Paper 2025/2095

FPS: Flexible Payment System

Abstract

Existing payment systems make fixed trade-offs between performance and security assumptions. Traditional centralized systems like Visa assume synchronous networks and crash faults to achieve high throughput, while blockchain-based systems (e.g., Algorand, Aptos) adopt Byzantine fault tolerance and partial synchrony for stronger security at the cost of performance. This rigid approach forces all users to accept the same security-performance trade-off regardless of their individual trust and threat models. We present a flexible payment system where clients independently choose assumptions about (i) network timing (bounded or partial synchrony), (ii) corruption (static or adaptive), and (iii) faults (crash or Byzantine), supporting eight assumption combinations simultaneously. Unlike traditional systems requiring consensus, our approach uses a novel flexible variant of consistent broadcast where clients external to the protocol verify delivery through cryptographic proofs, eliminating the need for global ordering. We implemented our system in Rust and demonstrated that clients choosing partially synchronous network and crash assumptions achieve $+242.1\%$ higher throughput and $+70.4\%$ better latency compared to clients with synchronous network and Byzantine assumptions, confirming that our system enables users to optimize their individual security-performance trade-offs.