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\author[1]{Rafael Belchior}%
\author[2]{Dimo Dimov}%
\author[2]{Zahary Karadjov}%
\author[2]{Jonas Pfannschmidt}%
\author[2]{André Vasconcelos}%
\author[2]{Miguel Correia}%
\affil[1]{Affiliation not available}%
\affil[2]{Instituto Superior Técnico INESC-ID Blockdaemon Metacraft Labs CONTENTS}%


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  \date{June 15, 2024}


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\begin{abstract}
The field of blockchain interoperability plays a pivotal role in
blockchain adoption. Despite these advances, a notorious problem
persists: the high number and success rate of attacks on blockchain
bridges. We propose Harmonia, a framework for building robust, secure,
efficient, and decentralized cross-chain applications. A main component
of Harmonia is DendrETH, a decentralized and efficient zero-knowledge
proof-based light client. DendrETH mitigates security problems by
lowering the attack surface by relying on the properties of
zero-knowledge proofs. The DendrETH instance of this paper is an
improvement of Ethereum's light client sync protocol that fixes critical
security flaws. This light client protocol is implemented as a smart
contract, allowing blockchains to read the state of the source
blockchain in a trust-minimized way. Harmonia and DendrETH support
several cross-chain use cases, such as secure cross-blockchain bridges
(asset transfers) and smart contract migrations (data transfers),
without a trusted operator. We implemented Harmonia in 9K lines of code.
Our implementation is compatible with the Ethereum Virtual Machine (EVM)
based chains and some non-EVM chains. Our experimental evaluation shows
that Harmonia can generate light client updates with reasonable latency,
costs (a dozen to a few thousand US dollars per year), and minimal
storage requirements (around 4.5 MB per year). We also carried out
experiments to evaluate the security of DendrETH. We provide an
open-source implementation and reproducible environment for researchers
and practitioners to replicate our results.%
\end{abstract}%



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