Offline-finding systems allow small devices that are not connected to a network to enable communication between a finder and the owner – the most well-known system is probably Apple's AirTag system. AirTags can be attached to anything, and constantly broadcast beacons that can be discovered by smartphones running the corresponding to communicate a geolocation to the owner of the tag.

Starting from an experiment at Teckids, we are building an open implementation, absed on ESP32, Rust, and elliptic-curve cryptography. In this talk, we would like to explain how such a ssytem works, our approach, and what we learnt while building it.