Blockchain as a Service is a new application method that embeds the blockchain framework into the cloud computingplatform,which can effectively use the cloud platform to improve the convenience and efficiency of the deployment and operation of the blockchain system.This paper mainly analyzes and summarizes the key techniques and existing platform systems of Blockchain as a Service.Firstly,this paper introduces the concept and function of BaaS platform,and analyzes the advantages of BaaS platform in improving safety performance,realizing personalized customization and reducing development cost.Then,based on the existing commercial BaaS platform,the system architecture and key technique architecture of BaaS platform are introduced in detail,and the characteristics,technologies and functions of the current mainstream BaaS platform,as well as the relevant application scenarios are introduced.In this paper,the challenges encountered by the current BaaS platform are summarized,and the future research direction of BaaS is prospected.

Blockchain technology can be extensively applied in diverse services,ranging from online micro-payment,supply chain tracking,digital forensics,health-care record sharing to insurance payment.Extending the technology to the crowdsourcing,we can obtain a verifiable and traceable crowdsourcing systems.Emerging research in crowdsourcing applications exploits blockchain technology to optimize the task assignment and reward payment using various consensus protocols and blockchain techniques,which can provide additional security,automatic,verifiable and traceable crowdsourcing platforms.In this paper,we conduct a systematic survey of the key components of crowdsourcing blockchain and compare a number of popular blockchain applications.In particular,we first give an architecture overview of popular crowdsourcing-blockchain systems by analyzing their network structures and protocols.Then,we discuss variant consensus protocols for crowdsourcing blockchains,and make comparisons among different consensus algorithms.Finally,we look forward to the future research problems in this field,and provide a large number of references.

At present,false information is prevalent on the Internet,and the authenticity of shared information cannot be guaranteed when patients communicate on the Internet.In order to solve this problem,a blockchain-based online patient communication model is proposed,and patients can anonymously share real medical data and communicate with other patients.Firstly,the patient's digital identity is used to protect privacy,the medical summary data needed for patient communication are uploded to the blockchain and published all the data for retrieval,and it allows patients to retrieve the required cases without locating a specific patient.Secondly,in order to avoid malicious uploading by authorized personnel,smart contracts are used to perform multiple authentication on data upload,and doctors and patients restrict each other to ensure that all the data on the blockchain is true and reliable.Finally,the improved RAFT consensus algorithm can quickly identify Byzantine nodes so as to better achieve consensus on the blockchain.The performance of the model is evaluated through experiments,and the results show that the medical data could be shared,and meet the needs of patients for online communication under the premise of ensuring patient privacy.

The coming of 5G era brings new challenges to the information security of emerging information industries.The exis-ting security technologies can't meet the requirements of millisecond level authentication and trusted authentication for specific scenarios in 5G era.Therefore,CFL technology is taken as the origin technology of information security.Based on the local modification of the security axioms of the original BLP model,combined with the Blockchain technology,CFL_BLP_BC model formally describes the basic elements,security axioms and state transition rules of the model.The model can support the construction of five aspects of information security,and has the attributes of millisecond level,instruction level and empirical system.The model belongs to endogenous safety,innate immunity and active defense technology.The model can provide important theoretical gui-dance for theemerging information industry.

Threshold signatures are fundamental tools for multi-party information security protocols.It is widely used in fields such as identity authentication,anti-counterfeiting and tamper-resistance.We introduce a new decentralized threshold signature protocol BHTSP which combines distributed key generation (DKG) and BLS signature.The protocol allows multi-party participation and generates a signature of constant size.We implement this protocol with smart contract as the communication layer for secure parameter exchange.Experimental simulation results show that BHTSP can generate threshold signature with constant size.It reduced the memory consumption for aggregated public key combinations needed in signature verification by 85.3% compared to Schnorr signature.In the experimental blockchain platform,BHTSP is able to support the generation of threshold signatures involving up to 50 participants,optimizing the execution process for blockchain multi-party transactions.

Blockchain technology has the advantages of decentralization,non-tampering,and traceability,and has been widely used in various fields.However,as a cutting-edge technology,blockchain has problems in development and high learning costs,and most developers are more familiar with application development methods based on relational databases,that is operating relational databases through SQL.Hyperledger Fabric is the most mainstream blockchain development framework.In response to this pro-blem,the article proposes a SQL access framework for Hyperledger Fabric.First of all,in view of the inconsistency of the underlying storage structure of Fabric and SQL,the conversion rules from relational model to key-value model are defined and implemented the model conversion algorithm.Second,the SQL execution contract is developed based on smart contract technology,which realizes the automatic transformation from SQL statement to CouchDB operation statement,and then operates the under-lying data of Fabric.Finally,from the perspective of application layer development,an application automation refactoring tool is designed.The tool consists of two parts,including the application refactoring tool and the blockchain.Fabric-Driver is an interactive middleware for the network.The experimental results show that compared with the existing scheme,using the proposed framework to develop blockchain applications can save about 82% of the time,the reading and writing performance of using this framework is less than 5%.

