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Received July 7, 2019, accepted July 28, 2019, date of publication August 19, 2019, date of current version September 4, 2019.
Digital Object Identifier 10.1109/ACCESS.2019.2936094
A Survey of Blockchain From the Perspectives of Applications, Challenges, and Opportunities AHMED AFIF MONRAT , OLOV SCHELÉN, (Member, IEEE), AND KARL ANDERSSON , (Senior Member, IEEE) Department of Computer Science, Electrical and Space Engineering, Lulea University of Technology, 931 87 Skelleftea, Sweden
Corresponding author: Ahmed Afif Monrat ([email protected])
The financial support for the research is provided by the Swedish Energy Agency under Grant 43090-2, and in part by the Cloudberry Datacenters.
ABSTRACT Blockchain is the underlying technology of a number of digital cryptocurrencies. Blockchain is a chain of blocks that store information with digital signatures in a decentralized and distributed network. The features of blockchain, including decentralization, immutability, transparency and auditability, make transactions more secure and tamper proof. Apart from cryptocurrency, blockchain technology can be used in financial and social services, risk management, healthcare facilities, and so on. A number of research studies focus on the opportunity that blockchain provides in various application domains. This paper presents a comparative study of the tradeoffs of blockchain and also explains the taxonomy and architecture of blockchain, provides a comparison among different consensus mechanisms and discusses challenges, including scalability, privacy, interoperability, energy consumption and regulatory issues. In addition, this paper also notes the future scope of blockchain technology.
INDEX TERMS Blockchain, distributed ledger, consensus procedures, cryptocurrency, smart contract, selfish mining, energy consumption.
I. INTRODUCTION Unlike traditional methods, blockchain enables peer-to-peer transfer of digital assets without any intermediaries . Blockchain was a technology originally created to support the famous cryptocurrency Bitcoin. Bitcoin was first proposed in 2008 and implemented in 2009 by Nakamoto . Since then, it has seen huge growth with the capital market, reaching 10 billion dollars in 2016. Blockchain is basically a chain of blocks that store all committed transactions using a public ledger . The chain grows continuously when new blocks are appended to it. Blockchain works in a decentralized environment that is enabled by comprising several core tech- nologies, such as digital signatures, cryptographic hash, and distributed consensus algorithms. All the transactions occur in a decentralized manner that eliminates the requirement for any intermediaries to validate and verify the transactions . Blockchain has some key characteristics, such as decentral- ization, transparency, immutability, and auditability .
Although Bitcoin is the most famous application of blockchain, it can be applied to diverse applications far beyond cryptocurrencies. Since it allows payments to be
The associate editor coordinating the review of this article and approving it for publication was Chien-Ming Chen.
finished without any bank or any intermediary, blockchain can be used in various financial services, such as digital assets, remittance and online payment . The blockchain itself has taken on a life of its own and permeated a broad range of applications across many industries, includ- ing finance, healthcare, government, manufacturing, and distribution . The blockchain is poised to innovate and transform a wide range of applications, including goods transfer (supply chain), digital media transfer (sale of art), remote services delivery (travel and tourism), plat- forms for example, moving computing to data sources and distributed credentialing . Additional applications of blockchain include distributed resources (power generation and distribution), crowdfunding, electronic voting, Identity management and governing public records.
Despite the fact that blockchain technology shows great potential that may replace many of the current digital plat- forms, it has some technical constraints. Scalability is a huge concern for blockchain based platforms . In Bitcoin, the limited size and frequency of the blocks along with the number of transactions the network can process can be con- sidered a scalability problem . The average block creation time in Bitcoin is 10 minutes, and the block size is limited to 1 megabyte which constrains the network’s throughput .
