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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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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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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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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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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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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 | 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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",",
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"##ble",
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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 | 715 |
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
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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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"selected",
"IBM",
"as",
"a",
"strategic",
"partner",
"for",
"integrating",
"a",
"Block",
"##cha",
"##in",
"technology",
"with",
"municipality",
"services",
"[",
"8",
"]",
".",
"It",
"is",
"worth",
"mentioning",
"that",
"the",
"beginnings",
"of",
"the",
"Block",
"##cha",
"##in",
"were",
"with",
"the",
"emergence",
"of",
"the",
"Bit",
"##co",
"##in",
"by",
"Na",
"##ka",
"##moto",
"[",
"9",
"]",
",",
"which",
"was",
"credited",
"with",
"its",
"fame",
".",
"Bit",
"##co",
"##in",
"eliminate",
"##s",
"the",
"need",
"for",
"a",
"trusted",
"third",
"-",
"party",
",",
"while",
"is",
"necessary",
"for",
"conventional",
"payment",
"methods",
"to",
"verify",
"##ing",
"the",
"transactions",
".",
"Though",
",",
"unfortunately",
",",
"this",
"third",
"-",
"party",
"can",
"be",
"a",
"weak",
"point",
"in",
"the",
"network",
"that",
"can",
"be",
"exploited",
"to",
"de",
"##ceive",
"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",
"verify",
"##ing",
"the",
"transaction",
"rather",
"than",
"one",
"center",
"[",
"10",
"]",
".",
"The",
"key",
"ingredient",
"in",
"the",
"Bit",
"##co",
"##in",
"is",
"a",
"ledge",
"##r",
"which",
"retains",
"each",
"verified",
"transaction",
".",
"In",
"respect",
"of",
"making",
"the",
"system",
"trust",
"##worthy",
",",
"the",
"ledge",
"##r",
"is",
"kept",
"on",
"several",
"network",
"nodes",
".",
"Sato",
"##shi",
"has",
"proposed",
"an",
"enhanced",
"design",
"for",
"this",
"ledge",
"##r",
"that",
"is",
"based",
"on",
"blocks",
".",
"The",
"block",
"contains",
"verified",
"transactions",
",",
"where",
"the",
"above",
"-",
"mentioned",
"ledge",
"##r",
"has",
"been",
"op",
"##ti",
"##mized",
"to",
"contain",
"many",
"blocks",
",",
"hence",
"the",
"name",
"of",
"\"",
"Block",
"##cha",
"##in",
"\"",
".",
"The",
"first",
"block",
"in",
"the",
"chain",
"called",
"\"",
"genes",
"##is",
"block",
"\"",
",",
"and",
"it",
"holds",
"the",
"initial",
"transactions",
"of",
"the",
"Bit",
"##co",
"##in",
"[",
"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",
"verify",
"##ing",
"the",
"transactions",
"causes",
"confusion",
"if",
"each",
"node",
"tries",
"to",
"broadcast",
"a",
"new",
"block",
"at",
"the",
"same",
"time",
".",
"In",
"order",
"to",
"avoid",
"this",
"case",
",",
"the",
"\"",
"Con",
"##sen",
"##sus",
"algorithms",
"\"",
"must",
"be",
"performed",
"between",
"nodes",
"to",
"manage",
"chains",
"update",
"processes",
".",
"There",
"are",
"many",
"proposed",
"Con",
"##sen",
"##sus",
"algorithms",
"which",
"vary",
"in",
"several",
"aspects",
"such",
"as",
"the",
"node",
"identity",
"management",
"method",
",",
"the",
"energy",
"saving",
"rate",
",",
"and",
"the",
"tolerate",
"##d",
"power",
"against",
"the",
"ad",
"##vers",
"##ary",
".",
"Such",
"variety",
"makes",
"each",
"algorithm",
"to",
"be",
"applicable",
"for",
"specific",
"scenarios",
"[",
"12",
"]",
".",
"Thus",
",",
"this",
"paper",
"focuses",
"on",
"studying",
"and",
"evaluating",
"the",
"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",
"block",
"##cha",
"##in",
"technology",
"including",
"the",
"definition",
",",
"types",
","
] | 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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"V",
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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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"##in",
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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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"specific",
"certain",
"amount",
"of",
"time",
"before",
"it",
"is",
"completed",
",",
"for",
"example",
",",
"each",
"Bit",
"##co",
"##in",
"transaction",
"is",
"processed",
"within",
"approximately",
"ten",
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".",
"Thus",
",",
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"period",
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"the",
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"##s",
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"##actions",
"are",
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"in",
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"##in",
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"after",
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"nodes",
"consensus",
"##es",
"on",
"their",
"validity",
".",
"An",
"attacker",
"could",
"benefit",
"from",
"this",
"scenario",
"by",
"man",
"##ip",
"##ulating",
"the",
"nodes",
"of",
"the",
"network",
"to",
"valid",
"##ate",
"a",
"false",
"transaction",
"[",
"21",
"]",
".",
"Therefore",
",",
"achieving",
"a",
"consensus",
"state",
"through",
"non",
"-",
"trust",
"##worthy",
"nodes",
"is",
"a",
"critical",
"problem",
"in",
"Block",
"##cha",
"##in",
"networks",
".",
"IV",
".",
"E",
"##X",
"##IS",
"##TI",
"##NG",
"B",
"##L",
"##OC",
"##K",
"##CH",
"##A",
"##IN",
"CO",
"##NS",
"##EN",
"##SU",
"##S",
"AL",
"##G",
"##OR",
"##IT",
"##H",
"##MS",
"The",
"beginning",
"of",
"the",
"consensus",
"algorithms",
"was",
"when",
"distributed",
"systems",
"emerged",
",",
"where",
"they",
"were",
"regulated",
"transitions",
"within",
"these",
"systems",
".",
"There",
"are",
"a",
"number",
"of",
"the",
"consensus",
"algorithms",
"utilized",
"to",
"Block",
"##cha",
"##in",
"technology",
"[",
"22",
"]",
".",
"There",
"are",
"two",
"types",
"of",
"consensus",
"algorithms",
":",
"proof",
"-",
"based",
"consensus",
"algorithms",
"and",
"voting",
"-",
"based",
"consensus",
"algorithms",
".",
"The",
"following",
"sub",
"-",
"sections",
"discuss",
"these",
"two",
"types",
"and",
"provide",
"examples",
"of",
"each",
".",
"Also",
",",
"in",
"Fi",
"##g",
".",
"5",
",",
"we",
"present",
"a",
"classification",
"of",
