Chatap: Decentralized Offline Messaging over Bluetooth Low Energy—Prototype, Proposed Mesh Relay and Security Design, and Evaluation Plan

Most chat applications depend on the internet and a central server, so they stop working when the network fails, in remote places, or after a disaster. This paper describes Chatap, an Android application written in Kotlin that lets nearby phones exchange text messages over Bluetooth Low Energy (BLE) with no internet, no Wi-Fi access point and no server. The current prototype discovers other phones running Chatap, connects to them automatically and supports real-time one-to-one chat through a simple interface. Around this working core, the paper specifies as proposed work a four-layer architecture, a local data model for the Room database, a managed-flooding relay protocol that uses message identifiers, a hop limit (TTL) and duplicate suppression, acknowledgements with retry, and an end-to-end security design based on X25519 key agreement and AES-256-GCM. We add a packet-size analysis, an upper bound on relay traffic, and a measurement plan for discovery time, delivery ratio, latency, multi-hop delivery, duplicate rate, battery drain and encryption overhead. No quantitative results have been collected yet and none are reported. Multi-hop relay, acknowledgements and encryption are not yet implemented, and we say so throughout. The contribution of this paper is a reproducible system design and evaluation protocol, not a demonstrated performance result.