Metaverse is drawing increasing attention from both academia and industry. Interoperability among different metaverse systems has become essential. A cross-metaverse interoperability protocol can enable interoperability across metaverses. However, cross-metaverse protocols often suffer significant cost overhead and transaction latency. For example, in STYLE, a leading cross-metaverse platform, 74% of transaction latency and 97% of the cost overhead are attributed to the relay blockchain rather than the two participating metaverses. To make cross-metaverse efficient, in this article, we propose a fast and cheap cross-metaverse interoperability protocol named CrossMeta. CrossMeta can enable direct communication among heterogeneous metaverses rather than depending on a relay blockchain. This is achieved through two components: i) a committee that relays transactions from the source metaverse to the destination metaverse, along with availability proofs, and ii) a smart contract that verifies the proofs provided by the committee. To ensure an honest majority within the selected committee, we propose a dynamic committee selection method based on the chain quality property. Furthermore, we demonstrate that honest brokers achieve a Nash equilibrium. Additionally, we prove that the proposed CrossMeta protocol satisfies the security properties of atomicity and liveness. To demonstrate the practicality of CrossMeta, we implemented a prototype of the CrossMeta using two real-world metaverse platforms, i.e., Axie Infinity and Sandbox. The evaluation results show that CrossMeta outperforms other cross-metaverse solutions regarding transaction latency and gas fees.

Permissioned blockchains play a significant role in various application scenarios. Applications built on heterogeneous permissioned blockchains need to migrate data from one chain to another, aiming to keep their competitiveness and security. Thus, data migration across heterogeneous chains is a building block of permissioned blockchains. However, existing data migration protocols across heterogeneous chains are rarely used in practice since data migration technologies are insecure. To this end, we propose a data migration protocol across permissioned blockchains, named DataFly. We design a peg consensus mechanism, which provides consistent data-migration functionality between any two permissioned blockchains. To preserve the confidentiality of data, we invoke two classical cryptographic methods, i.e., i) ECDSA feature and ii) the integrated signature and public key encryption scheme. Through combining those two methods, data can be securely migrated from one permissioned blockchain to another without exposing the migrated data to anyone except associated parties. To demonstrate the practicality of DataFly, we implement a prototype of DataFly using existing popular permissioned blockchains, i.e., Hyperledger Fabric and private enterprise Ethereum. Measurement results demonstrate that DataFly outperforms related works in terms of transaction latency and gas costs.

Numerous blockchain simulators have been proposed to allow researchers to simulate mainstream blockchains. However, we have not yet found a testbed that enables researchers to develop and evaluate their new consensus algorithms or new protocols for blockchain sharding systems. To fill this gap, we developed BlockEmulator, which is designed as an experimental platform, particularly for emulating blockchain sharding mechanisms. BlockEmulator adopts a lightweight blockchain architecture so developers can only focus on implementing their new protocols or mechanisms. Using layered modules and useful programming interfaces offered by BlockEmulator, researchers can implement a new protocol with minimum effort. Through experiments, we test various functionalities of BlockEmulator in two steps. Firstly, we prove the correctness of the emulation results yielded by BlockEmulator by comparing the theoretical analysis with the observed experiment results. Secondly, other experimental results demonstrate that BlockEmulator can facilitate measuring a series of metrics, including throughput, transaction confirmation latency, cross-shard transaction ratio, the queuing status of transaction pools, workload distribution across blockchain shards, etc. We have made BlockEmulator open-source in Github.

The proliferation of smart devices has propelled the advancement of IoT-based spatial crowdsourcing. The issue of location privacy in task allocation for IoT-based spatial crowdsourcing has attracted significant attention. Therefore, the main goals of privacy-preserving spatial crowdsourcing (PriSC) are: 1) achieving better location privacy for both participants and tasks and 2) achieving better allocation performance, i.e., accuracy and average moving distance. The homomorphic encryption-based approaches can achieve these goals, yet they suffer from heavy computation and large communication overhead. Although the differential privacy (DP)-based approaches are very efficient, these approaches leverage allocation performance to achieve better location privacy. Motivated by the deficiencies of these existing approaches, we propose a lightweight hash-based spatial crowdsourcing protocol, which not only protects both task location and participant location from the server but also reduces service providers' computation and communication overhead. Besides, our design is independent of the concrete hash function and thus can be instantiated by any collision-resistant cryptographic hash function. Experiment results demonstrate that our protocol outperforms related works in terms of accuracy and average moving distance.

