How I Built Encrypted Bluetooth Team Sync in Flutter — AES-256-GCM, ECDH, CRDTs, Zero Servers

There are plenty of places where cell towers don't exist. Mountains, forests, remote wilderness where the only infrastructure is what you carry. When your team spreads out across a few kilometers, knowing where everyone is isn't a nice-to-have — it's a safety issue. So I built Red Grid Link: an encrypted peer-to-peer team coordination app that syncs positions over Bluetooth and WiFi Direct. No servers, no subscriptions to a mesh network, no extra hardware. Just the phones already in everyone's pocket. This is the technical story of how it works under the hood. Part 1: The Problem Military and SAR teams need to share positions in the field. The gold standard is ATAK — the Android Team Awareness Kit. It's powerful, battle-proven, and used across the DoD. But ATAK requires a TAK Server for team sync, it's Android-only, and the learning curve is steep. You're not handing it to a volunteer search-and-rescue member and saying "figure it out." Hardware solutions exist too. goTenna mesh radios