"these",
"algorithms",
"along",
"with",
"a",
"c",
"##lar",
"##ification",
"of",
"pure"
] | 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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",",
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":",
"proof",
"-",
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"algorithms",
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"-",
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"-",
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"two",
"types",
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"provide",
"examples",
"of",
"each",
".",
"Also",
",",
"in",
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"##g",
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"5",
",",
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"present",
"a",
"classification",
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"##ification",
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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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",",
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",",
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"##ators",
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"amount",
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"##ation",
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".",
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"process",
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"require",
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"computing",
"power",
"compared",
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"of",
"energy",
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"[",
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"]",
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",",
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"[",
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"long",
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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].
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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].
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"##hr"
] | 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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"##onized",
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".",
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"As",
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"work",
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"##ating",
"F",
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"##s",
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",",
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"18",
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":",
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"1",
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"31",
"]",
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",",
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"'",
"t",
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"a",
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"[",
"11",
"]",
".",
"2",
")",
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"proposed",
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"manage",
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"replica",
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"logs",
"impose",
"##s",
"strong",
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"where",
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"leader",
"election",
"process",
"based",
"on",
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"##ized",
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".",
"This",
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"improve",
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"the",
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"level",
"through",
"constant",
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"for",
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"##s",
".",
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",",
"Ra",
"##ft",
"unable",
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"ma",
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"##ious",
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"up",
"to",
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"%",
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"fault",
"tolerance",
"[",
"32",
"]",
".",
"In",
"Table",
"II",
",",
"we",
"provide",
"a",
"comparison",
"of",
"the",
"consensus",
"algorithms",
"studied",
"in",
"this",
"paper",
".",
"The",
"comparison",
"will",
"cover",
"many",
"aspects",
",",
"such",
"as",
"node",
"identity",
"management",
"method",
",",
"energy",
"saving",
"level",
",",
"mining",
"incentives",
","
] | 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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"32",
"]",
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"In",
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"II",
",",
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",",
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"level",
",",
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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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"It",
"has",
"a",
"significant",
"impact",
"on",
"the",
"amount",
"of",
"power",
"consumption",
",",
"performance",
"and",
"network",
"security",
".",
"Po",
"##W",
"mining",
"involves",
"solving",
"a",
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"increases",
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"but",
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"##s",
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"power",
".",
"Because",
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".",
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"##T",
",",
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".",
"C",
".",
"Energy",
"saving",
"The",
"most",
"energy",
"-",
"consuming",
"algorithm",
"is",
"Po",
"##W",
".",
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"elect",
"##ing",
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"mine",
"##r",
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"this",
"algorithm",
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"on",
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",",
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"energy",
".",
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",",
"the",
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"lower",
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"##S",
",",
"D",
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"##S",