A cross-shard transaction (CTX) is parsed into two sub-transactions, which are then executed in the source and destination shards, respectively. However, the problem is that the client who submits the original transaction only pays one unit of the transaction fee. Thus, sub-transactions will experience much higher queueing delays than regular intra-shard transactions when they wait in shard transaction pools. This is unfair for those original transactions that will be parsed into sub-transactions from the perspective of a sharded blockchain. Therefore, how to ensure fairness for all CTXs while securing the atomicity of any pair of sub-transactions becomes a critical challenge. State-of-the-art solutions addressed the transaction atomicity challenge, but the literature still lacks a dedicated incentive mechanism to ensure the fairness of CTXs. To this end, we propose an incentive mechanism named Justitia, which aims to achieve fairness by motivating blockchain proposers to prioritize the CTXs queueing in transaction pools when they package transactions to generate a new block. We rigorously analyze that Justitia upholds the fundamental properties of a sharded blockchain, including security, atomicity, and fairness. We then implement a prototype of Justitia on an open-source sharding-enabled blockchain testbed. Our experiments using historical Ethereum transactions demonstrate that i) Justitia guarantees fairness while processing CTXs, ii) its token-issuance mechanism does not lead to unstable economic inflation, and iii) Justitia only yields 20%-80% of queueing latency for CTXs upon comparing with Monoxide protocol.

Blockchain interoperability has become an essential functionality, which enables asset/data transfers across different blockchains. Sidechains have been deemed as a key technique to provide interoperability. However, sidechains are rarely used in practice, this is because sidechain technologies are impractical and non cost-efficient. To make sidechains practical, in this paper, we design a universal sidechain construction named USSC, which applies to a variety of blockchains without forking them. USSC also enables interoperability across heterogeneous blockchains regardless of underlying consensus. This is facilitated by three components: i) a committee selection method, ii) a cross-chain certificate, and iii) a cross-chain bridge based on smart contracts. The proposed committee-selection method guarantees an honest majority within a committee. Through a concrete implementation of USSC, we outline how the proof-of-stake (PoS) and the proof-of-work (PoW) blockchains enable asset transfers. Furthermore, our USSC is more storage-efficient because it produces a smaller size of certificate and only needs partial nodes instead of all sidechain nodes following a blockchain. Thus, USSC can reduce the overhead of storage and communication sidechain construction with desirable security properties. Finally, we develop a proof-of-concept implementation of USSC using Cardano and Ethereum. Experimental results demonstrate that USSC outperforms PoW and PoS sidechains, in terms of the certificate size.

With the application of blockchain in Internet ofThings (IoT), various IoT scenario-oriented blockchains havebeen proposed, meaning that multichain future within IoT ecosys-tem is growing. Cross-chain interoperability thus is essential.Sidechains is one of the crucial technologies for cross-chain inter-operability, which allows blockchains to interoperate with eachother. However, sidechains encounters practicability challengesimpeding adoption, such as poor generality and inefficiency.In this article, we propose practical and secure sidechain con-struction (PSSC) in the form of parent-child chains, a practicaland secure sidechains construction for heterogeneous blockchainsorienting IoT scenarios. To achieve practicability, we first uti-lize succinct noninteractive argument of knowledge (SNARK),which is succinct noninteractive proof (arguments) of knowl-edge, to reproduce the state transition of blockchains, so thatPSSC can get rid of the constraints on consensus mechanisms ofblockchains; we then design an impawn-punish mechanism basedon smart contract, which transfers the work verifying SNARKproof from mainchain to sidechain, preventing fork threat tomainchain. Due to employing SNARK based on recursive proof composition to generate cross-chain proof, our PSSC is moreefficient with succinct cross-chain proof, reducing the over head of storage of nodes. Besides, we formally prove our PSSC sat-isfies the three fundamental security properties of sidechains:1) persistence; 2) liveness; and 3) firewall. Finally, we develop aproof-of-concept implementation of PSSC, and the experimental results show that the proof size of PSSC is about 28.8 kB, whichis roughly 2094x, 9.36x, 6.56x, 4.23x, and 3.27xsmaller than BTC Relay proof, PoW sidechains proof, PoS sidechains, zkRelayproof, and fast sidechains, respectively.

