tray (notification area).

How to Use

1. Right-click the SignalLock tray icon → Select Trusted Device…
2. Bring the device you want to use as your "key" close to the PC. Pick the entry with the strongest (least negative) RSSI. Click Select.
3. From now on, the app starts monitoring automatically. Walk away with the trusted device and the screen will lock once the configured away delay elapses.
4. Use Settings… to tune RSSI threshold, away delay, smoothing window, and Start at login.
5. Test Lock locks the workstation immediately — useful to verify behavior before relying on auto-lock.

How Proximity Detection Works

1. BluetoothScanner runs Windows.Devices.Bluetooth.Advertisement.BluetoothLEAdvertisementWatcher in active scanning mode and emits each advertisement with its RSSI.
2. ProximityMonitor keeps a moving average of recent RSSI samples (default window 5).
3. The trusted device is considered away when either:
   • The smoothed RSSI is below the configured threshold (default -80 dBm), or
   • No advertisement has been received for longer than the away delay (default 20 seconds).
4. The away condition must persist for the full away-delay grace period before a lock fires. Exactly one lock fires per away episode.
5. After locking, SignalLock listens for the Windows SessionSwitch / SessionUnlock event. When you unlock with your password / PIN / Hello, the proximity monitor is rearmed: the next walkaway will lock again.

How Locking Works

LockService invokes user32!LockWorkStation (Win32). It is the canonical Windows API for locking the current session, requires no elevated privileges, no Accessibility-style consent, and is the only place the project P/Invokes for the lock action.

Required Permissions

• Bluetooth access. Modern Windows 11 lets users gate apps via Settings → Privacy & security → Bluetooth. SignalLock does not request elevated privileges.
• No Location, Accessibility, or Full Disk Access requirements.
• No network access — the app never opens a socket.

Auto-start at Login

Toggling Start at login writes a value under HKCU\SOFTWARE\Microsoft\Windows\CurrentVersion\Run. Per-user, no admin required, no Task Scheduler dependency.

Diagnostics

Logs are written to %APPDATA%\SignalLock\signallock.log and also forwarded to the Visual Studio Debug Output and stdout. Categories: app, lock, proximity, bluetooth — same as the macOS port. Sample lines:

2026-05-05 21:08:34.121 [INFO ] [SignalLock][app] Trusted device selected: Galaxy Watch (id=AABBCCDDEEFF)
2026-05-05 21:08:35.014 [INFO ] [SignalLock][app] Monitoring started (device=Galaxy Watch, threshold=-80 dBm, awayDelay=20s)
2026-05-05 21:09:00.832 [INFO ] [SignalLock][proximity] Away condition started (signal=true, silence=false)
2026-05-05 21:09:20.010 [INFO ] [SignalLock][proximity] Confirmed AWAY after 20s; will trigger lock
2026-05-05 21:09:20.012 [INFO ] [SignalLock][lock] Locked via LockWorkStation
2026-05-05 21:09:34.420 [INFO ] [SignalLock][app] SessionUnlock → rearming proximity monitor

%APPDATA%\SignalLock\ also stores settings.json and trusted-device.json.

Architecture

Program.cs                       — entry point; single-instance mutex; auto-start
AppState.cs                      — orchestrator; marshals events to UI thread

Models.cs                        — AppSettings, TrustedDevice, DiscoveredDevice, BluetoothAvailability
Logging.cs                       — minimal file + Trace + Console logger

Services/StoragePaths.cs         — %APPDATA%\SignalLock\ paths
Services/SettingsStore.cs        — JSON persistence of AppSettings
Services/TrustedDeviceStore.cs   — JSON persistence of TrustedDevice
Services/BluetoothScanner.cs     — WinRT BluetoothLEAdvertisementWatcher wrapper
Services/ProximityMonitor.cs     — moving-average + grace-period away decision
Services/LockService.cs          — Win32 LockWorkStation P/Invoke
Services/LoginItemService.cs     — HKCU\…\Run registry toggle

UI/TrayController.cs             — NotifyIcon + ContextMenuStrip
UI/SettingsForm.cs               — WinForms settings window
UI/DeviceSelectionForm.cs        — WinForms BLE picker

State flow is one-directional: BluetoothScanner → AppState → ProximityMonitor → LockService. UI only reads via AppState.StateChanged and writes via public methods.

Current Limitations

• iPhones rotate BLE advertising addresses for privacy; tracking a stock iPhone reliably is hard. Apple Watch, Galaxy Watch, AirPods, fitness bands and dedicated BLE tags work well.
• The single-instance mutex is per-user; running the app under different users on the same machine produces independent instances.
• No notarized installer yet; running an unsigned .exe will trigger SmartScreen — choose More info → Run anyway the first time.
• Watcher recovery on transient Bluetooth failures is minimal: on BluetoothLEAdvertisementWatcherStoppedEventArgs.Error, we log the error; restarting monitoring is up to the user.

Safety Defaults

• Default RSSI threshold: -80 dBm.
• Default away delay: 20 seconds.
• Default smoothing window: 5 samples.
• Auto-start on launch is enabled when a trusted device is configured.
• The first lock cannot fire until the device has been seen at least once after monitoring starts.

License

SignalLock is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License v3.0 as published by the Free Software Foundation. See the LICENSE file in this directory for the full text, or https://www.gnu.org/licenses/gpl-3.0.html.

SignalLock is distributed in the hope that it will be useful, but without any warranty; without even the implied warranty of merchantability or fitness for a particular purpose.

Notes for forks and downstream builds

• The Inno Setup AppId GUID ({56B5F70A-5A1E-40B5-B9C0-31C437B72F54} in installer/SignalLock.iss) uniquely identifies upgrades. If you produce your own installer, generate a fresh GUID — otherwise the installer will treat your build as an upgrade of the upstream SignalLock.
• The publisher/support URL in the installer (https://signallock.app) points to the upstream project website; change it in your fork so that the Help and Update links in Apps & features point somewhere you control.
• This repository contains only the Windows app. The marketing website is maintained separately and is not GPL-licensed.