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"##A",
",",
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"##W",
".",
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"not",
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"##I",
",",
"Po",
"##L",
",",
"P",
"##BF",
"##T",
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"Ra",
"##ft",
".",
"D",
".",
"Inc",
"##ent",
"##ive",
"In",
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"Block",
"##cha",
"##in",
"networks",
",",
"mining",
"operations",
"depend",
"on",
"enterprise",
"resources",
".",
"Therefore",
",",
"no",
"need",
"to",
"give",
"a",
"reward",
"to",
"the",
"miners",
".",
"In",
"contrast",
",",
"in",
"public",
"Block",
"##cha",
"##in",
"networks",
",",
"there",
"are",
"incentives",
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"what",
"they",
"spend",
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"computing",
"and",
"electrical",
"power",
",",
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"encourages",
"them",
"to",
"participate",
"in",
"the",
"mining",
"processes",
".",
"E",
".",
"Performance",
"and",
"scope",
"Performance",
"is",
"the",
"biggest",
"di",
"##lemma",
"in",
"Block",
"##cha",
"##in",
"networks",
",",
"as",
"it",
"often",
"decreases",
"when",
"the",
"network",
"size",
"increases",
".",
"Also",
",",
"it",
"could",
"be",
"measured",
"by",
"five",
"factors",
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"are",
":",
"1",
".",
"V",
"##eri",
"##fication",
"speed",
":",
"In",
"##dicate",
"##s",
"the",
"time",
"required",
"in",
"seconds",
"to",
"complete",
"a",
"transaction",
"valid",
"##ation",
"process",
".",
"Main",
"##ly",
",",
"the",
"Block",
"##cha",
"##in",
"network",
"type",
"and",
"the",
"speed",
"of",
"the",
"computing",
"unit",
"will",
"affect",
"this",
"factor",
".",
"2",
".",
"Through",
"##put"
] | A Survey of Consensus Algorithms for Blockchain Techno | 728 |
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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"##SI",
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"A",
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"##W",
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".",
"2",
".",
"Through",
"##put"
] | 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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"5"
] | 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 | [
131,
1109,
1295,
1104,
14409,
14659,
1679,
1248,
119,
11389,
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"5"
] | 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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".",
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",",
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".",
"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",
".",
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"it",
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"##s",
"the",
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"of",
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"and",
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",",
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"algorithms",
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"in",
"the",
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",",
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] | 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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"factors",
"in",
"detail",
"1",
".",
"To",
"##ler",
"##ated",
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"##vers",
"##ary",
".",
"All",
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"##vers",
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".",
"The",
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"51",
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",",
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"##W",
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"##vers",
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"##BF",
"##T",
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".",
"3",
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".",
"However",
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".",
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",",
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".",
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".",
"Expo",
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".",
"The",
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"The",
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",",
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",",
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".",
"While",
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"##S",
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"Po",
"##A",
"the",
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",",
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"51",
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".",
"G",
".",
"Applications",
"Most",
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"deal",
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"cry",
"##pt",
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"-",
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",",
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".",
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",",
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"-",
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"require",
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".",
"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",
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"##s",
"the",
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"the",
"data",
"and",
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",",
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",",
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"the",
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"point",
"found",
"in",
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"central",
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"and",
"reduced",
"network",
"management",
"costs",
".",
"It",
"relies",
"on",
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"algorithms",
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"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 | 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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",",
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",",
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",",
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",",
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",",
