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ifetimeof the system [7]. The possibility to customize and style along with tech-nological enhancements towards small-scale electronics andmodern applications make handheld and wearable devices astrong contender in the Internet of Things (IoT) technologicalrace [8]. Almost one billion wearable devices are expected tojoin the IoT family by 2021 [9]. Said fascinating developmentis a driving force behind the convergence of the physical anddigital worlds that promises to create an unprecedented IoTmarket of 1
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Blockchain Technology for Smartphones and Constrained IoT Devices A Future Perspective and Implementation
700
9 trillion USD over the next decade, and it isexpected that a significant percentage of those devices willbe smartphones. There are currently about 2.71 billion smartphones in theworld1 and the average one can process 2 billion floatingpoint operations per second (FLOPS) 2, thus, leaving us with 1See "Number of smartphone users worldwide from 2014 to 2020(in billions) "by Statista, 2019: https: / / www.statista.com / statistics / 274774 /forecast-of-mobile-phone-users-worldwide / 2See "Processing power co
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Blockchain Technology for Smartphones and Constrained IoT Devices A Future Perspective and Implementation
701
mpared" by Experts Exchange, 2019: https: / /pages.experts-exchange.com / processing-power-compared 20 2019 IEEE 21st Conference on Business Informatics (CBI) 978-1-7281-0650-2 / 19 / $31.00 © 2019 IEEEDOI 10.1109 / CBI.2019.10092 constantly underused 5 EFLOPS. This power can be usedin the transaction publication and validation processes, smartcontracts, or distributed storage [10]. Based on the above,almost any modern smartphone already has the power to bea part of Proof of Activity (PoA) [11] and exec
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Blockchain Technology for Smartphones and Constrained IoT Devices A Future Perspective and Implementation
702
mpared" by Experts Exchange, 2019: https: / /pages.experts-exchange.com / processing-power-compared 20 2019 IEEE 21st Conference on Business Informatics (CBI) 978-1-7281-0650-2 / 19 / $31.00 © 2019 IEEEDOI 10.1109 / CBI.2019.10092 constantly underused 5 EFLOPS. This power can be usedin the transaction publication and validation processes, smartcontracts, or distributed storage [10]. Based on the above,almost any modern smartphone already has the power to bea part of Proof of Activity (PoA) [11] and exec
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Blockchain Technology for Smartphones and Constrained IoT Devices A Future Perspective and Implementation
703
ute relatedcryptographic primitives [12]. Fig. 1. Smartphones as part of blockchain ecosystem. However, deploying blockchain applications to mobile de-vices acting as actual miners faces many critical challenges.It is mainly due to the mining process habits, i.e., solving thePoW that requires not only computing power but also energyfrom the interacting mobile devices. To address this challenge,the edge computing paradigm was introduced by the researchcommunity for cases of mobile blockchain networks [13].
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Blockchain Technology for Smartphones and Constrained IoT Devices A Future Perspective and Implementation
704
However, it requires the use of more computational- andpower-independent nodes to take block generation actionsinstead of actual smartphones. There are, however, a num-ber of miner implementations of blockchain applications forsmartphones but it has been shown that the income of a singlemobile device acting as a miner in the blockchain networkis nonprofitable3. Worth noting, the use of constrained devices is generallyunderestimated concerning the blockchain. The mining featureis ultimately not the most ef
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Blockchain Technology for Smartphones and Constrained IoT Devices A Future Perspective and Implementation
705
ficient utilization of this class dueto the computational and power limitations, but the conceptknown as Proof-of-Stake (PoS) provided the first opportunityfor such constrained devices utilization [14]. Here, PoS nodesdo not act as miners to solve complex tasks. With PoS,stakeholders are used to confirming transactions and blocksbased on their "stake" in the system and the use of theresource-constrained device, for this reason, is a natural stepforward. The role of smartphones is to pay only the transacti
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Blockchain Technology for Smartphones and Constrained IoT Devices A Future Perspective and Implementation
706
onfees of the network without involvement in actual mining. Theprobability of being selected to take part in the next blockgeneration to the chain directly depends on the number ofcoins or tokens held by the relevant node. The most interesting from smartphone perspective concept,proposed in [11], is indeed PoA. The authors envision a newprotocol for a cryptocurrency constructed upon Bitcoin bycombining the PoW component with a PoS type of system.PoA recommended itself as more secure against known prac-tic
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Blockchain Technology for Smartphones and Constrained IoT Devices A Future Perspective and Implementation
707
al attacks with relatively low utilization of both commu-nications and storage resources. In PoA, mining is usually 3See "Is Mobile Mining Profitable?", by COINCENTRAL, 2018: https:/ / coincentral.com / is-mobile-mining-profitable / executed in a traditional PoW manner. However, the minedblock does not contain transactions, i.e., the block is composedof the header and the rewards address. After the miningprocess, the system operation changes to PoS mode. Severalstakeholders (smartphones, in our case) are
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Blockchain Technology for Smartphones and Constrained IoT Devices A Future Perspective and Implementation
708
randomly selectedto sign (verify) the newly generated block. After everyone inthe group signs the block, it is added to the blockchain. Ifsome of the 'validators' have not participated in the validationprocess, the block is discarded, and the next PoW-based oneis used, and the procedure is repeated. The reward is then splitbetween active PoS validators and PoW miners. The main contribution of this work is a protocol suitetitled 'Trinity'. It allows utilizing an intelligent combinationof PoA, PoS, and PoW
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Blockchain Technology for Smartphones and Constrained IoT Devices A Future Perspective and Implementation
709
aiming at the involvement of mobiledevices for blockchain operation. The underlying conceptsused in Trinity are as follows. The first one is ID-basedcryptography initially proposed in [15] during the times whenblockchain itself was brought to the research community'sattention. After 20 years, the first realization of this strategytook place in work [16] by C. Cocks et al. They proposed anovel approach on obtaining the public key of the recipientfor the signed message transmission employing Public KeyGener
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Blockchain Technology for Smartphones and Constrained IoT Devices A Future Perspective and Implementation
710
ator (PKG) and unique IDs of the participants. However,there is a number of challenges related to PKG utilization:(i) PKG can sign and decrypt all the messages; (ii) keyrevoking is not implemented; (iii) safe channel is requiredfor the key dissemination; and (iv) encryption and decryptionmechanisms are computationally different. Most of those couldbe mitigated by utilizing Shamir Secret Sharing [17] allowingfor the secret key dissemination and reconstruction based ononly a portion of previously distribute
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Blockchain Technology for Smartphones and Constrained IoT Devices A Future Perspective and Implementation
711