In recent years,relying on the research breakthrough and rapid development of blockchain technology,a variety of di-gital currencies are rising and flooding into the market.As the currency with the strongest privacy in the UTXO model of blockchain so far,the anonymity technology of Zcash not only provides a strong guarantee for users' privacy,but also has high scienti-fic research value and a wide range of application prospects.Therefore,in order to standardize the legal use of digital currency and explore the wider application prospect of digital currency anonymity technology,scholars from all walks oflife have also conducted research on the anonymity and anti-anonymity technology of Zcash from different angles.Focusing on Zcash,a new digital currency,we first introduce thegeneral framework of Zcash.Secondly,the anonymous technology adopted by Zcash:zk-SNARKs and shielded pool transaction technology,are sorted out.Then we summarize and analyzethe research on Zcash tracking technology by scholars from all walks of life.In the end,anonymous technology and tracking technology development of Zcash are prospected.

Mixing is an important way for privacy protection among digital currency such as Bitcoin.However,on the one hand,Bitcoin mixing protects user privacy,on the other hand,it facilitates the transfer of assets for illegal activities such as ransomware and Bitcoin theft.In this paper,we propose a traceable scheme for Bitcoin mixing.The scheme aims to protect the privacy of legi-timate users and can trace the illegal assets.The system is regulated by trusted third party,user anonymity and traceability based on the group signature which is constructed by bilinear groups and strong Diffie-Hellman assumption.When there is a need for tracing,the regulator can determine the signed user through the system private key,so as to determine the illegal asset transfer path.Security analysis shows that the scheme can trace the illegal asset transfer without modifying the current Bitcoin system,meanwhile,the solution provide privacy protection and asset safety for legitimate users.Furthermore,the scheme provides a refe-rence direction for the research on digital currency privacy protection.

Smart contract is an important component of blockchain platform to realize transactions,which provides an effective solution to the trust problem between multi-party transactions.Smart contracts not only manage high value tokens but also have the characteristics of immutable,which lead to the security threats of smart contracts many times in recent years.At present,a lot of researches have devoted to the security of smart contracts,among which the vulnerability detection of smart contracts has become the main concern.This paper analyzes the security of smart contract systematically.From the perspective of whether to execute the smart contract,vulnerability detection tools are divided into static detection tools and dynamic detection tools.In particular,the vulnerability detection ability of existing detection tools is analyzed,and the principles,advantages and disadvantages of 16 detection technologies are discussed.Finally,the paper gives a prospect of how to improve the security of intelligent contract,and puts forward three research directions which may improve the security of smart contract.

As the largest blockchain platform supporting smart contracts,millions of smart contracts have been deployed on Ethereum.Since the deployed smart contracts cannot be modified even if the contracts contain bugs,it is critical for developers to eliminate bugs prior to the deployment.Many smart contract analysis tools have been proposed.These tools either use bytecode-based symbolic execution to detect bugs,or convert the source code to an intermediate representation and then detect bugs.The tools based on symbolic execution usually cannot cover many types of bugs in source code.Converting the source code to an intermediate representation negatively impacts the detection speed.Moreover,these tools are bug detectors,which cannot automatically fix bugs based on analysis results.To address these limitations,we propose an approach named SolidityCheck,which employs regular expressions,program instrumentation and statement replacement in source code to quickly detect bugs and fix certain types of bugs.We conduct extensive experiments to evaluate SolidityCheck.The experimental results show that,compared with existing approaches,SolidityCheck demonstrates excellent performances on multiple indicators.

Cloud storage enables users to obtain cheap online storage services on demand through network connection anytime and anywhere.However,due to the untrustability of cloud service providers,third-party institutions and users as well as the inevitable malicious attacks,there are many security vulnerabilities of cloud storage.Blockchain has the potential to build a trusted platform with its characteristics of decentralization,persistence,anonymity and auditability.Therefore,the research on cloud storage security mechanism based on blockchain technology has become a research trend.Based on this,the security architecture of cloud sto-rage system and the security of blockchain technology are first outlined,then the literature review and comparative analysis are conducted from four aspects of access control,integrity verification,data deduplication and data provenance.Finally,the technical challenges of blockchain-based cloud storage security mechanism are analyzed,summarized and prospected.

Blockchain technology has evolved rapidly in recent years,as a result,many organizations and enterprises have started using decentralized applications (DApps) based on blockchain and smart contracts to enhance the functionality and security of their information systems,or to expand new businesses.However,DApps may also introduce new problems due to the possible security and performance issues of blockchain and smart contracts.In order to deeply study and analyze the data and behavioral phenomena of DApps so as to help users better apply blockchain and DApps,a total of 2 565 DApps in 21 categories are first collected,as well as data related to these DApps from July 30,2015 to May 4,2020 (about 10 million block heights),including 16 302 smart contracts,7 678 185 EOA,95 889 930 external transactions,and 30 833 719 internal transactions.Then the DApp distribution is deeply analyzed from four perspectives:number,time,category,and smart contract,and some findings are summarized from them,which can provide valuable references for DApp developers and blockchain researchers.

