Off Chat: A Decentralized Digital Concierge

Bachelor Thesis Project | CTU FEE, Prague

Off Chat is an offline-first, peer-to-peer communication platform built with Flutter. It utilizes Bluetooth Low Energy (BLE) to enable device discovery, relative location tracking (Radar), and secure messaging without any reliance on internet or cellular connectivity.

This project was developed as part of a Bachelor Thesis at the Czech Technical University in Prague (CTU), Faculty of Electrical Engineering (FEE).

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🌟 Key Features

• The Nodes (Discovery): Real-time scanning of nearby active devices. Displays user profiles (avatar, alias) and proximity metadata.
• The Radar (Location): A custom-painted visualization showing the relative bearing and distance of peers using a combination of GPS coordinates and device compass data.
• Off-Grid Messaging: Reliable text and media exchange (images up to 512x512) over BLE GATT characteristics.
• Identity Protocol: Secure local profile management with automated image compression and background synchronization.
• Aurelian Noir Aesthetic: A premium "Digital Concierge" design system featuring deep black surfaces, gold primary accents, and fluid geometry.

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🏗 Architecture

The project follows a Feature-First Architecture, ensuring high cohesion and low coupling between modules. Each feature is self-contained with its own Presentation, Domain, and Data layers.

lib/src/
├── features/
│   ├── onboarding/   # Multi-step security & setup protocol
│   ├── discovery/    # BLE Scanning & Peer management
│   ├── location/     # Radar math, Compass, & UI Canvas
│   ├── profile/      # Local identity & Settings
│   └── chat/         # Messaging logic & Media chunking
├── core/
│   ├── routing/      # Reactive GoRouter with Onboarding protection
│   ├── theme/        # Centralized Aurelian Noir ThemeData
│   ├── database/     # Isar local persistence layer
│   └── notifications/# Local background alerts
└── app.dart          # Root Material application

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🛰 The Off-Chat Protocol

To overcome standard BLE advertisement limits (31 bytes) and iOS background restrictions, this project implements a Hybrid Discovery Strategy:

1. The 13-Byte Payload

We pack essential metadata into a byte-level structure to fit within Manufacturer Data:

• Byte 0: Flags (Platform: Android/iOS, Visibility: Enabled/Disabled).
• Bytes 1-4: Profile Hash (Deterministic CRC32 of username/avatar).
• Bytes 5-8: Latitude (Float32).
• Bytes 9-12: Longitude (Float32).

2. Hybrid Strategy

• Primary Advertisement: Broadcasts only the Service UUID and essential flags to minimize packet size and maximize discovery reliability (especially for iOS background).
• Scan Response: The 13-byte custom metadata is moved to the Scan Response packet.
• Local Name: Disabled to free up primary advertisement bandwidth.
• Background (iOS): When Scan Response data is stripped by the OS, the app initiates a silent GATT connection to read the "Identity Characteristic," fetching the 13-byte payload before disconnecting.

3. Intelligent Motion (Flight Mode)

To ensure stability in high-speed environments (airplanes, fast trains), the protocol implements Adaptive Throttling:

• Normal Mode (< 72 km/h): Updates location and restarts advertising every 10 meters.
• High-Speed Mode (≥ 72 km/h): Switches to a stable 2-minute timer, ignoring distance changes. This prevents rapid Bluetooth stack resets and allows peers a reliable window to connect.
• Heartbeat: Forced update every 5 minutes even if the device is stationary.

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🛠 Tech Stack & Dependencies

• Framework: Flutter (Dart)
• State Management: Riverpod (Generator & Annotation)
• Navigation: GoRouter
• Database: Isar (NoSQL)
• Communication: flutter_blue_plus (Client) & ble_peripheral (Server)
• Permissions: permission_handler (Runtime Android 12+ BLE/Location support)
• Logging: logging (Structured output for debugging)
• Sensors: geolocator (GPS) & flutter_compass
• Design: Google Fonts (Plus Jakarta Sans)

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🚀 Getting Started

Prerequisites

• Flutter SDK (Latest Stable)
• Two physical devices OR Android Emulators with Netsim enabled.
• Android 12+ (API 31) for full BLE Advertising/Scanning support.

Installation

1. Clone the repository.
2. Install dependencies:

   flutter pub get

3. Run code generation (required for Riverpod and Isar):

   dart run build_runner build --delete-conflicting-outputs

4. Deploy to a device:

   flutter run

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🧪 Testing & Verification

Developer Terminal (In-App)

A hidden debug console is available for real-time monitoring of BLE status and system logs:

1. Navigate to the Discovery (Home) screen.
2. Triple-tap the "OFFCHAT" logo in the header.
3. The terminal displays the last 200 logs with color-coded severity levels.

Multi-Emulator Testing (Netsim)

You can test BLE discovery and GATT sync locally using Netsim (Android Network Simulator):

1. Launch two Android emulators (API 31+).
2. Ensure Netsim is enabled in emulator settings.
3. Open the Netsim GUI: http://localhost:7681/gui to manage device proximity and signal strength.

Continuous Integration

A GitHub Action is configured to build release artifacts on every push to master.

• APK: off-chat-release.apk
• App Bundle: off-chat-release.aab Builds are pushed automatically to the android branch for deployment.

Automated Checks

The project strictly follows Dart style guidelines (lowerCamelCase constants, no print in production). Verify code quality and run tests:

flutter analyze
flutter test

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Author: Student at CTU FEE, Prague Academic Year: 2025/2026