ator (PKG) and unique IDs of the participants. However,there is a number of challenges related to PKG utilization:(i) PKG can sign and decrypt all the messages; (ii) keyrevoking is not implemented; (iii) safe channel is requiredfor the key dissemination; and (iv) encryption and decryptionmechanisms are computationally different. Most of those couldbe mitigated by utilizing Shamir Secret Sharing [17] allowingfor the secret key dissemination and reconstruction based ononly a portion of previously distribute
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Blockchain Technology for Smartphones and Constrained IoT Devices A Future Perspective and Implementation
712
d shares. The rest of the paper is organized as follows. Section IIprovides the main primitives and constructs used to constructthe Trinity. Next, Section III provides a detailed descriptionof the developed cryptographic protocols. Section V providesa quantitative performance evaluation of the developed sys-tem. The last section concludes the paper and lists futureresearch directions. II. MAIN SYSTEM COMPONENTS This section provides a brief overview of the main systemcomponents used during Trinity develop
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Blockchain Technology for Smartphones and Constrained IoT Devices A Future Perspective and Implementation
713
ment. To start with, theblock creation process begins when there are enough pendingtransactions to start assembling a block. Analyzing specifiedparameters of each transaction, miners determine its value forthe system and add it into a corresponding block. Minimum-size blocks can be created to reach the minimum delay in speedper operation while possible, and as the load on the networkincreases, the block size grows. In circumstances where auser needs a block size larger than 4 MB, the system alsosupports c
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Blockchain Technology for Smartphones and Constrained IoT Devices A Future Perspective and Implementation
714
ombining any number of blocks (microblocks) intoa macroblock, thereby allowing the storage of large volumesof data on the blockchain. Bitcoin-NG protocol is selected to handle ma
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Blockchain Technology for Smartphones and Constrained IoT Devices A Future Perspective and Implementation
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A Survey of Consensus Algorithms for Blockchain Technology 978-1-5386-8125-1/19/$31.00 ©2019 IEEE A Survey of Consensus Algorithms for Blockchain Technology Shikah J. Alsunaidi Department of Computer Science. College of Computer Sciences & Information Technology Imam Abdulrahman Bin Faisal Univarsity P.O.Box 1982, Dammam 31441, Saudi Arabia [email protected] Fahd A. Alhaidari Department of Computer Science. College of Computer Sciences & Information Technology Imam
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A Survey of Consensus Algorithms for Blockchain Techno
716
Abdulrahman Bin Faisal Univarsity P.O.Box 1982, Dammam 31441, Saudi Arabia [email protected] Abstract—Blockchain technology is a new shift in the Internet world, it has brought about changes in many sectors. Where transactions can be securely accomplished via point-to- point connections in a distributed system without the need for a third-party, with the help of consensus algorithms. In this paper, we will conduct a comprehensive survey on the Blockchain technology with focusing in
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A Survey of Consensus Algorithms for Blockchain Techno
717
the popular consensus algorithms, in order to figure out their features, and the factors that affect the performance and security. Also, we will provide a classification of the consensus algorithms and we will perform a comprehensive comparison of studied consensus algorithms. In addition, we will provide a detailed discussion of the consensus algorithms that studied with an analysis of the main factors affecting these algorithms. Finally, we will refer to some recommendations that must be
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A Survey of Consensus Algorithms for Blockchain Techno
718
the popular consensus algorithms, in order to figure out their features, and the factors that affect the performance and security. Also, we will provide a classification of the consensus algorithms and we will perform a comprehensive comparison of studied consensus algorithms. In addition, we will provide a detailed discussion of the consensus algorithms that studied with an analysis of the main factors affecting these algorithms. Finally, we will refer to some recommendations that must be
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A Survey of Consensus Algorithms for Blockchain Techno
719
considered and which can contribute to the development of this area. Keywords—Blockchain, consensus algorithm, Proof-based, Vote-based I. INTRODUCTION In 1991, Haber and Stornetta introduced a structure similar to the Blockchain [1]. However, the technique of Blockchain became quite popular in the recent era, and researchers are studying the possibility of applying it in many areas including IoT security [2] and smart city [3], digital rights management [4] and smart contracts [5]. Great
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A Survey of Consensus Algorithms for Blockchain Techno
720
considered and which can contribute to the development of this area. Keywords—Blockchain, consensus algorithm, Proof-based, Vote-based I. INTRODUCTION In 1991, Haber and Stornetta introduced a structure similar to the Blockchain [1]. However, the technique of Blockchain became quite popular in the recent era, and researchers are studying the possibility of applying it in many areas including IoT security [2] and smart city [3], digital rights management [4] and smart contracts [5]. Great
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A Survey of Consensus Algorithms for Blockchain Techno
721
efforts were made in the seek of utilizing Blockchain technology for preserving data and sharing it in the medical field [6], [7]. A Blockchain technology has received considerable attention from companies and governments in many countries, for instance, Riyadh Municipality has selected IBM as a strategic partner for integrating a Blockchain technology with municipality services [8]. It is worth mentioning that the beginnings of the Blockchain were with the emergence of the Bitcoin by Nakamoto
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A Survey of Consensus Algorithms for Blockchain Techno
722
efforts were made in the seek of utilizing Blockchain technology for preserving data and sharing it in the medical field [6], [7]. A Blockchain technology has received considerable attention from companies and governments in many countries, for instance, Riyadh Municipality has selected IBM as a strategic partner for integrating a Blockchain technology with municipality services [8]. It is worth mentioning that the beginnings of the Blockchain were with the emergence of the Bitcoin by Nakamoto
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A Survey of Consensus Algorithms for Blockchain Techno
723
[9], which was credited with its fame. Bitcoin eliminates the need for a trusted third-party, while is necessary for conventional payment methods to verifying the transactions. Though, unfortunately, this third-party can be a weak point in the network that can be exploited to deceive network clients since it is the center for all network transaction. Therefore, the solution for this issue is to rely on multiple nodes or parties for verifying the transaction rather than one center [10]. The key i
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A Survey of Consensus Algorithms for Blockchain Techno
724
[9], which was credited with its fame. Bitcoin eliminates the need for a trusted third-party, while is necessary for conventional payment methods to verifying the transactions. Though, unfortunately, this third-party can be a weak point in the network that can be exploited to deceive network clients since it is the center for all network transaction. Therefore, the solution for this issue is to rely on multiple nodes or parties for verifying the transaction rather than one center [10]. The key i
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A Survey of Consensus Algorithms for Blockchain Techno