This paper proposes a mobile edge computing (MEC) assisted blockchain system in which mobile terminals (MT) do not have enough local computation resources to solve the proof of work (PoW) puzzle.By combining the computation resource allocation of MTs and edge server (ES) with the revenue sharing of MTs,a joint optimization problem is formulated to maximize the system-wide utility of all MTs and the ES.To solve the optimization problem efficiently,a multi-layer decomposition algorithm based on cyclic block coordinate descent (CBCD) is proposed.First,given the revenue sharing variables in advance,the corresponding sub-problem can be solved to obtain the computation resource allocation results of both MTs and ES.Then,with the obtained computation resource allocation,the revenue sharing variables of MTs are optimized.Finally,this paper optimizes the two sub-problems alternately until the algorithm reaches convergence.The numerical results show that the proposed algorithm can obtain the optimal solution of the joint optimization problem effectively and improve the system-wide utility.

Focusing on the problems of poor scalability,the random selection of primary nodes,and high network overhead in the practical-Byzantine-fault-tolerant consensus algorithm,this paper proposes a Byzantine-fault-tolerant consensus algorithm based on credit evaluation mechanism.First of all,the system sets different roles for the nodes in the cluster and assigns different permissions to the nodes according to different roles,so that the nodes with different permissions can enter and exit the network dynamically.Secondly,the voting mechanism and follow-the-satoshi algorithm based on credibility are designed to ensure the security and fairness of the election.Finally,in the aspect of the consensus process,an optimized two-stage Byzantine-fault-tolerant consensus is proposed to reduce the network overhead in the PBFT consensus process.Experiments show that the consensus algorithm based on the credit evaluation mechanism proposed in this paper has the characteristics of high dynamic,election security,and low cost compared with the PBFT algorithm which is suitable for the alliance chain.

As the core of blockchain technology,consensus mechanism determines the performance,scalability,and security of blockchain systems.Aiming at the performance and scalability issues of current blockchains and the high cost of the incentives used to maintain the security of the systems,a trust-based dual-layer scalable consensus protocol (TDSCP) is proposed.First,the trust model and consensus algorithm of dual-layer cooperation are designed through a structured network.The trust model determines whether a node gets the right to generate blocks based on its trustworthiness to avoid the high cost of mining.Secondly,the dual-layer consensus algorithm within the partitions improves consensus efficiency,expands the number of nodes involved in consensus,and avoids the problem of system centralization.Finally,the verifiable random function and the multilevel graph partitioning algorithm are combined for partitioning nodes,which can effectively prevent malicious nodes from gathering and reduce the number of cross-partition transactions.The experimental results show that TDSCP improves the scalability of the blockchain system,the latency of its intra-partition algorithm consensus is lower,and the partition method significantly reduces the number of cross-partition transactions.

Faced with a large number of IoT transactions,efficient consensus algorithm plays a key role in the application of blockchain technology into IoT.In this paper,according to the problems of long consensus time delay and low throughput in practical Byzantine fault tolerant algorithm (PBFT),we propose a practical Byzantine fault tolerant algorithm based on clustering (C-PBFT).Firstly,the nodes are clustered according to location features to form a network structure with multiple centers and layers.Then,consensus tasks are divided to conduct consensus in bottom and top network,thereby reducing the communication cost needed by consensus.Finally,credibility of dynamic credit model evaluation node is introduced to reduce the participation of abnormal nodes and increase the security and reliability of the system.Experimental results show that the C-PBFT algorithm can effectively reduce communication overhead,consensus delay and improve throughput.

Bitcoin is one of the most mature public chain application systems,the user key is the critical factor to the process of determining the ownership of Bitcoin,the security of Bitcoin is guaranteed by the safe management of the user key,and the loss of the key will lead to the loss of a large number of user assets.So it is an urgent problem to recover the lost assets.This paper proposes a key update mechanism in Bitcoin based on the improved P2PKHCA (pay-to-public-key-hash-with-conditional-anonymity) script scheme to solve above problems.Firstly,the key generation algorithm in the P2PKHCA scheme is improved by introducing the key life cycle and random number to solve its key leakage problem.Secondly,the two new opcodes,OP_KEYUPDATE and OP_TSELECTION,are proposed to design the new key update script to realize the user key update of the Bitcoin system.Finally,two types of key update schemes based on the key update script are constructed to make the script suitable for the requirements of different key update applications.The security analysis and performance analysis of the key update mechanism show that the proposed mechanism realizes the recovery of lost Bitcoins in the Bitcoin system on the premise of the effective completion of update of user's key.