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Bitcoin’s ability to scale depends on the size of the block and is limited to the complexity of the mathematical puz- zle independent of the nodes in the network. In general, the transaction processing capacity of Bitcoin is between 3.3 to 7 transaction per second . However, due to the increased size of recently generated blocks, the transaction throughput is being effectively limited to 2-4 transactions per second, which is incapable of high-frequency trading. At present, there are more than 36 million wallet users, and with time, it will increase and create an adverse impact on the network’s throughput. Different issues such as the blockchain congestion problem, transaction delays, and increased trans- action fees will raise concerns. As a result, the technology may not be a sustainable approach for government or private sectors to build their business model upon the blockchain platform. Moreover, increased block size requires substantial storage space and cause slower propagation in the blockchain network , which will also lead towards centralization and trust issues as users would like to operate and maintain such a large blockchain. Therefore, it has become a great challenge to deal with the tradeoff between blockchain size and trust.
Blockchain has some other issues regarding interoper- ability, privacy, energy consumption, selfish mining, secu- rity, and regulation policy. The interoperability issue arises due to the lack of standard protocol for adopting and integrating blockchain-based solutions among companies. Privacy leakage may also happen within the blockchain, although the system claims to be extensively secured as users only make transactions with digital signatures that associate public-private key encryption . Furthermore, it is possible to track the user’s real IP address. Consensus mechanisms such as proof-of-work (PoW) and proof-of-stake (PoS) are also facing serious concerns. For instance, PoW is known for consuming a large extent of electrical energy due to the competitive nature of miners for creating blocks by solving complex mathematical puzzles . In PoS, the rich become gradually richer as the chance of obtaining a block depends on how much stake the miners have . Another drawback of blockchain technology is selfish mining, where miners can gain better revenue than their fair share by keeping their blocks private . Blockchain can also suffer from 51% attacks, where some node attains the majority in a network and abuses it. Furthermore, it is believed that blockchain technology may not reach its peak or anticipated large-scale adoption by stakeholders because of uncertainties that arise with potential government regulations . One of the major underlying reasons could be that the decentralized nature of blockchain eliminates intermediary links to central banks to control the economy, which does not bode well with the government. Hence, some measures need to be put forward to address these issues in blockchain.
This survey paper focuses on state-of-art blockchain stud- ies including blockchain architecture, consensus algorithms, applications of blockchains, trade-off and challenges. The rest of this survey paper is organized as follows. Section II introduces blockchain architecture. Section III shows typical
consensus algorithms used in the blockchain. Section IV introduces several typical blockchain applications. Section V summarizes the tradeoffs and technical challenges, in this area. Section VI discusses some possible future directions and Section VII concludes the paper.
II. BLOCKCHAIN ARCHITECTURE A node initiates a transaction in a decentralized blockchain network by employing a digital signature using private key cryptography. A transaction can be considered as a data struc- ture that represents transfer of digital assets between peers on the blockchain network. All the transactions are stored in an unconfirmed transaction pool and propagated in the network by using a flooding protocol known as Gossip protocol. Then, peers need to choose and validate these transactions based on some preset criteria. For example, the nodes try to verify and validate these transactions by checking whether an initiator has sufficient balance to trigger a transaction or by trying to fool the system by enforcing double spending. Double spending refers to using the same input amount for two or more different transactions . Once the transaction is verified and validated by the miners, it is included in a block. Peers who use their computational power to mine for blocks are called miners . Miner nodes need to solve a computa- tional puzzle and spent a sufficient amount of their computing resources to publish a block. The miner who can solve the puzzle first will become a winner and obtains the opportunity to create a new block. A small amount of incentive is given upon successfully creating a new block. All the peers in the network then verify the new block using a consensus mechanism, which is a technique that assist a decentralized network comes to an agreement on certain matters. After that the new block will be added to the existing chain and the local copy of each peer’s immutable ledger. At this point, the trans- action is confirmed. The next block links itself with the newly created block by using a cryptographic hash pointer. Now the block obtains its first confirmation while the transaction obtains the second confirmation. Similarly, with every time a new block is appended to the chain, the transaction will be reconfirmed. In general, a transaction needs six confirmations in the network to be considered final .