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",",
"D",
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"##S",
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"DD",
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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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"been",
"reached",
"on",
"Block",
"##cha",
"##in",
"technology",
"and",
"consensus",
"algorithms",
".",
"We",
"present",
"these",
"facts",
"in",
"this",
"section",
"with",
"our",
"recommendation",
"to",
"consider",
"them",
"in",
"future",
"work",
",",
"as",
"we",
"believe",
"that",
"they",
"will",
"have",
"a",
"good",
"impact",
"on",
"the",
"development",
"of",
"this",
"area",
".",
"(",
"1",
")",
"As",
"the",
"Block",
"##cha",
"##in",
"is",
"a",
"new",
"technology",
",",
"there",
"is",
"a",
"need",
"to",
"provide",
"definitions",
",",
"standards",
",",
"and",
"good",
"descriptions",
"of",
"the",
"new",
"terminology",
"that",
"it",
"contains",
".",
"(",
"2",
")",
"There",
"are",
"no",
"guarantees",
"in",
"the",
"public",
"Block",
"##cha",
"##in",
"network",
"about",
"the",
"identity",
"of",
"the",
"miners",
",",
"which",
"in",
"turn",
"may",
"open",
"a",
"door",
"for",
"several",
"threats",
"and",
"attacks",
"including",
"Do",
"##S",
".",
"As",
"a",
"future",
"work",
"related",
"to",
"this",
"point",
"of",
"findings",
",",
"we",
"do",
"suggest",
"improving",
"one",
"of",
"the",
"best",
"consensus",
"algorithms",
",",
"like",
"Po",
"##W",
",",
"Po",
"##S",
",",
"D",
"##P",
"##o",
"##S",
"and",
"Po",
"##A",
"that",
"work",
"for",
"public",
"Block",
"##cha",
"##in",
"networks",
",",
"by",
"adding",
"a",
"lightweight",
"process",
"of",
"identification",
"that",
"differentiate",
"##s",
"the",
"machine",
"##d",
"node",
"from",
"the",
"normal",
"node",
".",
"Such",
"improvement",
"will",
"reduce",
"the",
"risk",
"of",
"attacks",
"such",
"as",
"Do",
"##S",
"/",
"DD",
"##o",
"##S",
"attacks",
".",
"(",
"3",
")",
"Most",
"efforts",
"are",
"made",
"to",
"provide",
"consensus",
"algorithms",
"to",
"control",
"cry",
"##pt",
"##o",
"-",
"currency",
"transactions",
"and",
"smart",
"contracts",
".",
"Therefore",
",",
"a",
"need",
"to",
"study",
"the",
"app",
"##lica",
"##bility",
"of",
"these",
"algorithms",
"on",
"lightweight",
"devices",
"and",
"to",
"manage",
"real",
"-",
"time",
"transactions",
"that",
"require",
"minimal",
"execution",
"time",
"while",
"providing",
"a",
"high",
"level",
"of",
"security",
"and",
"minimum",
"energy",
"consumption",
".",
"(",
"4",
")",
"The",
"best",
"value",
"for",
"the",
"power",
"of",
"tolerance",
"against",
"the",
"ad",
"##vers",
"##ary",
"achieved",
"by",
"some",
"of",
"the",
"proposed",
"consensus",
"algorithms",
"is",
"50",
"%",
".",
"For",
"some",
"scenarios",
",",
"this",
"value",
"considered",
"low",
",",
"algorithms",
"must",
"be",
"developed",
"to",
"be",
"stronger",
"against",
"any",
"potential",
"attack",
".",
"AC",
"##K",
"##N",
"##OW",
"##LE",
"##D",
"##GM",
"##EN",
"##T",
"All",
"thanks",
"must",
"be",
"sent",
"to",
"the",
"management",
"of",
"Imam",
"Abdul",
"##rah",
"##man",
"Bin",
"F",
"##ais",
"##al",
"University",
"(",
"I",
"##A",
"##U",
")",
"and",
"its",
"College",
"of",
"Computer",
"Science",
"and",
"Information",
"Technology",
"(",
"CC",
"##SI",
"##T",
")",
"to",
"facilitate",
"access",
"##ing",
"the",
"resources",
"used",
"in",
"this",
"study",
".",
"R",
"##EF",
"##ER",
"##EN",
"##CE",
"##S",
"[",
"1",
"]",
"S",
".",
"Ha",
"##ber",
"and",
"W",
"."
] | 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.
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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.
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] | A Survey of Consensus Algorithms for Blockchain Techno | 738 |
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 | [
8284,
118,
1106,
118,
153,
8284,
10952,
10511,
3910,
117,
107,
1369,
119,
164,
10523,
166,
119,
11651,
8009,
2165,
131,
18630,
131,
120,
120,
2113,
2528,
1394,
119,
8916,
120,
2113,
2528,
1394,
119,
185,
1181,
2087,
119,
164,
11737,
1174,
131,
1406,
118,
23181,
118,
1857,
166,
119,
164,
1275,
166,
138,
119,
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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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"##tov"
] | 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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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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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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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-
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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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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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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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/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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training dataset) and train the model. The training is composed
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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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",",
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"Today",
",",
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"##ins",
",",
"various",
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"-",
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"reach",
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"up",
"##dating",
"what",
"is",
"often",
"referred",
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"as",
"a",
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"##r",
".",
"Very",
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"has",
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"devoted",
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"its",
"theoretical",
"ram",
"##ifications",
".",
"As",
"a",
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"existing",
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"are",
"sometimes",
"mi",
"##sun",
"##ders",
"##to",
"##od",
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"is",
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"unclear",
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"during",
"their",
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"are",
"due",
"to",
"implementation",
"bugs",
"or",
"more",
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".",
"In",
"this",
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",",
"we",
"discuss",
"the",
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"block",