Efficient and secure data sharing in the Internet of Vehicles (IoV) is critical for the advancement of intelligent transportation, particularly in the context of emerging 5G/6G networks and driverless technology. This has prompted researchers to explore Blockchain-based Internet of Vehicles (BIoV) solutions to address common issues in intelligent transportation systems (ITS). However, at the edge layer, existing reputation and consensus mechanisms in the BIoV model face challenges such as significant energy consumption, limited scalability, and pronounced centralization. To tackle these issues, we introduce TRCO, a Permissioned Blockchain-based Trusted and Robust Consensus Optimization orienting ITS. TRCO is a three-layer BIoV architecture applicable to any BIoV scenario, offering robust scalability with the addition of new sub-regions. To improve the quality of data sharing for edge layer, we propose a decentralized reputation evaluation mechanism and integrate PoS and PBFT consensus algorithms to enhance performance by evaluating the communication behavior of edge servers. Security analysis and simulation experiments demonstrate that TRCO achieves high scalability and reliability, effectively filters Byzantine edge servers, and enhances the decentralization and consensus efficiency of the system.

Blockchain sharding is a significant technical area, improving the scalability of blockchain systems. It is regarded as one of the potential solutions that can achieve on-chain scaling, and significantly improve the scalability of blockchains without alleviating the decentralization feature of blockchain. To provide a reference and inspire participation from both the academic and industrial sectors in the area of blockchain sharding, we have researched the state-of-the-art studies published in the past three years. We have also conducted experiments to show the performance of representative sharding protocols such as Monoxide, LBF, Metis, and BrokerChain. We envision the potential challenges and promising future of sharding techniques in terms of the urgent demands of high throughput required by emerging applications such as Web3, Metaverse, and Decentralized Finance (DeFi). We hope that this article is helpful to researchers, engineers, and educators, and will inspire subsequent studies in the field of blockchain sharding.

The emergence of 5G/6G networks has sparked new potentials for Internet of Things (IoT) scenarios, such as vehicle ad hoc networks (VANETs), inspiring numerous scholars to leverage Blockchain-based Internet of Vehicles (BIoV) solutions to address prevailing issues in VANETs. However, the dynamic and decentralized nature of VANETs presents significant challenges in terms of security and privacy, hindering data providers from engaging in the data-sharing process. Furthermore, the reliability of edge nodes and system architecture in the BIoV paradigm faces several challenges, including limited consensus participation, high resource consumption, poor scalability, and centralization. To mitigate these challenges, we propose RTSP, a robust and trusted service protocol for VANETs, based on a distributed ledger technology. RTSP advocates a novel three-tier BIoV architecture suitable for any permissioned BIoV application scenario. To improve the quality of data sharing, we design a decentralized reputation mechanism. This mechanism mitigates the performance bottleneck induced by consensus transactions by measuring the historical behavior of roadside units (RSUs). Empirical evidence from simulation experiments and security performance analyses substantiates RTSP's capabilities. It can hinder the number of faulty RSUs from increasing while simultaneously improving data-sharing efficiency, simplifying communication complexity, and enhancing system scalability and consensus stability.

Metaverse brings unlimited space and tremendous potential since it is an integrated application of multiple fundamental technologies such as artificial intelligence, blockchain, networking, Internet of Things, and interactivity. During those building blocks of metaverse, blockchain is a type of technology operated by a group of individual participants and known for its immutability feature. The massive adoption of blockchain has been severely prevented by various security and scalability issues in blockchain-based applications due to the inherent characteristics of this technology. To accelerate the massive adoption of blockchain, many previous studies have been carried out to address the security and scalability issues. This article reviews blockchain-related publications collected from four major security conferences (i.e., NDSS, CCS, S&P, and USENIX Security) published in the past three years. Through this overview, we disclose the security and scalability issues of mainstream blockchains such as Bitcoin and Ethereum. Our study aims to help researchers better understand the bottleneck of blockchain-empowered metaverse, and how to address user requirements for security and scalability from the perspective of blockchains.