725
ngredient in the Bitcoin is a ledger which retains each verified transaction. In respect of making the system trustworthy, the ledger is kept on several network nodes. Satoshi has proposed an enhanced design for this ledger that is based on blocks. The block contains verified transactions, where the above-mentioned ledger has been optimized to contain many blocks, hence the name of "Blockchain". The first block in the chain called "genesis block", and it holds the initial transactions of the Bitc
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A Survey of Consensus Algorithms for Blockchain Techno
726
ngredient in the Bitcoin is a ledger which retains each verified transaction. In respect of making the system trustworthy, the ledger is kept on several network nodes. Satoshi has proposed an enhanced design for this ledger that is based on blocks. The block contains verified transactions, where the above-mentioned ledger has been optimized to contain many blocks, hence the name of "Blockchain". The first block in the chain called "genesis block", and it holds the initial transactions of the Bitc
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A Survey of Consensus Algorithms for Blockchain Techno
727
oin [11]. Furthermore, when a new transaction appears, its validity will be checked by some nodes. Afterwards, if the transaction has been marked as a valid transaction, it will be inserted into the block. Then, the block must associate with the chain. All nodes must be aware of any updates occurring in the network, any node that has a new block must inform all other nodes to update their local chains [10]. However, using this method for verifying the transactions causes confusion if each node
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A Survey of Consensus Algorithms for Blockchain Techno
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oin [11]. Furthermore, when a new transaction appears, its validity will be checked by some nodes. Afterwards, if the transaction has been marked as a valid transaction, it will be inserted into the block. Then, the block must associate with the chain. All nodes must be aware of any updates occurring in the network, any node that has a new block must inform all other nodes to update their local chains [10]. However, using this method for verifying the transactions causes confusion if each node
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A Survey of Consensus Algorithms for Blockchain Techno
729
tries to broadcast a new block at the same time. In order to avoid this case, the "Consensus algorithms" must be performed between nodes to manage chains update processes. There are many proposed Consensus algorithms which vary in several aspects such as the node identity management method, the energy saving rate, and the tolerated power against the adversary. Such variety makes each algorithm to be applicable for specific scenarios [12]. Thus, this paper focuses on studying and evaluating the
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A Survey of Consensus Algorithms for Blockchain Techno
730
tries to broadcast a new block at the same time. In order to avoid this case, the "Consensus algorithms" must be performed between nodes to manage chains update processes. There are many proposed Consensus algorithms which vary in several aspects such as the node identity management method, the energy saving rate, and the tolerated power against the adversary. Such variety makes each algorithm to be applicable for specific scenarios [12]. Thus, this paper focuses on studying and evaluating the
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A Survey of Consensus Algorithms for Blockchain Techno
731
available consensus algorithms by providing a comprehensive discussion and analysis for the common consensus algorithms as well as providing recommendations for future work. The rest of the paper is organized as follows. In Section 2, we present a background on the blockchain technology including the definition, types, and how does it work. Section 3 presents the two main problems of blockchain that required a perfect design of consensus algorithms. In Section 4, the consensus algorithms appli
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A Survey of Consensus Algorithms for Blockchain Techno
732
ed with Blockchain technology are reviewed and compared. Afterwards, the analysis and discussions of the reviewed consensus algorithms are provided in Section 5. The conclusion of the paper is given in Section 6. Finally, Section 7 presents the findings of the study along with notions for future work. II. BACKGROUND A. What is a Blockchain? The Blockchain is a peer-to-peer network transaction system that provides secure execution of operations without the need for a trusted third party. In th
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[ "factors", "in", "detail", "1", ".", "To", "##ler", "##ated", "power", "of", "the", "ad", "##vers", "##ary", ".", "All", "proposed", "algorithms", "have", "identified", "a", "certain", "percentage", "of", "the", "network", "power", "that", "may", "be", "owned", "by", "an", "ad", "##vers", "##ary", "to", "attack", "network", "security", ".", "The", "highest", "rate", "was", "51", "%", ",", "while", "the", "worst", "case", "found", "with", "Po", "##W", "as", "the", "tolerate", "##d", "power", "of", "the", "ad", "##vers", "##ary", "was", "less", "than", "25", "%", "followed", "by", "the", "P", "##BF", "##T", "the", "percentage", "was", "less", "than", "33", ".", "3", "%", ".", "However", ",", "these", "ratios", "cannot", "be", "trusted", "in", "all", "cases", ".", "Therefore", ",", "the", "consensus", "algorithms", "must", "be", "improved", "to", "increase", "the", "value", "of", "the", "tolerate", "##d", "power", "of", "the", "ad", "##vers", "##ary", ".", "2", ".", "Expo", "##sure", "likelihood", ".", "The", "decent", "##ral", "##ized", "architecture", "of", "Block", "##cha", "##in", "networks", "increases", "the", "challenge", "of", "achieving", "a", "good", "level", "of", "security", ".", "The", "network", "can", "be", "attacked", "in", "several", "ways", ",", "for", "example", ",", "transactions", "are", "confirmed", "based", "on", "what", "is", "agreed", "upon", "by", "most", "nodes", "in", "the", "network", ".", "Therefore", ",", "if", "the", "attacker", "able", "to", "manage", "an", "adequate", "portion", "of", "these", "nodes", ",", "then", "it", "will", "have", "a", "significant", "impact", "on", "the", "mining", "decision", "(", "performing", "of", "the", "transaction", ")", ".", "However", ",", "one", "of", "the", "main", "goals", "of", "the", "Block", "##cha", "##in", "network", "is", "to", "eliminate", "the", "central", "##ization", "and", "therefore", "each", "algorithm", "seeks", "to", "prevent", "attack", "by", "increasing", "its", "costs", ".", "For", "example", ",", "in", "Po", "##W", ",", "an", "attacker", "needs", "to", "have", "more", "than", "25", "%", "of", "the", "power", "of", "the", "network", ",", "since", "it", "will", "be", "expensive", "for", "him", "to", "provide", "high", "-", "computing", "devices", "and", "enough", "power", "to", "operate", "them", ",", "noting", "that", "these", "requirements", "are", "increasing", "over", "time", ".", "While", "in", "Po", "##S", "and", "Po", "##A", "the", "attacker", "needs", "to", "own", "more", "than", "51", "%", "of", "the", "total", "of", "the", "network", "stake", "to", "perform", "a", "successful", "attack", ".", "Ultimately", ",", "most", "of", "the", "proposed", "algorithms", "succeeded", "in", "eliminating", "the", "two", "general", "security", "problems", "which", "are", ":", "51", "%", "attack", "and", "double", "spending", ".", "G", ".", "Applications", "Most", "of", "the", "proposed", "algorithms", "designed", "to", "deal", "with", "cry", "##pt", "##o", "-", "currency", "systems", ",", "and", "there", "are", "also", "many", "efforts", "that", "have", "been", "provided", "consensus", "algorithms", "to", "manage", "smart", "contract", "transactions", ".", "However", ",", "the", "quality", "and", "performance", "of", "these", "algorithms", "should", "be", "studied", "if", "they", "implemented", "on", "real", "-", "time", "applications", "and", "scientific", "applications", "that", "require", "high", "computing", "capabilities", ".", "VI", ".", "CO", "##NC", "##L", "##US", "##ION", "Block", "##cha", "##in", "technology", "will", "be", "a", "promising", "technology", "in", "the", "future", "for", "many", "sectors", ",", "not", "just", "for", "digital", "currency", "exchange", ".", "As", "it", "enhance", "##s", "the", "way", "of", "distributing", "the", "data", "and", "assets", ",", "also", ",", "it", "has", "eliminated", "the", "single", "failure", "point", "found", "in", "the", "central", "environment", "and", "reduced", "network", "management", "costs", ".", "It", "relies", "on", "consensus", "algorithms", "to", "manage", "performing", "all", "transactions", "in", "the", "network", ",", "as", "there", "are", "several", "proposed", "algorithms", ".", "Therefore", ",", "in", "this", "paper", ",", "a", "classification", "of", "these", "algorithms", "was", "presented" ]
