SurferMonkey A Decentralized Anonymous Blockchain Intercommunication System via Zero Knowledge Proofs

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SurferMonkey: A Decentralized Anonymous Blockchain

Intercommunication System via Zero Knowledge Proofs

MIGUEL DÍAZ MONTIEL,

École Polytechnique Fédérale de Lausanne (EPFL), Switzerland and Uni-

versitat Politècnica de Catalunya (UPC), Spain

RACHID GUERRAOUI, École Polytechnique Fédérale de Lausanne (EPFL), Switzerland

PIERRE-LOUIS ROMAN, École Polytechnique Fédérale de Lausanne (EPFL), Switzerland

Blockchain intercommunication systems enable the exchanges of messages between blockchains. This interoperability

promotes innovation, unlocks liquidity and access to assets. As of March 2022, the total value locked (TVL) in these

systems was of $21.8 billion [

]. However, blockchains are isolated systems that originally were not designed for

interoperability. This makes cross-chain communication, or bridges for short, insecure by nature. More precisely,

cross-chain systems face security challenges in terms of selsh rational players such as maximal extractable value

(MEV) and censorship. As of July 2022, the top 3 bridge hacks account for more than $1.5 billion in losses [

] and the

aggregated value extracted from the users using MEV techniques is $663 million [12].

We propose to solve these challenges using zero knowledge proofs (ZKPs) for cross-chain communication. Securing

cross-chain communication is remarkably more complex than securing single-chain events as such a system must

preserve user security against both on- and o-chain analysis.

To achieve this goal, we propose the following pair of contributions: the DACT protocol and the SurferMonkey

infrastructure that supports the DACT protocol. The decentralized anonymous agnostic cross-chain transfer (DACT)

protocol is a global solution for the anonymity and security challenges of agnostic blockchain intercommunication.

DACT breaks on- and o-chain analysis thanks to the use of ZKPs. SurferMonkey is a decentralized infrastructure

that implements DACT in practice. Since SurferMonkey works at the blockchain application layer, any decentralized

application (dApp) can use SurferMonkey to send any type of message to a dApp on another blockchain. With

SurferMonkey, users can neither be censored nor be exposed to MEV. By applying decentralized proactive security, we

obtain resilience against selsh rational players, and raise the security bar against cyberattacks. We have implemented

a proof of concept (PoC) of SurferMonkey by reverse engineering Tornado Cash and by applying IDEN3 ZKP circuits.

SurferMonkey enables new usecases, ranging from anonymous voting and gaming, to a new phase of anonymous

decentralized nance (aDeFi).

This report is based on the Master’s thesis of Miguel Díaz Montiel defended on July 13, 2022 at EPFL.

Authors’ addresses: Miguel Díaz Montiel, miguel.diaz.montiel@estudiantat.upc.edu; Rachid Guerraoui, rachid.guerraoui@ep.ch;

Pierre-Louis Roman, pierre-louis.roman@ep.ch.

Permission to make digital or hard copies of part or all of this work for personal or classroom use is granted without fee provided that

copies are not made or distributed for prot or commercial advantage and that copies bear this notice and the full citation on the rst

page. Copyrights for third-party components of this work must be honored. For all other uses, contact the owner/author(s).

arXiv:2210.13242v1 [cs.CR] 24 Oct 2022

Miguel Díaz Montiel, Rachid Guerraoui, and Pierre-Louis Roman

Abstract 1

1 Introduction 4

1.1 Cross-Chain Ecosystem Drawbacks 4

1.2 Solution: DACT and SurferMonkey 6

2 Previous Work: Blockchain Intercommunication Protocols 10

2.1 Types of Intercommunication Security Designs 10

2.2 Multiplexer Blockchains 12

3 Previous Work: ZKPs Applied to Single-chain Blockchain Privacy 12

3.1 Zerocoin 12

3.2 Zerocash 13

3.3 Tornado Cash 14

4 Background: Zero Knowledge Proofs 14

4.1 Dierent Types of ZKPs 15

4.2 Choosing Zk-SNARKs 16

4.3 SNARKs Arithmetic Circuit 16

4.4 Pedersen Hash with 4-bit Windows 17

4.5 Applied zk-SNARKs with Circom 17

5 The DACT Protocol 19

5.1 DACT Data Structures 19

5.2 DACT Proof of Membership 21

5.3 DACT ZKP Circuits 21

5.4 DACT Security 22

5.5 DACT Creation and Settlement 25

5.6 DACT Unlinked Finality and Atomicity 26

6 The SurferMonkey Infrastructure 26

6.1 System Requirements and Design Principles 28

6.2 System Architecture 29

6.3 Settlement Transaction Flow 32

6.4 Revert Transaction Flow 34

6.5 Actors Authentication 35

7 System Resilience and Analysis 37

7.1 O-chain Attacks and Denial of Service 37

7.2 Oracle Attacks 37

7.3 dApp Attacks 38

SurferMonkey: A Decentralized Anonymous Blockchain Intercommunication System via Zero Knowledge Proofs

7.4 User Attacks 39

7.5 MEV Background 40

7.6 SurferMonkey MEV Resilience 41

7.7 Privacy 41

7.8 System Constraints 41

8 Proof of Concept Implementation 42

8.1 Reverse engineering Tornado Cash 43

8.2 Complications and Learnings 43

9 Evaluation 44

9.1 Unit Tests 45

9.2 System Tests 45

9.3 Gas Execution Cost 45

10 Design Decisions 47

11 Future Research and Development 49

11.1 Accumulators 49

11.2 Distributed Key Generation 49

11.3 Blockchain Reorganisations 49

11.4 Compromised Intercommunication Blockchain Transactions due to Heuristics 50

11.5 Minimize Gas Transaction 50

12 Conclusion 50

References 51

Contents

Miguel Díaz Montiel, Rachid Guerraoui, and Pierre-Louis Roman

1 INTRODUCTION

The industry has developed multiple blockchain systems to tackle dierent problems. As more blockchains

systems began to appear on the market, the need for a cross-blockchain communication system became

evident. This led to the development of the so called blockchain bridges.

