🗨️ Fluent - Secure Chat App with End-to-End Encryption

Fluent is a secure, real-time Android chat application built with Jetpack Compose and Kotlin. It features end-to-end encrypted messaging, token-based authentication, and real-time communication using Ktor WebSockets — all backed by a FastAPI server.

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✨ Features

• 🔐 End-to-End Encrypted Messaging (RSA via Android Keystore)
• 🌐 Real-Time Messaging using Ktor WebSockets
• 🔑 JWT Authentication with Refresh Token Support
• 📱 Jetpack Compose UI (Material 3) with smooth UX
• 💬 Message List & One-on-One Chat
• 🧩 Receiver-Side RSA Decryption via AndroidKeyStore
• 🚀 Message Streaming via WebSocket Flow Collector
• 🎯 Auto Key Generation + Storage via Android Keystore
• ☁️ EncryptedSharedPreferences for Secure Token Storage
• 👤 User Profiles & Sidebar Drawer
• 🧭 Custom Bottom Navigation with Slide Menu
• 🧠 MVVM + Clean Architecture
• 🔧 Dependency Injection via Koin
• 📦 Structured Modular Codebase

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📸 Screenshots

🔐 Login & Register

💬 Chat List & Messages

🔍 User Search

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🧱 Architecture Overview

presentation/
    ├── authentication/
    ├── chatList/
    ├── message/
    ├── splashOpening/
    ├── profile/
data/
    ├── dto/
    ├── remote/
    ├── mapper/
    ├── network/
domain/
    ├── models/
    ├── repository/
    ├── utility/
di/
    ├── Koin modules (network, encryption, viewmodel, repository)

• Jetpack Compose for UI
• Ktor HTTP/WebSocket for networking
• Android Keystore for RSA encryption
• StateFlow & SharedFlow for reactive state management

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🔐 End-to-End Encryption Details

Each device generates an RSA keypair securely stored via the Android Keystore System.

• Messages are encrypted with the recipient's public key.
• Messages are decrypted using the user’s private key on-device.
• No plaintext is stored or transmitted at any point.
• Message decryption runs on a dedicated coroutine scope with SupervisorJob and Mutex for serialized, thread-safe access.

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🧪 Real-Time Communication

• Messages are received over WebSocket (Ktor client)
• Decryption occurs on a background coroutine (Dispatchers.Default) using a Mutex
• Flow collector emits to the UI using SharedFlow to support real-time updates

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⚙️ Tech Stack

• 🧬 Kotlin + Jetpack Compose
• 🌐 Ktor HTTP/WebSocket Client
• 🧩 Koin (DI)
• 🛡️ EncryptedSharedPreferences
• 🔐 Android Keystore (RSA)
• 🧠 StateFlow + SharedFlow
• 🖥️ FastAPI (Python Backend)
• 📜 JWT Authentication (Access + Refresh Tokens)

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🧱 Improvements Made

• ✅ RSA decryption now safely runs in a background coroutine with a Mutex to avoid IllegalBlockSizeException
• ✅ Decryption bug fixed: messages now decrypt properly when received live (even without screen navigation)
• ✅ SupervisorJob added to isolate failure of decrypt jobs
• ✅ Scoped coroutine (wsScope) handles WebSocket lifecycle reliably
• ✅ WebSocket reconnect and lifecycle cleanup improved

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🚧 Future Work

• User Search API integration
• Real-Time Message Sync using Flow
• Push Notifications (FCM integration)
• Typing Indicators
• Media Sharing (images, files, voice notes)
• Offline-first with Room DB
• Read receipts, chat status

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▶️ How to Run

1. Clone this repo:

   git clone https://github.com/jishnu70/Fluent-Secure-E2E-Chat-App-Kotlin-Compose.git

2. Open in Android Studio (Kotlin 1.9+, Compose Compiler 1.5+)

3. Run the FastAPI backend

   (Backend repo: cryptalkfastapi)

4. Launch on a device or emulator (Android 8.0+ required)

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🤝 Contributing

This project is built solo for now.
Feel free to open issues, suggest improvements, or send PRs if you're passionate about privacy-first messaging apps.