A Survey of Consensus Algorithms for Blockchain Techno
733
ed with Blockchain technology are reviewed and compared. Afterwards, the analysis and discussions of the reviewed consensus algorithms are provided in Section 5. The conclusion of the paper is given in Section 6. Finally, Section 7 presents the findings of the study along with notions for future work. II. BACKGROUND A. What is a Blockchain? The Blockchain is a peer-to-peer network transaction system that provides secure execution of operations without the need for a trusted third party. In th
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A Survey of Consensus Algorithms for Blockchain Techno
734
is technique, all transactions are performed on a decentralized ledger database shared between many nodes that build the Blockchain network. In Fig. 1, we present a simple model to represent the Blockchain network. The ledger database consists of blocks where the block structure is shown in Fig. 2. The database entries are permanent not changeable, only new entries can append to the database [11]. B. Types of Blockchain As mentioned in [13], Blockchain techniques can be divided into three t
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A Survey of Consensus Algorithms for Blockchain Techno
735
is technique, all transactions are performed on a decentralized ledger database shared between many nodes that build the Blockchain network. In Fig. 1, we present a simple model to represent the Blockchain network. The ledger database consists of blocks where the block structure is shown in Fig. 2. The database entries are permanent not changeable, only new entries can append to the database [11]. B. Types of Blockchain As mentioned in [13], Blockchain techniques can be divided into three t
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A Survey of Consensus Algorithms for Blockchain Techno
736
ypes which we present in Table I. C. How Blockchain Works? The initial transaction is stored in the first block of the chain called the genesis block. The other coming transactions must be verified before they are accepted, where this is done by two things: 1. Verification that is performed by some other nodes in the same chain. 2. Digital signature of the sender. The valid transaction will be appended to the block and distributed to other nodes to complete the replication process. Perfo
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A Survey of Consensus Algorithms for Blockchain Techno
737
ypes which we present in Table I. C. How Blockchain Works? The initial transaction is stored in the first block of the chain called the genesis block. The other coming transactions must be verified before they are accepted, where this is done by two things: 1. Verification that is performed by some other nodes in the same chain. 2. Digital signature of the sender. The valid transaction will be appended to the block and distributed to other nodes to complete the replication process. Perfo
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A Survey of Consensus Algorithms for Blockchain Techno
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rming transactions in this manner may cause some confusion if each node tries to broadcast its blocks updates. In order to avoid this state, an agreement must take place among nodes to determine which block will append and via which node, this achieved by applying a consensus algorithm [18]. Fig. 1. Blockchain Structure Fig. 2. Block Structure TABLE I. TYPES OF BLOCKCHAIN Type Public Private Consortium or (Federated) Configured for Everyone Specific organization Institutions
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A Survey of Consensus Algorithms for Blockchain Techno
739
that have a partnership where data can be public or private Managed by Anyone Organization itself Selected authority node Centralized No Yes Partial Efficiency Low High High Example Bitcoin [14], Ethereum [15] Blockchain for accounting departments within a company Hyperledger [16], R3CEV [17] D. Blockchain Challenges Protection and security are the biggest concerns that were introduced by the Internet and the abundance of information. Owe to that, information and transaction
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A Survey of Consensus Algorithms for Blockchain Techno
740
that have a partnership where data can be public or private Managed by Anyone Organization itself Selected authority node Centralized No Yes Partial Efficiency Low High High Example Bitcoin [14], Ethereum [15] Blockchain for accounting departments within a company Hyperledger [16], R3CEV [17] D. Blockchain Challenges Protection and security are the biggest concerns that were introduced by the Internet and the abundance of information. Owe to that, information and transaction
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A Survey of Consensus Algorithms for Blockchain Techno
741
s on the Internet must be protected from hackers and saboteurs. The Blockchain is one of the security techniques that can be applied to achieve the required level of protection. There are many issues and challenges faced by Blockchain technology [19] we summarize them in Fig. 3. To meet these challenges, we must work to improve the consensus algorithms that are
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A Survey of Consensus Algorithms for Blockchain Techno
742
s on the Internet must be protected from hackers and saboteurs. The Blockchain is one of the security techniques that can be applied to achieve the required level of protection. There are many issues and challenges faced by Blockchain technology [19] we summarize them in Fig. 3. To meet these challenges, we must work to improve the consensus algorithms that are
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A Survey of Consensus Algorithms for Blockchain Techno
743
Energy-recycling Blockchain with Proof-of-Deep-Learning Energy-recycling Blockchain with Proof-of-Deep-Learning Changhao Chenli∗, Boyang Li∗, Yiyu Shi, Taeho Jung Department of Computer Science and Engineering University of Notre Dame Notre Dame, Indiana, USA {cchenli,Boyang.Li.258,yshi4,tjung}@nd.edu Abstract—An enormous amount of energy is wasted in Proof- of-Work (PoW) mechanisms adopted by popular blockchain applications (e.g., PoW-based cryptocurrencies) because miners must conduct a large amou
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Energy recycling Blockchain with Proof of Deep Learning
744
Energy-recycling Blockchain with Proof-of-Deep-Learning Energy-recycling Blockchain with Proof-of-Deep-Learning Changhao Chenli∗, Boyang Li∗, Yiyu Shi, Taeho Jung Department of Computer Science and Engineering University of Notre Dame Notre Dame, Indiana, USA {cchenli,Boyang.Li.258,yshi4,tjung}@nd.edu Abstract—An enormous amount of energy is wasted in Proof- of-Work (PoW) mechanisms adopted by popular blockchain applications (e.g., PoW-based cryptocurrencies) because miners must conduct a large amou
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Energy recycling Blockchain with Proof of Deep Learning
745