Later in this segment, Section II-A discusses the trans- action process of blockchain with some example platforms, such as Bitcoin and Ethereum, Section II-B introduce the basic block structure and the process of cryptographic hash functions while Blockchain key characteristics are explained in Section II-C and Section II-D represents the taxonomy of blockchain.
A. BLOCKCHAIN TRANSACTION PROCESS A Blockchain transaction can be defined as a small unit of a task that is stored in public records. These records are also known as blocks . These blocks are executed, imple- mented and stored in blockchain for validation by all miners involved in the blockchain network. Each previous transac- tion can be reviewed at any time but cannot be updated .
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FIGURE 1. Functional diagram of a Blockchain network.
Blockchain is the underlying technology of Bitcoin, and it facilitates transactions that occur within a peer to peer global network in a decentralized fashion. That makes Bitcoin a borderless, censorship-resistant digital currency. In general, trust may be the main concern regarding traditional central- ized systems, such as a banks, where people need to put their solemn confidence in the system. This is the sweet spot for public blockchain technology, in that it does not require any trust while handing over the ownership of digital assets from one peer to another. Blockchain is a trustless system that provides trust through the functions that propagate all the activities within the network . Security is another aspect to consider while initiating transactions. Blockchain mining and consensus mechanisms that rely heavily on a cryptographic hash function can address the security issues. For example, Bitcoin uses a 256 bits’ secure hash algorithm known as SHA-256 . Bitcoin can take any type of input, such as text, numbers, string or even a computer-generated file of any length, to produce 256 bits or the 64 characters output called hash . Given the same input, the converted hash output will always remain exactly similar. However, a small change to the input will change the output completely, which is also called a one-way function, meaning that from the output, it is not feasible to calculate the input. One can only guess what the input was, and the odds of guessing it right are rather astronomical, in other words, it is secure.
The first step of the transaction process is to verify the identity of the sender, which means the transaction between the sender and the receiver is requested by the sender, and not by anyone else. Figure 2 demonstrates the verification pro- cess with a simple example of a transaction between Bob and Alice. Let us assume both Alice and Bob has Bitcoin balance, and Alice wants to pay 10 Bitcoins to Bob. Now, to send the
money, Alice will broadcast a message with the information for the transaction in the blockchain network. To do this, Blockchain employs digital signatures (public and private keys) . For the broadcast, Alice provides Bob’s infor- mation, such as his public address and transaction amount, along with her public key and digital signature. Alice used her private key to make that digital signature. Transaction validation is carried out independently by all miners based on different criteria that we have discussed later in this section. Elliptic curve digital signature algorithm (ECDSA) is used by blockchain . This algorithm ensures that the funds can only be spent by their true possessors.
The signature in each transaction contains 256 bits, if any- one wants to fake this signature to make a fraudulent transac- tion, he or she has to guess 2256 cases, which is infeasible and waste of resources for a malicious peer/attacker . In addition to checking the validity of the sender, the verifier also has to check the validity of the transaction regarding whether the sender has enough money to send to the receiver, or not. It could be performed by looking at the ledger, which holds information about every past successful transaction.
1) BITCOIN TRANSACTION According to the original Bitcoin whitepaper, the main pur- pose of this digital cryptocurrency was to allow a decen- tralized electronic cash payment system between different parties by eliminating central intermediaries . A Bitcoin transaction transfers the ownership of some bitcoin amount to another bitcoin address. Generally, it is initiated by a bitcoin wallet of a client and later broadcast to the network. The nodes on the network will rebroadcast the transaction and include it in the block they are mining only if the transaction is valid. It takes approximately 10 minutes to include the
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transaction along with other transactions in a block . The receiver should see the amount of transaction in their wallet by this point.
