FreeTime uses a defense-in-depth model. This document describes the design principles at a high level.

Sender                         Server                         Receiver
   │                              │                              │
   ├─ Generate per-file key ─────►                              │
   ├─ Encrypt media locally       │                              │
   ├─ Send encrypted_file + ─────►│── Blind storage ───────────►│
   │  encrypted_key               │                              │
   │                              ├─ Store (no plaintext)       │
   │                              ├─ Forward to recipient       │

• Per-file random encryption keys (AES-256-GCM)
• Server never has access to plaintext media
• Unique IV per encryption operation

• Android Keystore (hardware-backed on supported devices)
• AES-256-GCM via Google Tink for authenticated encryption
• Session tokens and credentials encrypted at rest

• TLS 1.3 for all API and WebSocket communication
• JWT-based auth tokens with auto-refresh
• Device tracking to prevent concurrent session abuse

Note: The server code included in this repository (master-server/ and peer/) has been sanitized for public release. All production domain names, IP addresses, and credentials have been replaced with CHANGE_ME placeholders or generic values (YOUR_DOMAIN.com, YOUR_SERVER_IP). The peer-to-master authentication domain and API keys are intentionally censored — configure your own in config/.env when self-hosting.

• Production server domain names and IP addresses
• Real JWT secrets, API keys, and passwords
• Peer-to-master authentication challenge-response details
• SSL/TLS private keys and certificate paths tied to production infrastructure
• Firebase service account credentials

All omitted values are replaced with clearly marked placeholders in configuration files.