nt of computation. Owing to this, one serious rising concern is that the energy waste not only dilutes the value of the blockchain but also hinders its further application. In this paper, we propose a novel blockchain design that fully recycles the energy required for facilitating and maintaining it, which is re-invested to the computation of deep learning. We realize this by proposing Proof-of-Deep-Learning (PoDL) such that a valid proof for a new block can be generated if and only if a proper deep learn
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Energy recycling Blockchain with Proof of Deep Learning
746
ing model is produced. We present a proof-of-concept design of PoDL that is compatible with the majority of the cryptocurrencies that are based on hash- based PoW mechanisms. Our benchmark and simulation results show that the proposed design is feasible for various popular cryptocurrencies such as Bitcoin, Bitcoin Cash, and Litecoin. Index Terms—Blockchain, Sustainability, Deep Learning I. INTRODUCTION In the past decade, blockchain technology has been suc- cessfully applied in different fields and pot
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Energy recycling Blockchain with Proof of Deep Learning
747
ing model is produced. We present a proof-of-concept design of PoDL that is compatible with the majority of the cryptocurrencies that are based on hash- based PoW mechanisms. Our benchmark and simulation results show that the proposed design is feasible for various popular cryptocurrencies such as Bitcoin, Bitcoin Cash, and Litecoin. Index Terms—Blockchain, Sustainability, Deep Learning I. INTRODUCTION In the past decade, blockchain technology has been suc- cessfully applied in different fields and pot
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Energy recycling Blockchain with Proof of Deep Learning
748
entially will be applied in more critical areas. However, the current mecha- nism consumes a huge amount of energy for conducting the computation needed for maintaining the security guarantee of the system, and various concerns are rising accordingly. "The cryptocurrency uses as much CO2 a year as 1 million transat- lantic flights. We need to take it seriously as a climate threat" according to the Guardian [1]. Because of such concerns, "Bitcoin's need for electricity is its Achilles Heel" according to th
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Energy recycling Blockchain with Proof of Deep Learning
749
e Forbes [2]. According to [3], the energy consumption of Bitcoin has been steadily increasing until late November 2018 when the Bitcoin price dropped suddenly. Even after the drop, more than 50 TWh (trillion watt-hour) per year is consumed just for maintaining the blockchain underlying Bitcoin. This amount is from Bitcoin only, and the total energy consumption by all applications based on blockchain will be much more than that. The main issue is that all energy is wasted to some ex- tent. The majority o
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Energy recycling Blockchain with Proof of Deep Learning
750
f the energy is being consumed by the hash calculation in PoW-based blockchains. Proof-of-Capacity (PoC) is proposed to address this issue (e.g., Burstcoin [4]), * Authors with equal contributions listed alphabetically with last names. however it does not completely solve the problem because storage resources are wasted instead. Primecoin [5] uses prime number finding instead of hash calculation as PoW, and miners seek special sequences of prime numbers (Cunningham chains). However, the appli- cation o
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Energy recycling Blockchain with Proof of Deep Learning
751
f those numbers is limited in cryptographic pro- tocols. Gridcoin [6], Golem [7], and FoldingCoin [8] are cryptocurrencies that distribute rewards to miners based on the amount of scientific computation they performed. Though being similar, the Proof-of-Deep-Learning (PoDL) proposed in this paper differ from them substantially. Miners' computing ability does not make blockchain secure in those systems, however PoDL is an improved PoW-like consensus mechanism with which deep learning power of honest miners
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Energy recycling Blockchain with Proof of Deep Learning
752
provide tamper-proofness. Owing to this, PoDL can be deployed in any PoW-based blockchain applications, recycling their miners' energy for deep learning. Proof-of-Stake (e.g., Nxt [9]) or Proof-of-Important (e.g., NEM [10]) are alternative consensus mechanisms with less energy consumption. However, their principle is orthogonal to that of PoW, and the 'block mining' does not involve computation. Therefore, these are orthogonal to our work. We present a novel design of blockchain which reinvests the ener
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Energy recycling Blockchain with Proof of Deep Learning
753
gy consumed by blockchain maintenance in computation tasks of deep learning. This is done by introducing Proof- of-Deep-Learning (PoDL) mechanism which forces miners to perform deep learning training and present trained models as proofs. The contributions of this paper are summarized as follows. (1) We present the first consensus mechanism, PoDL, that maintains blockchain via deep learning instead of useless hash calculation; (2) Our PoDL can be applied to any cryptocur- rency based on PoW mechanisms bec
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Energy recycling Blockchain with Proof of Deep Learning
754
ause we only incremen- tally add components to block headers; (3) Our experiment shows that the design is feasible for cryptocurrencies whose block intervals are much greater than 10 seconds. II. PRELIMINARIES A. Proof of work (PoW) and Block Mining Proof of work (PoW) [11] is used commonly in many cryp- tocurrencies (Bitcoin, ZCash, Monero, Litecoin etc.), where miners need to find a hash value smaller than some threshold, and this involves brute-force search over a large search space.978-1-7281-1328-9
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Energy recycling Blockchain with Proof of Deep Learning
755
/19/$31.00 c© 2019 IEEE 19 Therefore, plenty of computation resource is needed in the PoW mechanism of those cryptocurrencies. Block mining is the process of creating a block with a valid hash value (i.e., less than a small threshold). Miners who create blocks with valid hash values are rewarded for their work. More specifically, they are allowed to insert one Coinbase transaction which creates and sends certain amount of block reward to any address specified by the miner. B. Deep Learning (DL) and i
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Energy recycling Blockchain with Proof of Deep Learning
756
ts Training Deep learning [12] (DL) outperforms traditional machine learning algorithms dramatically in many areas. To achieve a proper DL model, one needs to provide a dataset (called training dataset) and train the model. The training is composed of two algorithms, feed-forwarding and back-propagation, that are alternately executed. We say an epoch
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Energy recycling Blockchain with Proof of Deep Learning
757
From blockchain consensus back to Byzantine consensus AUTHORS ABSTRACT Consensus is a fundamental problem of distributed computing. While this problem has been known to be unsolvable since 1985, existing protocols were designed these past three decades to solve consensus under various assumptions. Today, with the recent advent of blockchains, various consensus implemen- tations were proposed to make replicas reach an agreement on the order of transactions updating what is often referred to as a distrib
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From blockchain consensus back to Byzantine consensus
758
From blockchain consensus back to Byzantine consensus AUTHORS ABSTRACT Consensus is a fundamental problem of distributed computing. While this problem has been known to be unsolvable since 1985, existing protocols were designed these past three decades to solve consensus under various assumptions. Today, with the recent advent of blockchains, various consensus implemen- tations were proposed to make replicas reach an agreement on the order of transactions updating what is often referred to as a distrib
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From blockchain consensus back to Byzantine consensus