The main element of a bitcoin structure is unspent trans- action output (UTXO), which refers to the output amount of a transaction that is received by a user and the capability of spending it in the future . Consider that cash or coins in a physical wallet get mixed up, which is not in the case of the received amount in Bitcoin. All the received amount in a Bitcoin wallet remains as a separate entity. For example, when we receive two distinct amounts ($2 and $3) and keep it in the same physical or online wallet, it will obtain summed up to $5. Whereas in the Bitcoin wallet, it will still show the exact amounts and remain as individual entities. Let us consider that Alice has three separate UTXO (0.01, 0.2 and 3) in her wallet, and she wants to send 0.15 BTC to Bob. To do that, the wallet needs to select a spend candidate from these three output UTXO. If the wallet chooses 0.2 as an output, then it will unlock this amount and spend the whole amount as an input UTXO for the 0.15 BTC transaction. Then, 0.15 BTC will be transferred to Bob’s address wallet as an output UTXO.
Miners will be incentivized by their effort in managing and validating all these transactions and creating a new block that will eventually add to the existing chain . A successful miner obtains the block creation rewards and transaction fees . While sending transactions, users usually assign a transaction fee upon successful block creation for the miners. There will not be any header information regarding the trans- action fee. The users can attach a transaction fee by sending a lesser amount to the recipients than the total input UTXO. This unassigned transaction amount can be considered as transaction fee as depicted in Eq. 1.
Inputs−outputs = Transactionfees (1)
Miners include their individual coinbase transaction along with the transaction data that they are trying to verify and validate while mining a block. A coinbase transaction is a unique type of bitcoin transaction that can only be created by a miner. This type of transaction has only outputs, and there is one created with each new block that is mined on the network. This is the transaction that rewards a miner with the block reward for their work. Any transaction fees collected by the miner are also sent in this transaction. The peers in the network check whether the transaction is level out and then decide to put this record in the distributed ledger. The coinbase transaction will send the block reward and the sum of the transaction fees to the given address of the miner. That shows that a miner has to assign his reward while creating a block. However, every node in the network will check whether the block adheres to the requirement, and as shown in Eq. 2. Therefore, a miner is eligible to use the block reward and transaction fees only after the block is verified.
sum(BlockOutputs) ≤ sum(BlockInputs)+BlockReward
2) ETHEREUM TRANSACTION The Bitcoin state is defined in the terms of UTXO, a ref- erence implementation of the wallet application that held the account reference. However, Ethereum introduced the concept of an account as a part of the protocol that is the originator and target of a transaction. Hence, transactions directly update the account balances as opposed to main- taining the state, such as in the Bitcoin UTXOs, allowing transfer of values, messages and data between the accounts that may result in the state transitions . Ethereum has two types of account: Externally Owned Account (EOA) and Contract Account (CA). While EOA is owned by private keys, CA is controlled by the code and activated only by an EOA . EOA is needed to participate in the Ethereum network and interacts with the blockchain using transactions, whereas, CA represents a smart contract (SC). SC is a piece of code deployed in the blockchain’s node and adds a layer of logic and computation to the trust infrastructure . Exe- cution of an SC is initiated by a message embedded in the transactions.
In Ethereum, the transferable amount is known as ether. The denomination of ether is known as Wei . An Ethereum transaction has fields for transferring ether as well as messages to trigger smart contracts . Ethereum uses attributes similar to Bitcoin, for instance, previous block hash, nonce, and transaction details. Additionally, it uses some other fields such as fees limit, state of SC, and so on. For a simple ether transfer, the amount to transfer and the target address are specified, together with the fees, gas points, and the respective accounts. All the transac- tions generated will be validated by checking time stamp, nonce combination, and availability of sufficient fees for execution.
Ethereum also uses an incentive based model for block cre- ation. Any action in Ethereum requires crypto fuel or gas. Gas is used as fees instead of ether for ease of computation. The main reason behind that is that gas is a cryptocurrency inde- pendent of valuation for the transaction fee and computation fee. Ether, as a cryptocurrency, varies in value with market swings, but gas points do not vary. The mining process com- putes gas points required for the execution of a transaction. If the fee specified in the gas points in transaction is not suf- ficient, it is rejected. The gas points needed for the execution must be in the account balance and the proposed transaction for the execution to happen. The leftover amount after execut- ing the transaction will be returned to the originating account. Etherreum has a mining incentive model where the miners are competing for block creation. The miner who solves the puzzle first is called the winner and the miners who solve it afterwards are called ommers . The winner block is added to the main chain and ommer blocks are added as side blocks in the main chain. The winner block receives three ethers as a base fee along with the transaction fees as gas points. The ommers block receives a small percentage of total gas points.
