A system to segregate notification tokens for privacy protection

Privacy-focused notification system with end-to-end encrypted FCM token management.

Architecture

Android App → App Backend (8081) → Notification Backend (8080) → Firebase FCM

Privacy by Design: The app-backend acts as a zero-knowledge intermediary that cannot decrypt device tokens, ensuring organizational separation between user data and notification infrastructure.

Features

• End-to-End Encrypted Notifications: RSA + AES hybrid encryption
• Zero-Knowledge Intermediary: App-backend cannot decrypt tokens
• Durable Storage: Exoscale SOS integration with automatic cleanup
• Public Key Hash Namespacing: Prevents key collision in multi-tenant scenarios
• Modern FCM API: Uses Firebase Cloud Messaging API v1

Quick Start

1. Generate RSA Keypair

# Generate private key
openssl genrsa -out private_key.pem 4096

# Generate public key from private key
openssl rsa -in private_key.pem -pubout -out public_key.pem

2. Deploy Keys

• Android App: Replace demo-app/app/src/main/assets/public_key.pem
• Notification Backend: Place private_key.pem in notification-backend/
• Never commit private_key.pem to version control

3. Start Services

# Terminal 1: Start notification backend
cd notification-backend
go run main.go  # Runs on :8080

# Terminal 2: Start app backend  
cd app-backend
go run main.go  # Runs on :8081

4. Configure Firebase

See notification-backend/README.md for Firebase setup instructions.

Security Architecture

Hybrid Encryption Flow

1. Android App: Token → AES-256-GCM → RSA-4096(AES-key) → Base64 → Network
2. App Backend: Pass-through (zero-knowledge)
3. Notification Backend: Base64 → RSA-decrypt(AES-key) → AES-GCM-decrypt → Token → FCM

Privacy Guarantees

• Zero-Knowledge Relay: App-backend cryptographically cannot access tokens
• Just-in-Time Decryption: Tokens decrypted only when sending notifications
• Memory Security: Keys wiped immediately after use
• Private Key Isolation: Private key never leaves notification-backend
• Per-Token Keys: Each token encrypted with unique AES key

Components

• demo-app: Android FCM client with hybrid encryption
• app-backend: Zero-knowledge intermediary service
• notification-backend: FCM notification service with token decryption

See individual component READMEs for detailed setup and API documentation.