OpenSource at: https://github.com/balzack/databag

Features include:
- Public-private key based identity (not bound to any blockchain or hosting domain)
- Federated (accounts on different nodes can communicate)
- Lightweight (server runs on a raspberry pi zero v1.3)
- No server-side replication (direct communication between app and contact's node)
- Low latency (use of websockets for push events to avoid polling)
- Responsive (renders well in phone, tablet and PC)
- Unlimited accounts per node (host for your whole family)