A blockchain bridge is just a

message passing system between dierent blockchains. As more blockchains communicate with each other,

more value can be accessed. This improves the liquidity from the assets and it brings new projects into the

table, ranging from data services, gaming, to decentralized nance (DeFi).

A typical cross-chain communication system is depicted in Fig. 1. First, the user submits a transaction

request into the source blockchain where it calls the dApp. Second, oracle nodes listen in to the events

from the dApp and locally transform the source blockchain event into a destination blockchain transaction.

Third, the oracles submit the transaction into the destination blockchain.

Fig. 1. Traditional cross-chain communication protocol.

1.1 Cross-Chain Ecosystem Drawbacks

The interconnection of dierent blockchain systems brings new problems into the industry (game theory

and censorship), inherits the drawbacks from a single blockchain ecosystem (privacy), and maximizes

pitfalls from the isolated blockchain environments (maximal extractable value (MEV)).

Through game theory standards, the actors of a system are seen as rational players that always look for

their best interests. Censorship is the ability that an entity has to silence a user voice or transaction. MEV

is a rational permissionless move where the players extract as much as possible value from their rivals

without requiring their consent. Finally, in a blockchain system, users cannot have privacy nor anonymity,

therefore this data can be used as an attack vector.

1.1.1 First problem: game theory. The new problem that arises with cross-blockchain communication

systems is: How to obtain cross-chain message replication on environments that weren’t designed to be

interconnected? To solve this, the industry created a set of actors called oracles. The oracles hold the

cryptography keys to input data on the destination blockchains. The oracles are machines run by persons

In the industry, a blockchain bridge is commonly reserved for applications that solely make value transfers from one chain to another

and that have their own liquidity pools. However, the nomenclature of cross-chain messaging system is reserved for protocols that

decentralized applications (dApps) use to transfer any type of message. For simplicity, we are addressing “blockchain bridges” as equal

as “cross-chain messaging systems”.

SurferMonkey: A Decentralized Anonymous Blockchain Intercommunication System via Zero Knowledge Proofs

or entities that, simply put, listen to the events that happened on blockchain A, and submit those events

into blockchain B. The solution brings a new variable into the equation: How can we trust the oracles?

All together, the oracles are the new entities that store the keys to secure the cross-chain messaging

system. However, oracles are subject to bribery, rational thinking, and cyberattacks.

1.1.2 Second problem: MEV. The maximal extractable value (MEV) gaming vector is performed by having

nodes change the order of transactions executed. For example, an MEV player can frontrun or sandwich

other transactions [

]. An MEV player relies on knowing the negotiation parameters from a rival, to extract

as much value from the rival as possible, before the rival transaction settles. This is possible, because the

MEV players know beforehand how much more a specic rival is willing to pay for a certain asset. The

MEV players can be the blockchain miners, oracles or another user.

In an isolated blockchain scenario we have a single MEV point: the blockchain mempool (Fig. 2). While

on a cross-chain communication system we have three MEV opportunities: the source chain mempool, the

source red event, and the destination chain mempool (Fig. 3). In a cross-chain system, the MEV players

have more time to orchestrate a move prior to the rival transaction settlement. The MEV players can go even

further and, if the business logic allows it, they can potentially extract value on each step of the cross-chain

system.

1.1.3 Third problem: privacy. By default blockchains have a public register (ledger) of all the actions

that happened, as well as the deeds that are about to happen (mempool). Both ledger and mempool are

transparent. Therefore the whole user history can be traced. This has an impact on the privacy of the users

as this data can be exploited as an attack vector and can negatively impact an individual. For example,

maybe a user does not want to be publicly exposed while casting a vote within a Decentralized Autonomous

Organization (DAO).

Furthermore, in a cross-chain ecosystem, we can also trace not only the on-chain data, but also the

o-chain ow. Which means that we can know that Alice transferred a message from blockchain A to

blockchain B, and performed certain computational steps on blockchain B. We are able to trace the o-chain

data from the cross-communication system because the user intent is transparent on the source chain. Even

if that intent is obfuscated in the source chain, the oracles can still trace the o-chain footprint as they

know which specic package to push from blockchain A to blockchain B which breaks the users privacy.

All in all, transparent data on and o chain can be mined. This data can be used to train specic machine

learning models in order to weaponize articial intelligence.

1.1.4 Fourth problem: censorship. As we move from single blockchain environments into the multi-chain

ecosystems, we nd ourselves with censorship problems by the oracles against the users, dApps, and

blockchains. The oracles can see a specic user request and destiny to perform a cross-chain transfer; the

oracles have the power to silence this specic user by rejecting its cross-chain transfer requests. The oracles

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SurferMonkey:ADecentralizedAnonymousBlockchainIntercommunicationSystemviaZeroKnowledgeProofsMIGUELDÍAZMONTIEL,ÉcolePolytechniqueFédéraledeLausanne(EPFL),SwitzerlandandUni-versitatPolitècnicadeCatalunya(UPC),SpainRACHIDGUERRAOUI,ÉcolePolytechniqueFédéraledeLausanne(EPFL),SwitzerlandPIERRE-LOUISROMAN,...

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SurferMonkey A Decentralized Anonymous Blockchain Intercommunication System via Zero Knowledge Proofs

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