Campus Grid: A Decentralized, Offline-First, andCryptographically Isolated Mesh MessagingArchitecture for Campus Android Devices

Campus communities depend on messaging forcoordination, announcements, and emergency alerts, yetmainstream messengers need an Internet connection and a centralserver that can be congested, unavailable, or a privacy risk. Thispaper presents Campus Grid, a decentralized, offline-first meshcommunication application for Android that delivers chat, groupmessaging, announcements, and file transfer without any centralserver. The application is written in Kotlin following CleanArchitecture and uses Bluetooth Low Energy (BLE), Wi-Fi Direct, andLocal IP transports selected by a policy that reserves BLE for controltraffic and uses Wi-Fi Direct or LAN links for large payloads.Confidentiality and integrity rest on hardware-backed identity keysin the Android Keystore, an authenticated signed ephemeral Diffie-Hellman handshake for forward secrecy, HKDF-SHA256 keyderivation, AES-256-GCM authenticated encryption, a sliding-window replay protector, and a Room database encrypted withSQLCipher. Mesh resilience comes from a bounded-memoryduplicate detector, hop-limited forwarding, reactive route learning,and a pure forwarding decision engine, and is exercised through amesh-congestion simulation covering peer flooding, duplicatestorms, and TTL exhaustion. A central rule of the system is secretisolation: private keys never leave the Keystore, and no plaintextreaches the database or the radio.