With the popularity of the mobile Internet, data is increasingly becoming a new resource. Therefore, the trading of such data resources has become an increasing demand. In this paper, we propose a fair privacy-preserving data trading protocol based on blockchain. Firstly, our data trading protocol achieves fairness by carefully combining the probabilistic approaches and the fully homomorphic encryption techniques. Moreover, our protocol allows online arbitration when misbehavior occurs in the trading process is detected. Note that previous data trading protocols need a Trusted Third Party (TTP) or an offline arbitrator to solve disputes, weakening the trust of those protocols. Secondly, the data validity verification process of our protocol is more flexible. Most Importantly, different from all previous designs which only achieve privacy against communication channel eavesdroppers, our protocol achieves privacy against any eavesdropper and the passive arbitrator. The above-distinguishing properties of our protocol are mainly benefited from the homomorphic encryption and double encryption techniques. In addition, our data trading protocol can be instantiated with post-quantum primitives and thus achieves post-quantum security. To demonstrate the feasibility of the proposed protocol, we conduct a comprehensive evaluation with the instantiated cryptographic primitives based on the Ethereum test network.

Economic systems play pivotal roles in the metaverse. However, we have not yet found an overview that systematically introduces economic systems for the metaverse. Therefore, we review the state-of-the-art solutions, architectures, and systems related to economic systems. When investigating those state-of-the-art studies, we keep two questions in mind: (1) What is the framework of economic systems in the context of the metaverse? and (2) What activities would economic systems engage in the metaverse? This article aims to disclose insights into the economic systems that work for both the current and the future metaverse. To have a clear overview of the economic system framework, we mainly discuss the connections among three fundamental elements in the metaverse, i.e., digital creation, digital assets, and the digital trading market. After that, we elaborate on each topic of the proposed economic system framework. Those topics include incentive mechanisms, monetary systems, digital wallets, decentralized finance activities, and cross-platform interoperability for the metaverse. For each topic, we mainly discuss three questions: (a) the rationale of this topic, (b) why the metaverse needs this topic, and (c) how this topic will evolve in the metaverse. Through this overview, we wish readers can better understand what economic systems the metaverse needs and the insights behind the economic activities in the metaverse.

Various metaverse applications have entered our daily life and show a promising trend that will occupy people's attention in the era of Web3. This makes interoperability across metaverses become one of the fundamental technologies in the context of multiple metaverse platforms. The aim of interoperability is to provide a seamless service for users when their requests interact with multiple metaverses. However, the development of cross-metaverse interoperability is still in its initial stage in both industry and academia. In this article, we review the state- of-the-art cross-metaverse interoperability solutions, which are designed for a dedicated purpose but do not apply to all metaverse platforms. To this end, we propose MetaOpera, a generalized cross-metaverse interoperability protocol. Connecting to MetaOpera by means of wireless communication, users and digital objects across different metaverses that rely on centralized servers or decentralized blockchains are capable of interacting with each other. We also implement a proof-of-concept mechanism for Meta- Opera, aiming at evaluating its performance with a state-of-the-art cross-metaverse solution based on the Sidechains technique. Simulation results demonstrate that the size of cross-metaverse proof and the average latency of cross-metaverse transactions using the proposed solution are about eight to three times smaller, respectively, than those of the Sidechains solution. This article also suggests a number of open issues and challenges faced by cross-metaverse interoperability that may inspire future research.

Criminal forensics in an Internet-of-Things (IoT) environment often requires complex investigations because IoT devices usually generate a large amount of electronic data, especially image data, which brings great difficulties to traditional digital forensic methods. Therefore, designing a secure management solution for image data and making it to be image evidence available in court is a new challenge. In this paper, we present a new secure image evidence management framework based on multi-bits digital watermarking and blockchain. In this framework, we first propose a flexible and robust self-learning based watermark embedding algorithm, which can embed both image marks and binary messages into the latent space of the input images and also improve the resistance of a broad range of attacks (geometric transform, JPEG, noise, etc.) to the watermarked images. And then, we design a smart contract resided in blockchain, which can achieve safe storage and automatically authentication of embedded watermarks. The experimental results on both watermarking algorithm and smart contracts have demonstrated the feasibility and efficacy of the proposed framework.