"##cha",
"##in",
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"algorithms",
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"##vis",
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"##cha",
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".",
"In",
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",",
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"-",
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"-",
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"illustrate",
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"-",
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"-",
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".",
"Based",
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",",
"we",
"warn",
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"using",
"these",
"block",
"##cha",
"##ins",
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"understanding",
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"guarantees",
"their",
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".",
"In",
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",",
"we",
"survey",
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"against",
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"Bit",
"##co",
"##in",
"and",
"the",
"E",
"##ther",
"##eum",
"consensus",
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".",
"We",
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"##cha",
"##in",
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"consensus",
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",",
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"##E",
"##Y",
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"Al",
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"##ith",
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",",
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"-",
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",",
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",",
"Sc",
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":",
"IN",
"##TR",
"##OD",
"##UC",
"##TI",
"##ON",
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"##cha",
"##in",
"technology",
"[",
"1",
"]",
"promises",
"to",
"radical",
"##ly",
"transform",
"the",
"way",
"individuals",
"and",
"companies",
"exchange",
"digital",
"assets",
"and",
"track",
"secure",
"##ly",
"ownership",
"of",
"these",
"assets",
"without",
"the",
"control",
"of",
"a",
"central",
"authority",
".",
"At",
"its",
"heart",
"lies",
"a",
"distributed",
"ledge",
"##r",
"that",
"is",
"consistent",
"with",
"high",
"probability",
"when",
"particular",
"assumptions",
"are",
"fulfilled",
".",
"In",
"particular",
",",
"the",
"distributed",
"set",
"of",
"participants",
"g",
"##ua",
"##ran",
"-",
"tee",
"its",
"consistency",
"despite",
"potentially",
"ma",
"##lic",
"##ious",
"participants",
"that",
"behave",
"a",
"##rb",
"##it",
"##rar",
"##ily",
",",
"also",
"called",
"Byzantine",
"failures",
"[",
"2",
"]",
".",
"The",
"novelty",
"of",
"block",
"##cha",
"##in",
"is",
"a",
"genuine",
"combination",
"of",
"well",
"-",
"known",
"research",
"results",
"taken",
"from",
"distributed",
"computing",
",",
"cry",
"##pt",
"##ography",
"and",
"game",
"theory",
".",
"Its",
"distributed",
"nature",
"guarantees",
"the",
"per",
"##sistence",
"of",
"the",
"ledge",
"##r",
"data",
".",
"Its",
"public",
"key",
"cry",
"##pt",
"##o",
"-",
"system",
"offers",
"the",
"capabilities",
"for",
"a",
"user",
"to",
"sign",
"transactions",
"that",
"transfer",
"assets",
"from",
"her",
"account",
"to",
"other",
"accounts",
".",
"Its",
"incentive",
"mechanisms",
"guarantee",
"that",
"a",
"subset",
"of",
"participants",
"maintain",
"the",
"validity",
"of",
"the",
"transactions",
".",
"And",
"finally",
",",
"a",
"Byzantine",
"tolerant"
] | 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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".",
"While",
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",",
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"Today",
",",
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"##cha",
"##ins",
",",
"various",
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"-",
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"##dating",
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"a",
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".",
"Very",
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"has",
"however",
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"its",
"theoretical",
"ram",
"##ifications",
".",
"As",
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"result",
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"are",
"sometimes",
"mi",
"##sun",
"##ders",
"##to",
"##od",
"and",
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"In",
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",",
"we",
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"##cha",
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"In",
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",",
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"##cha",
"##ins",
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".",
"In",
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",",
"we",
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"##co",
"##in",
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"the",
"E",
"##ther",
"##eum",
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"We",
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"-",
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",",
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":",
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"[",
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"promises",
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"transform",
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"and",
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"exchange",
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"assets",
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"track",
"secure",
"##ly",
"ownership",
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"assets",
"without",
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"control",
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".",
"At",
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"heart",
"lies",
"a",
"distributed",
"ledge",
"##r",
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"is",
"consistent",
"with",
"high",
"probability",
"when",
"particular",
"assumptions",
"are",
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".",
"In",
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",",
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"g",
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"-",
