EchoPay — Audio-Powered Offline Payment System

Offline P2P payments encoded as audio tones between phones — no internet required.

I built an offline P2P payment system that encodes transaction data as audio tones between phones (MFSK modulation) — no internet required — with a hybrid UDP fallback and an offline-first Room sync queue.

KotlinAndroid SDKExpress.jsRailwayMFSKHMAC-SHA256

Live Demo →

The Honest Part

My native C++ audio engine is stubbed — all the real signal work happens in pure Kotlin. My backend has real deploy-debugging history behind it: 5 fix commits just for a Railway crash loop.

Highlights

Custom Pure Kotlin MFSK engine built from scratch using the Goertzel algorithm
Hybrid Audio+UDP transceiver with offline-first Room sync queue
Native C++ audio engine is stubbed — all real signal work happens in pure Kotlin
Backend has real deploy-debugging history: 5 fix commits for Railway crash loops