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B. BLOCK STRUCTURE The Blockchain comprises a sequence of blocks, which stores the information of all the transactions, similar to a public ledger. These blocks are linked to each other via a refer- ence hash that belongs to the previous block known as the parent block. The starting block is called the genesis block, which does not have any parent block. A block consists of the block header and the block body . The block header includes metadata such as block version, parent block hash, Merkle tree root hash, timestamp, nBits, and nonce as shown in Table 1 and Fig. II-B.
TABLE 1. Block header attributes.
FIGURE 2. Block structure.
The block body is composed of a transaction counter and transactions. The transaction counter refers to how many transactions follow, and transactions represent the list of recorded transactions in the block. The maximum number of transactions that a block can contain depends on the block size and the size of each transaction. Blockchain uses an asymmetric cryptography mechanism to validate the authen- tication of transactions. A digital signature based on asym- metric cryptography is used in an untrustworthy environment such as the blockchain network. In this process, each par- ticipant in the network owns a private key and public key pair. The private key is used for signing or encrypting the transaction while the public key is distributed throughout the network and is visible to everyone, which helps to decrypt the following transaction.
C. CHARACTERISTICS OF BLOCKCHAIN 1) DECENTRALIZATION In conventional centralized transaction systems, each transac- tion needs to be validated through the central trusted agency (e.g., the central bank). Therefore, decentralization requires trust, which is the main issue, along with lift resilience, avail- ability and fail over, where the decentralized peer-to-peer blockchain architecture could be a better solution. Unlike a centralized system, a transaction in the blockchain network can be conducted between any two peers (P2P) without the authentication by the central agency. In this manner, blockchain can reduce the trust concern by using various consensus procedures. Moreover, it can reduce the server costs (including the development cost and the operation cost) and mitigate the performance bottlenecks at the central server. In contrast, in many cases, blockchain has some trade- offs. For example, PoW cases such as Bitcoin and Ethereum, the server and energy cost are orders of magnitude higher, while the performance are also several orders of magnitude lower.
2) PERSISTENCY Blockchain provides the infrastructure by which truth can be measured  and enables the producers as well as con- sumers to prove their data are authentic and not altered. For example, if a Blockchain consists of 10 blocks, then block no. 10 contains the hash of the previous subsequent block, and to create a new block, the information of the current block is used. Therefore, all the blocks are linked and connected with each other in the existing chain. Even the transactions are related to the prior transaction. Now, a simple update on any transaction will significantly change the hash of the block. If someone wants to modify any information, he has to change all the previous block’s hash data which is considered an astronomically difficult task considering the amount of work that needs to be done. In addition, after generating a block by a miner, it is confirmed by other users in the net- work. Hence, any manipulation or falsification of data will be detected by the network. For this reason, blockchain is almost tamper proof and considered as an immutable distributed ledger.
3) ANONYMITY It is possible to interact with the blockchain network with a randomly generated address . A user can have many addresses within a Blockchain network to avoid the exposure of his identity. As it is a decentralized system, no central authority is monitoring or recording users’ private informa- tion. Blockchain provides a certain amount of anonymity through its trustless environment.
4) AUDITABILITY All the transactions that occur in a blockchain network are recorded by a digital distributed ledger and validated by a digital timestamp. As a result, it is possible to audit and trace
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previous records by accessing any node in the network . For example, all the transactions could be traced iteratively in Bitcoin which facilitates auditability and transparency of the data state in the blockchain. However, by tumbling money through many accounts, it becomes very hard to trace the money to its origin.
D. TAXONOMY OF BLOCKCHAIN SYSTEMS There are three types of blockchain: public, private and consortium . These systems can be compared using dif- ferent perspective as described below.