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"[",
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",",
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".",
"Its",
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"##r",
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".",
"Its",
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"##pt",
"##o",
"-",
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"Its",
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"maintain",
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"the",
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".",
"And",
"finally",
",",
"a",
"Byzantine",
"tolerant"
] | 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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"##cha",
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",",
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"##E",
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"##cha",
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"capital",
"##ization",
".",
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",",
"we",
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"the",
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"at",
"the",
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".",
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"to",
"decades",
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"research",
"on",
"the",
"topic",
"of",
"distributed",
"computing",
".",
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",",
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"##ther",
"##eum",
",",
"which",
"cope",
"##s",
"with",
"some",
"of",
"these",
"attacks"
] | 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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"consensus",
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"aims",
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"##rs",
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".",
"Put",
"into",
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"##cha",
"##in",
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",",
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"fault",
"##y",
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"one",
"block",
"of",
"transaction",
"at",
"a",
"given",
"index",
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"a",
"chain",
"of",
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".",
"This",
"consensus",
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"be",
"stated",
"along",
"three",
"properties",
":",
"(",
"i",
")",
"agreement",
":",
"no",
"two",
"correct",
"processes",
"decided",
"different",
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";",
"(",
"ii",
")",
"validity",
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"ii",
"##i",
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"termination",
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".",
"A",
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"necessary",
"to",
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"that",
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"are",
"totally",
"ordered",
",",
"hence",
"preventing",
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"app",
"##ended",
"blocks",
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".",
"Today",
",",
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"the",
"recent",
"advent",
"of",
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"##cha",
"##ins",
",",
"various",
"consensus",
"implementation",
"##s",
"were",
"proposed",
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"##s",
"reach",
"an",
"agreement",
"on",
"the",
"order",
"of",
"blocks",
"of",
"transactions",
"up",
"##dating",
"the",
"distributed",
"ledge",
"##r",
".",
"However",
",",
"consensus",
"has",
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"known",
"to",
"be",
"un",
"-",
"so",
"##l",
"##vable",
"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",
"block",
"##cha",
"##in",
"consensus",
"algorithms",
"and",
"what",
"are",
"the",
"necessary",
"conditions",
"for",
"these",
"guarantees",
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"be",
"satisfied",
".",
"While",
"the",
"source",
"code",
"of",
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"block",
"##cha",
"##in",
"protocols",
"is",
"publicly",
"available",
",",
"the",
"theoretical",
"ram",
"##ifications",
"of",
"the",
"block",
"##cha",
"##in",
"abstract",
"##ion",
"are",
"rather",
"informal",
".",
"As",
"main",
"block",
"##cha",
"##in",
"systems",
",",
"like",
"Bit",
"##co",
"##in",
"[",
"1",
"]",
"and",
"E",
"##ther",
"##eum",
"[",
"3",
"]",
",",
"are",
"now",
"used",
"to",
"trade",
"millions",
"of",
"US",
"$",
"every",
"day",
",",
"1",
"it",
"has",
"become",
"crucial",
"to",
"precisely",
"identified",
"its",
"theoretical",
"ram",
"##ifications",
"to",
"anti",
"##ci",
"##pate",
"the",
"situations",
"where",
"large",
"volume",
"of",
"assets",
"could",
"be",
"lost",
".",
"In",
"this",
"paper",
",",
"we",
"illustrate",
"the",
"danger",
"of",
"using",
"proof",
"-",
"of",
"-",
"work",
"block",
"##cha",
"##in",
"without",
"understanding",
"precisely",
"their",
"guarantees",
"by",
"listing",
"v",
"##ul",
"##ner",
"##abi",
"##lities",
"that",
"affect",
"the",
"predominant",
"proof",
"-",
"of",
"-",
"work",
"block",
"##cha",
"##in",
"systems",
",",
"namely",
"Bit",
"##co",
"##in",
"and",
"E",
"##ther",
"##eum",
"F",
"##O",
"##OT",
"##N",
"##OT",
"##E",
":",
"Bit",
"##co",
"##in",
"and",
"E",
"##ther",
"##eum",
"are",
"the",
"current",
"most",
"important",
"block",
"##cha",
"##in",
"systems",
"in",
"terms",
"of",
"market",
"capital",
"##ization",
".",
"To",
"this",
"end",
",",
"we",
"describe",
"the",
"consensus",
"algorithms",
"at",
"the",
"heart",
"of",
"these",
"two",
"block",
"##cha",
"##in",
"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",
"block",
"##cha",
"##in",
"consensus",
"is",
"violated",
"by",
":",
"(",
"i",
")",
"presenting",
"a",
"survey",
"of",
"existing",
"at",
"-",
"ta",
"##cks",
"against",
"the",
"Bit",
"##co",
"##in",
"consensus",
"protocol",
"and",
"(",
"ii",
")",
"showing",
"how",
"E",
"##ther",
"##eum",
",",
"which",
"cope",
"##s",
"with",
"some",
"of",
"these",
"attacks"
] | 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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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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"B",
"##L"
] | 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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",",
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"b",
"##1",
"and"
] | 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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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.