759
uted ledger. Very little work has however been devoted to explore its theoretical ramifications. As a result existing proposals are sometimes misunderstood and it is often unclear whether the problems arising during their executions are due to implementation bugs or more fundamental design issues. In this paper, we discuss the mainstream blockchain consensus algorithms and how the classic Byzantine consensus can be revisited for the blockchain context. In particular, we discuss proof-of-work consensus a
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From blockchain consensus back to Byzantine consensus
760
uted ledger. Very little work has however been devoted to explore its theoretical ramifications. As a result existing proposals are sometimes misunderstood and it is often unclear whether the problems arising during their executions are due to implementation bugs or more fundamental design issues. In this paper, we discuss the mainstream blockchain consensus algorithms and how the classic Byzantine consensus can be revisited for the blockchain context. In particular, we discuss proof-of-work consensus a
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From blockchain consensus back to Byzantine consensus
761
nd illustrate the differences between the Bitcoin and the Ethereum proof-of-work consensus algorithms. Based on these definitions, we warn about the dangers of using these blockchains without understanding precisely the guarantees their consensus algorithm offers. In particular, we survey attacks against the Bitcoin and the Ethereum consensus algorithms. We finally discuss the advantage of the recent Blockchain Byzantine consensus definition over previous definitions, and the promises offered by emerging
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From blockchain consensus back to Byzantine consensus
762
nd illustrate the differences between the Bitcoin and the Ethereum proof-of-work consensus algorithms. Based on these definitions, we warn about the dangers of using these blockchains without understanding precisely the guarantees their consensus algorithm offers. In particular, we survey attacks against the Bitcoin and the Ethereum consensus algorithms. We finally discuss the advantage of the recent Blockchain Byzantine consensus definition over previous definitions, and the promises offered by emerging
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From blockchain consensus back to Byzantine consensus
763
consistent blockchains. KEYWORDS Algorithms, Proof-of-work, Blockchain consensus, Scalability SECTION: INTRODUCTION The blockchain technology [1] promises to radically transform the way individuals and companies exchange digital assets and track securely ownership of these assets without the control of a central authority. At its heart lies a distributed ledger that is consistent with high probability when particular assumptions are fulfilled. In particular, the distributed set of participants guaran
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From blockchain consensus back to Byzantine consensus
764
consistent blockchains. KEYWORDS Algorithms, Proof-of-work, Blockchain consensus, Scalability SECTION: INTRODUCTION The blockchain technology [1] promises to radically transform the way individuals and companies exchange digital assets and track securely ownership of these assets without the control of a central authority. At its heart lies a distributed ledger that is consistent with high probability when particular assumptions are fulfilled. In particular, the distributed set of participants guaran
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From blockchain consensus back to Byzantine consensus
765
- tee its consistency despite potentially malicious participants that behave arbitrarily, also called Byzantine failures [2]. The novelty of blockchain is a genuine combination of well-known research results taken from distributed computing, cryptography and game theory. Its distributed nature guarantees the persistence of the ledger data. Its public key crypto-system offers the capabilities for a user to sign transactions that transfer assets from her account to other accounts. Its incentive mechanisms
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From blockchain consensus back to Byzantine consensus
766
- tee its consistency despite potentially malicious participants that behave arbitrarily, also called Byzantine failures [2]. The novelty of blockchain is a genuine combination of well-known research results taken from distributed computing, cryptography and game theory. Its distributed nature guarantees the persistence of the ledger data. Its public key crypto-system offers the capabilities for a user to sign transactions that transfer assets from her account to other accounts. Its incentive mechanisms
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From blockchain consensus back to Byzantine consensus
767
guarantee that a subset of participants maintain the validity of the transactions. And finally, a Byzantine tolerant consensus protocol aims at guaranteeing the integrity of the ledgers by defining a total order on newly appended blocks of transactions. Put into the blockchain context, the consensus problem is for the non-faulty or correct processes of a distributed system to agree on one block of transaction at a given index of a chain of block. This consensus problem can be stated along three propertie
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From blockchain consensus back to Byzantine consensus
768
guarantee that a subset of participants maintain the validity of the transactions. And finally, a Byzantine tolerant consensus protocol aims at guaranteeing the integrity of the ledgers by defining a total order on newly appended blocks of transactions. Put into the blockchain context, the consensus problem is for the non-faulty or correct processes of a distributed system to agree on one block of transaction at a given index of a chain of block. This consensus problem can be stated along three propertie
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From blockchain consensus back to Byzantine consensus
769
s: (i) agreement: no two correct processes decided different blocks; (ii) validity: the decided block is a block that was proposed by one process; (iii) termination: all correct processes eventually decide. A protocol solving the consensus problem is necessary to guarantee that blocks are totally ordered, hence preventing concurrently appended blocks from containing conflicting transactions. Today, with the recent advent of blockchains, various consensus implementations were proposed to make replicas rea
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From blockchain consensus back to Byzantine consensus
770
s: (i) agreement: no two correct processes decided different blocks; (ii) validity: the decided block is a block that was proposed by one process; (iii) termination: all correct processes eventually decide. A protocol solving the consensus problem is necessary to guarantee that blocks are totally ordered, hence preventing concurrently appended blocks from containing conflicting transactions. Today, with the recent advent of blockchains, various consensus implementations were proposed to make replicas rea
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From blockchain consensus back to Byzantine consensus
771
ch an agreement on the order of blocks of transactions updating the distributed ledger. However, consensus has been known to be un- solvable since 1985. While existing protocols were designed these past three decades to solve consensus under various assumptions, it remains unclear what are the guarantees offered by blockchain consensus algorithms and what are the necessary conditions for these guarantees to be satisfied. While the source code of most blockchain protocols is publicly available, the theore
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From blockchain consensus back to Byzantine consensus
772
