Catching the Fly: Practical Challenges in Making Blockchain FlyClient Real

TL;DR

This paper advances FlyClient, a lightweight blockchain verification protocol, by addressing deployment challenges, providing a formal adversary model, implementing a practical prover for Zcash, and optimizing proof sizes.

Contribution

It introduces a formal adversary model, implements a practical FlyClient prover for Zcash, and proposes proof size optimizations without requiring consensus changes.

Findings

Formal adversary model with economic interpretation

Practical FlyClient prover implementation for Zcash

Two proof size optimization techniques

Abstract

FlyClient is a lightweight blockchain verification protocol that enables proof-of-work validation using minimal data, making it ideal for resource-constrained environments like mobile wallets, Internet-of-Things devices or cross-chain bridges implemented with smart contracts. Despite its strong potential for enabling lightweight blockchain verification, FlyClient protocol is still in the experimental stages, with limited real-world deployments and performance evaluations under diverse conditions. In this paper we bridge the gap between theory and deployment, by addressing several technical challenges to advance FlyClient to a production-ready solution. Namely, our contribution is three-fold: (i) we formally introduce an adversary model alternative to the original FlyClient one, that allows us to parametrize a verifier under a concrete economic interpretation, while also saving some proof space; (ii) we provide the first practical FlyClient prover implementation for a production blockchain (Zcash), and we estimate its performance under different configurations; (iii) we introduce and evaluate two optimizations that minimize the size of FlyClient proofs, the first of which does not require any consensus change.