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"variations",
"in",
"order",
"to",
"guarantee",
"a",
"minimal",
"probability",
"of",
"success",
".",
"Unfortunately",
",",
"existing",
"applications",
"simply",
"rely",
"on",
"fixed",
"p",
"##a",
"-",
"ram",
"##eter",
"##s",
"regardless",
"of",
"these",
"factors",
",",
"like",
"waiting",
"for",
"a",
"fixed",
"number",
"m",
"of",
"blocks",
"to",
"be",
"mined",
"in",
"order",
"to",
"reach",
"termination",
".",
"As",
"we",
"will",
"see",
"in",
"Section",
"5",
".",
"1",
",",
"there",
"are",
"multiple",
"ways",
"Byzantine",
"processes",
"can",
"vary",
"the",
"delays",
"and",
"mining",
"power",
"to",
"double",
"spend",
"in",
"Bit",
"##co",
"##in",
"and",
"E",
"##ther",
"##eum",
",",
"and",
"some",
"already",
"translated",
"in",
"significant",
"financial",
"losses",
".",
"As",
"we",
"describe",
"in",
"Section",
"##s",
"5",
".",
"2",
"and",
"5",
".",
"3",
",",
"some",
"of",
"these",
"issues",
"are",
"not",
"inherent",
"to",
"the",
"Bit",
"##co",
"##in",
"consensus",
"protocol",
"but",
"could",
"also",
"occur",
"with",
"the",
"E",
"##ther",
"##eum",
"consensus",
"protocol",
".",
"With",
"the",
"advent",
"of",
"consortium",
"and",
"private",
"block",
"##cha",
"##ins",
",",
"some",
"of",
"these",
"factors",
"are",
"even",
"simply",
"produced",
"by",
"a",
"mi",
"##sco",
"##n",
"##fi",
"##gu",
"##ration",
"of",
"the",
"deployed",
"block",
"##cha",
"##in",
"systems",
".",
"S",
"##U",
"##BS",
"##EC",
"##TI",
"##ON",
":",
"AT",
"##TA",
"##C",
"##KS",
"AG",
"##A",
"##IN",
"##ST",
"B",
"##IT",
"##CO",
"##IN",
"Traditional",
"attacks",
"against",
"Bit",
"##co",
"##in",
"consist",
"of",
"waiting",
"for",
"some",
"external",
"action",
",",
"like",
"shipping",
"goods",
",",
"in",
"response",
"to",
"a",
"transaction",
"before",
"disc",
"##ard",
"##ing",
"the",
"transaction",
"from",
"the",
"main",
"branch",
".",
"As",
"the",
"transaction",
"is",
"revoked",
",",
"the",
"issue",
"##r",
"of",
"the",
"transaction",
"can",
"re",
"##use",
"the",
"coins",
"of",
"the",
"transaction",
"in",
"another",
"transaction",
".",
"As",
"the",
"side",
"effects",
"of",
"the",
"external",
"action",
"cannot",
"be",
"revoked",
",",
"the",
"second",
"transaction",
"appears",
"as",
"a",
"‘",
"‘",
"double",
"spending",
"’",
"’",
".",
"Perhaps",
"the",
"most",
"basic",
"form",
"of",
"such",
"an",
"attack",
"assumes",
"that",
"an",
"application",
"takes",
"an",
"external",
"action",
"as",
"soon",
"as",
"a",
"transaction",
"is",
"included",
"in",
"a",
"block",
"[",
"29",
"–",
"31",
"]",
".",
"The",
"first",
"attack",
"of",
"this",
"kind",
"is",
"called",
"Finn",
"##ey",
"’",
"s",
"attack",
"and",
"consists",
"of",
"solo",
"-",
"mining",
"a",
"block",
"with",
"a",
"transaction",
"that",
"sends",
"coins",
"to",
"itself",
"without",
"broadcasting",
"it",
"before",
"issuing",
"a",
"transaction",
"that",
"double",
"-",
"spends",
"the",
"same",
"coin",
"to",
"a",
"merchant",
".",
"When",
"the",
"goods",
"are",
"delivered",
"in",
"exchange",
"of",
"the",
"coins",
",",
"the",
"attacker",
"broadcasts",
"its",
"block",
"to",
"over",
"##ride",
"the",
"payment",
"to",
"the",
"merchant",
".",
"The",
"vector",
"##7",
"##6",
"attack",
"[",
"32",
"]",
"consists",
"of",
"an",
"attacker",
"solo",
"-",
"mining",
"after",
"block",
"b",
"##0",
"a",
"new",
"block",
"b",
"##1",
"containing",
"a",
"transaction",
"to",
"a",
"merchant",
"to",
"purchase",
"goods",
".",
"Once",
"another",
"block",
"b",
"′",
"1",
"is",
"mined",
"after",
"b",
"##0",
",",
"the",
"attacker",
"quickly",
"sends",
"b",