ch an agreement on the order of blocks of transactions updating the distributed ledger. However, consensus has been known to be un- solvable since 1985. While existing protocols were designed these past three decades to solve consensus under various assumptions, it remains unclear what are the guarantees offered by blockchain consensus algorithms and what are the necessary conditions for these guarantees to be satisfied. While the source code of most blockchain protocols is publicly available, the theore
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From blockchain consensus back to Byzantine consensus
773
tical ramifications of the blockchain abstraction are rather informal. As main blockchain systems, like Bitcoin [1] and Ethereum [3], are now used to trade millions of US$ every day,1 it has become crucial to precisely identified its theoretical ramifications to anticipate the situations where large volume of assets could be lost. In this paper, we illustrate the danger of using proof-of-work blockchain without understanding precisely their guarantees by listing vulnerabilities that affect the predominan
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From blockchain consensus back to Byzantine consensus
774
tical ramifications of the blockchain abstraction are rather informal. As main blockchain systems, like Bitcoin [1] and Ethereum [3], are now used to trade millions of US$ every day,1 it has become crucial to precisely identified its theoretical ramifications to anticipate the situations where large volume of assets could be lost. In this paper, we illustrate the danger of using proof-of-work blockchain without understanding precisely their guarantees by listing vulnerabilities that affect the predominan
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From blockchain consensus back to Byzantine consensus
775
t proof-of-work blockchain systems, namely Bitcoin and Ethereum FOOTNOTE: Bitcoin and Ethereum are the current most important blockchain systems in terms of market capitalization. To this end, we describe the consensus algorithms at the heart of these two blockchain systems. We also relate these consensus algorithms to decades of research on the topic of distributed computing. More precisely, we identify situations where proof-of-work blockchain consensus is violated by: (i) presenting a survey of exist
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From blockchain consensus back to Byzantine consensus
776
t proof-of-work blockchain systems, namely Bitcoin and Ethereum FOOTNOTE: Bitcoin and Ethereum are the current most important blockchain systems in terms of market capitalization. To this end, we describe the consensus algorithms at the heart of these two blockchain systems. We also relate these consensus algorithms to decades of research on the topic of distributed computing. More precisely, we identify situations where proof-of-work blockchain consensus is violated by: (i) presenting a survey of exist
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From blockchain consensus back to Byzantine consensus
777
ing at- tacks against the Bitcoin consensus protocol and (ii) showing how Ethereum, which copes with some of these attacks, may suffer from recent attacks, namely the blockchain anomaly [4] and the balance attack [5]. We elaborate on the risks for users to misconfigure proof-of-work blockchain systems when deploying them as a private and consortium blockchains and our own experience with the settings of the R3 Ethereum testbed. The fact that both main proof- of-work blockchains are vulnerable allows us t
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From blockchain consensus back to Byzantine consensus
778
ing at- tacks against the Bitcoin consensus protocol and (ii) showing how Ethereum, which copes with some of these attacks, may suffer from recent attacks, namely the blockchain anomaly [4] and the balance attack [5]. We elaborate on the risks for users to misconfigure proof-of-work blockchain systems when deploying them as a private and consortium blockchains and our own experience with the settings of the R3 Ethereum testbed. The fact that both main proof- of-work blockchains are vulnerable allows us t
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From blockchain consensus back to Byzantine consensus
779
o conclude that more research is necessary to design safe consensus algorithms suited for blockchains. The rest of the paper is organized as follows. Section 2 presents the general blockchain model. Section 3 introduces the classic Byzantine consensus problem and the probabilistic variant of it. Section 4 specifies the differences of the consensus algorithms used in Bitcoin and Ethereum. Section 5 describes the attacks against Bitcoin and two recent attacks against the Ethereum consensus al- gorithm. Sec
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From blockchain consensus back to Byzantine consensus
780
o conclude that more research is necessary to design safe consensus algorithms suited for blockchains. The rest of the paper is organized as follows. Section 2 presents the general blockchain model. Section 3 introduces the classic Byzantine consensus problem and the probabilistic variant of it. Section 4 specifies the differences of the consensus algorithms used in Bitcoin and Ethereum. Section 5 describes the attacks against Bitcoin and two recent attacks against the Ethereum consensus al- gorithm. Sec
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From blockchain consensus back to Byzantine consensus
781
tion 6 redefines the Byzantine consensus in the light of the blockchain context. Section 7 discusses the consortium model and recent reliable consensus proposals. Section 8 concludes. SECTION: THE GENERAL PROOF-OF-WORK BLOCKCHAIN MODEL In this section we model a simple distributed system as a communication graph that implements a blockchain abstraction as a directed acyclic graph. We propose a high-level pseudocode representation of proof-of-work blockchain protocols in this model. SUBSECTION: A simple
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From blockchain consensus back to Byzantine consensus
782
tion 6 redefines the Byzantine consensus in the light of the blockchain context. Section 7 discusses the consortium model and recent reliable consensus proposals. Section 8 concludes. SECTION: THE GENERAL PROOF-OF-WORK BLOCKCHAIN MODEL In this section we model a simple distributed system as a communication graph that implements a blockchain abstraction as a directed acyclic graph. We propose a high-level pseudocode representation of proof-of-work blockchain protocols in this model. SUBSECTION: A simple
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From blockchain consensus back to Byzantine consensus
783
distributed model for blockchains We consider a communication graph G = ⟨V , E ⟩ with processes V connected to each other through fixed communication links E. Processes are part of a blockchain system S. In this paper, we only consider proof-of-work blockchain systems and focus our attention to Bitcoin and Ethereum. Processes can act as clients by issuing transactions to the system and/or servers by mining, the action of trying to combine transactions into a block as de- scribed in Section 2.2. For the
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From blockchain consensus back to Byzantine consensus
784
distributed model for blockchains We consider a communication graph G = ⟨V , E ⟩ with processes V connected to each other through fixed communication links E. Processes are part of a blockchain system S. In this paper, we only consider proof-of-work blockchain systems and focus our attention to Bitcoin and Ethereum. Processes can act as clients by issuing transactions to the system and/or servers by mining, the action of trying to combine transactions into a block as de- scribed in Section 2.2. For the
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From blockchain consensus back to Byzantine consensus
785
sake of simplicity, we consider that each process possesses a single account (or address) and that a transaction issued by process pi is a transfer of digital assets or coins from the account of the source process pi to the account of a destination process pj ̸= pi. Each transaction is uniquely identified and broadcast to all processes in a best-effort manner. We assume that a process re-issuing the same transfer multiple times creates as many distinct transactions. Processes that initiate the consensus