Economic systems play pivotal roles in the metaverse. However, we have not yet found an overview that systematically introduces economic systems for the metaverse. Therefore, we review the state-of-the-art solutions, architectures, and systems related to economic systems. When investigating those state-of-the-art studies, we keep two questions in our mind: (1) what is the framework of economic systems in the context of the metaverse? (2) What activities would economic systems engage in the metaverse? This chapter aims to disclose insights into the economic systems that work for both the current and the future metaverse. To have a clear overview of the economic system framework, we mainly discuss the connections among three fundamental elements in the metaverse, i.e., digital creation, digital assets, and the digital trading market. After that, we elaborate on each fundamental technology of the economic system for metaverse. Those technologies include incentive mechanisms, monetary systems, digital wallets, decentralized finance (DeFi), and cross-platform interoperability for the metaverse. For each technology, we mainly discuss three topics: (a) the rationale of this topic, (b) why the metaverse needs this technology, and (c) how this technology will evolve in metaverse. Through this chapter, we wish readers can better understand what economic systems the metaverse needs and the insights behind the economic activities in metaverse.

The directed acyclic graph (DAG) technique applied in asynchronous Byzantine fault tolerance (BFT) consensus has tremendously improved the system throughput in practice. However, there are still two critical limitations on performance. First, most work relies on the transaction batching technique to achieve high throughput while maintaining low amortized communication complexity, which places a huge bandwidth burden on the relevant single party. Second, due to the concurrency of the DAG-based multi-leader, it is inevitable for them to mix duplicate transactions in their proposal blocks. This paper presents Hydra, an efficient asynchronous DAG-based BFT protocol that is the first work to address these problems. The core design of Hydra is to leverage a request transaction pre-processing scheme, which fairly distributes the transactions to different node for proposing to eliminate the transaction duplication. And an AVID-M protocol with a dispersal-then-retrieval workflow to split the bulky proposal blocks into tiny segments for dispersing and recovering, which achieves optimal amortized O(N) communicate complexity and alleviates the bandwidth pressure on a single party.

We propose FBSChain, a faster blockchain sharding protocol based on quorum-based BFT algorithm (running as sharding consensus) and use a vertical committee structure to assist in the decoupling of transaction confirmation and block generation. We utilize a set of clients to drive all these related processes. In addition, we deploy an improved common-coin scheme without any private setup to generate the randomness for reconfiguration. The protocol can perform better with less computational and communication costs and improve system scalability without compromising security performance.

Data is a kind of important asset in the digital economy and is driving the rise of data markets. Meanwhile, data markets promote data trading efficiently and improve the utilization of data. However, several challenges about data trading need to be addressed. Here, we resolve these challenges via our blockchain-based fair and decentralized data trading model. Disputes about data correctness is settled by the decentralized arbitration mechanism in our model. To ensure the fairness of data trading, we integrate a sale contract and a deterministic public-key encryption algorithm. The decentralization feature of blockchain cuts off the single-point failure for the data trading platform. In addition, we prove that the proposed protocol achieves the desirable security properties that a secure data trading protocol should have. Moreover, utilizing the smart contract in Solidity and program in Java, we implement our model and then evaluate its performance.

Since the advent of sidechains in 2014, they have been acknowledged as the key enabler of blockchain interoperability and upgradability. However, sidechains suffer from significant challenges such as centralization, inefficiency and insecurity, meaning that they are rarely used in practice. In this paper, we present SEPoW, a secure and efficient sidechains construction that is suitable for proof of work (PoW) sidechain systems. The drawbacks for the centralized exchange of cross-chain assets in the participating blockchains are overcome by our decentralized SEPoW. To reduce the size of a cross-chain proof, we introduce merged mining into our SEPoW such that the proof consists of two Merkle tree paths regardless of the size of the current blockchain. We prove that the proposed SEPoW achieves the desirable security properties that a secure sidechains construction should have. As an exemplary concrete instantiation we propose SEPoW for a PoW blockchain system consistent with Bitcoin. We evaluate the size of SEPoW proof and compare it with the state-of-the-art PoW sidechains protocols. Results demonstrate that SEPoW achieves a proof size of 416 bytes which is roughly 123 × , 510 × and 62000 × smaller than zkRelay proof, PoW sidechains proof and BTCRelay proof, respectively.