1) CONSENSUS DETERMINATION All the nodes can participate in the consensus process in the public blockchain such as Bitcoin, while only a few selected set of nodes are being responsible for confirming a block in the consortium blockchain. In the private blockchain, a cen- tral authority will decide the delegates who could determine the validated block.
2) READ PERMISSION Public blockchain allows read permission to the users, where the private and consortium can make restricted access to the distributed ledger. Therefore, the organization or consortium can decide whether the stored information needs to be kept public for all or not.
3) IMMUTABILITY In the decentralized blockchain network, transactions are stored in a distributed ledger and validated by all the peers, which makes it nearly impossible to modify in the public Blockchain. In contrast, the consortium and private Blockchain ledger can be tampered by the desire of the dominant authority.
4) EFFICIENCY In the public blockchain, any node can join or leave the network which makes it highly scalable. However, with the increasing complexity for the mining process and the flexible access of new nodes to the network, it results in limited throughput and higher latency. However, with fewer valida- tors and elective consensus protocols, private and consor- tium blockchain can facilitate better performance and energy efficiency .
5) CENTRALIZED The significant difference among these three types of Blockchain is that the public blockchain is decentralized, while the consortium is partially centralized and private blockchain is controlled by a centralized authority.
Since public blockchain is open to the world, it can attract many users. Communities are also very active. Many public blockchains emerge day-by-day. For the consortium blockchain, it could be applied to many business applica- tions. Currently, Hyperledger is developing business consor- tium blockchain frameworks. Ethereum has also has provided
TABLE 2. Comparison among different blockchain infrastructure.
tools for building consortium blockchains. For the private blockchain, there are still many companies implementing it for efficiency and auditability.
III. CONSENSUS PROCEDURES In blockchain, how to reach consensus among the untrustwor- thy nodes is a transformation of the Byzantine Generals (BG) Problem . In the BG problem, a group of generals who command a portion of a Byzantine army circle the city. The attack would fail if only part of the generals attack the city. Generals need to communicate to reach an agreement on whether to attack or not. However, there might be traitors within the generals. The traitor could send different decisions to different generals. This is a trustless environment. How to reach a consensus in such an environment is a challenge. It is also a challenge for blockchain as the blockchain network is distributed. In blockchain, there is no central node that ensures ledgers on distributed nodes are all the same. Nodes need not trust other nodes. Thus, some protocols are needed to ensure …
Blockchain Applications in Health Care and Public Health: Increased Transparency
Pedro Elkind Velmovitsky1, BSc, MSc; Frederico Moreira Bublitz1,2, BSc, MSc, PhD; Laura Xavier Fadrique1, MSc,
PMP; Plinio Pelegrini Morita1,3,4,5,6, PEng, MSc, PhD 1School of Public Health and Health Systems, University of Waterloo, Waterloo, ON, Canada 2Center for Strategic Technologies in Health (NUTES), State University of Paraiba (UEPB), Campina Grande, Brazil 3Institute of Health Policy, Management, and Evaluation, University of Toronto, Toronto, ON, Canada 4Research Institute for Aging, University of Waterloo, Waterloo, ON, Canada 5Department of Systems Design Engineering, University of Waterloo, Waterloo, ON, Canada 6eHealth Innovation, Techna Institute, University Health Network, Toronto, ON, Canada
Corresponding Author: Plinio Pelegrini Morita, PEng, MSc, PhD School of Public Health and Health Systems University of Waterloo 200 University Ave W Waterloo, ON, N2L 3G1 Canada Phone: 1 15198884567 ext 41372 Email: [email protected]
Background: Although big data and smart technologies allow for the development of precision medicine and predictive models in health care, there are still several challenges that need to be addressed before the full potential of these data can be realized (eg, data sharing and interoperability issues, lack of massive genomic data sets, data ownership, and security and privacy of health data). Health companies are exploring the use of blockchain, a tamperproof and distributed digital ledger, to address some of these challenges.
Objective: In this viewpoint, we aim to obtain an overview of blockchain solutions t
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