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From blockchain consensus back to Byzantine consensus
786
sake of simplicity, we consider that each process possesses a single account (or address) and that a transaction issued by process pi is a transfer of digital assets or coins from the account of the source process pi to the account of a destination process pj ̸= pi. Each transaction is uniquely identified and broadcast to all processes in a best-effort manner. We assume that a process re-issuing the same transfer multiple times creates as many distinct transactions. Processes that initiate the consensus
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From blockchain consensus back to Byzantine consensus
787
protocol are called miners, they initiate the consensus through a propose function depicted at lines 7–12 of Alg. 1 allowing them to propose new blocks. Processes decide upon a new block at a given index at line 18 depending on a function get-main-branch that is specific to the type of proof-of-work blockchain system in use (cf. Sections 4.3.1 and 4.3.2 for Bitcoin and Ethereum corresponding function, re- spectively). We refer to the computational power of a miner as its mining power and we denote the to
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From blockchain consensus back to Byzantine consensus
788
tal mining power t as the sum of the mining powers of all miners in V . Each miner tries to group a set T of transactions it heard about into a block b ⊇ T as long as transactions of T do not conflict and that the account balances remain non-negative. For the sake of simplicity in the presentation, the graph G is static meaning that no processes can join and leave the system, however, processes may fail as described in Section 2.3. SUBSECTION: MINERS MUST SOLVE A CRYPTOPUZZLE TO CREATE A NEW BLOCK Mine
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From blockchain consensus back to Byzantine consensus
789
tal mining power t as the sum of the mining powers of all miners in V . Each miner tries to group a set T of transactions it heard about into a block b ⊇ T as long as transactions of T do not conflict and that the account balances remain non-negative. For the sake of simplicity in the presentation, the graph G is static meaning that no processes can join and leave the system, however, processes may fail as described in Section 2.3. SUBSECTION: MINERS MUST SOLVE A CRYPTOPUZZLE TO CREATE A NEW BLOCK Mine
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From blockchain consensus back to Byzantine consensus
790
rs provably solve a hashcash cryptopuzzle [6] before creating a new block. Given a global threshold and the block of largest index the miner knows (Alg. 1, line 10), the miner repeatedly selects a nonce and applies a pseudo-random function to this block and the selected nonce until it obtains a result lower than the threshold, this mechanism is hidden within the solve-cryptopuzzle function at line 11 for clarity. Upon success the miner creates a block that contains the successful nounce as a proof-of-work
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From blockchain consensus back to Byzantine consensus
791
rs provably solve a hashcash cryptopuzzle [6] before creating a new block. Given a global threshold and the block of largest index the miner knows (Alg. 1, line 10), the miner repeatedly selects a nonce and applies a pseudo-random function to this block and the selected nonce until it obtains a result lower than the threshold, this mechanism is hidden within the solve-cryptopuzzle function at line 11 for clarity. Upon success the miner creates a block that contains the successful nounce as a proof-of-work
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From blockchain consensus back to Byzantine consensus
792
as well as the hash of the previous block, hence fixing the index of the block, and broadcasts the block (line 12). This broadcast function with some block and pointer parameter at process pi triggers a corresponding deliver function with the same parameter that is invoked at each correct process pj (including pi if it is correct) upon reception of this broadcast message (line 13). As there is no known strategy to solve the cryptopuzzle, the miners simply keep testing whether randomly chosen numbers sol
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From blockchain consensus back to Byzantine consensus
793
as well as the hash of the previous block, hence fixing the index of the block, and broadcasts the block (line 12). This broadcast function with some block and pointer parameter at process pi triggers a corresponding deliver function with the same parameter that is invoked at each correct process pj (including pi if it is correct) upon reception of this broadcast message (line 13). As there is no known strategy to solve the cryptopuzzle, the miners simply keep testing whether randomly chosen numbers sol
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From blockchain consensus back to Byzantine consensus
794
ve the cryptopuzzle with brute force. The difficulty of this cryp- topuzzle, defined by the threshold, limits the rate at which new blocks can be generated by the network. The value of coins in blockchain systems incentivizes participants to act maliciously if they can maximize their gain by for example executing a double-spending attack, spending the same coins in two distinct transactions. Malicious behaviors are gen- erally modeled by an arbitrary or Byzantine failure model that is named after the pro
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From blockchain consensus back to Byzantine consensus
795
ve the cryptopuzzle with brute force. The difficulty of this cryp- topuzzle, defined by the threshold, limits the rate at which new blocks can be generated by the network. The value of coins in blockchain systems incentivizes participants to act maliciously if they can maximize their gain by for example executing a double-spending attack, spending the same coins in two distinct transactions. Malicious behaviors are gen- erally modeled by an arbitrary or Byzantine failure model that is named after the pro
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From blockchain consensus back to Byzantine consensus
796
blem of generals attempting to reach an agreement in the presence of traitors [2]. In the Byzantine failure model, such a problem is usually referred to as the Byzantine consensus problem. Some solutions like PBFT [7] were proposed, however, there are known not to scale to a number of participants as large as mainstream public blockchain systems [8,9]. There are different failure models that separate faulty processes from correct processes. In a crash failure model, the faulty processes may crash at whi
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From blockchain consensus back to Byzantine consensus
797
blem of generals attempting to reach an agreement in the presence of traitors [2]. In the Byzantine failure model, such a problem is usually referred to as the Byzantine consensus problem. Some solutions like PBFT [7] were proposed, however, there are known not to scale to a number of participants as large as mainstream public blockchain systems [8,9]. There are different failure models that separate faulty processes from correct processes. In a crash failure model, the faulty processes may crash at whi
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From blockchain consensus back to Byzantine consensus
798
ch point they stop computing and stop communicating for the rest of the execution. In a Byzantine failure model, the faulty processes may not follow the protocol specification by behaving arbitrarily. When not stated otherwise, we consider a Byzantine failure model and assume the presence of an adversary (or attacker) that can control processes that together own a relatively small fraction ρ < 0.5 of the total mining power of the system. The processes controlled by the adversary are called malicious or B
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From blockchain consensus back to Byzantine consensus
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