"##1",
"to",
"the",
"merchant",
"for",
"an",
"external",
"action",
"to",
"be",
"taken",
".",
"If",
"b",
"##1",
"is",
"accepted",
"by",
"the",
"system",
",",
"the",
"attacker",
"can",
"issue",
"another",
"transaction",
"with",
"the",
"coins",
"spent",
"in"
] | 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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",",
"a",
"formal",
"##ization",
"of",
"which",
"can",
"be",
"find",
"in",
"[",
"27",
",",
"28",
"]",
",",
"respectively",
".",
"Building",
"upon",
"the",
"trade",
"##off",
"between",
"termination",
"and",
"agreement",
"mentioned",
"in",
"Section",
"3",
".",
"3",
",",
"a",
"solution",
"would",
"be",
"to",
"make",
"the",
"termination",
"dependent",
"on",
"environmental",
"factors",
",",
"like",
"network",
"delays",
"and",
"mining",
"power",
"variations",
"in",
"order",
"to",
"guarantee",
"a",
"minimal",
"probability",
"of",
"success",
".",
"Unfortunately",
",",
"existing",
"applications",
"simply",
"rely",
"on",
"fixed",
"p",
"##a",
"-",
"ram",
"##eter",
"##s",
"regardless",
"of",
"these",
"factors",
",",
"like",
"waiting",
"for",
"a",
"fixed",
"number",
"m",
"of",
"blocks",
"to",
"be",
"mined",
"in",
"order",
"to",
"reach",
"termination",
".",
"As",
"we",
"will",
"see",
"in",
"Section",
"5",
".",
"1",
",",
"there",
"are",
"multiple",
"ways",
"Byzantine",
"processes",
"can",
"vary",
"the",
"delays",
"and",
"mining",
"power",
"to",
"double",
"spend",
"in",
"Bit",
"##co",
"##in",
"and",
"E",
"##ther",
"##eum",
",",
"and",
"some",
"already",
"translated",
"in",
"significant",
"financial",
"losses",
".",
"As",
"we",
"describe",
"in",
"Section",
"##s",
"5",
".",
"2",
"and",
"5",
".",
"3",
",",
"some",
"of",
"these",
"issues",
"are",
"not",
"inherent",
"to",
"the",
"Bit",
"##co",
"##in",
"consensus",
"protocol",
"but",
"could",
"also",
"occur",
"with",
"the",
"E",
"##ther",
"##eum",
"consensus",
"protocol",
".",
"With",
"the",
"advent",
"of",
"consortium",
"and",
"private",
"block",
"##cha",
"##ins",
",",
"some",
"of",
"these",
"factors",
"are",
"even",
"simply",
"produced",
"by",
"a",
"mi",
"##sco",
"##n",
"##fi",
"##gu",
"##ration",
"of",
"the",
"deployed",
"block",
"##cha",
"##in",
"systems",
".",
"S",
"##U",
"##BS",
"##EC",
"##TI",
"##ON",
":",
"AT",
"##TA",
"##C",
"##KS",
"AG",
"##A",
"##IN",
"##ST",
"B",
"##IT",
"##CO",
"##IN",
"Traditional",
"attacks",
"against",
"Bit",
"##co",
"##in",
"consist",
"of",
"waiting",
"for",
"some",
"external",
"action",
",",
"like",
"shipping",
"goods",
",",
"in",
"response",
"to",
"a",
"transaction",
"before",
"disc",
"##ard",
"##ing",
"the",
"transaction",
"from",
"the",
"main",
"branch",
".",
"As",
"the",
"transaction",
"is",
"revoked",
",",
"the",
"issue",
"##r",
"of",
"the",
"transaction",
"can",
"re",
"##use",
"the",
"coins",
"of",
"the",
"transaction",
"in",
"another",
"transaction",
".",
"As",
"the",
"side",
"effects",
"of",
"the",
"external",
"action",
"cannot",
"be",
"revoked",
",",
"the",
"second",
"transaction",
"appears",
"as",
"a",
"‘",
"‘",
"double",
"spending",
"’",
"’",
".",
"Perhaps",
"the",
"most",
"basic",
"form",
"of",
"such",
"an",
"attack",
"assumes",
"that",
"an",
"application",
"takes",
"an",
"external",
"action",
"as",
"soon",
"as",
"a",
"transaction",
"is",
"included",
"in",
"a",
"block",
"[",
"29",
"–",
"31",
"]",
".",
"The",
"first",
"attack",
"of",
"this",
"kind",
"is",
"called",
"Finn",
"##ey",
"’",
"s",
"attack",
"and",
"consists",
"of",
"solo",
"-",
"mining",
"a",
"block",
"with",
"a",
"transaction",
"that",
"sends",
"coins",
"to",
"itself",
"without",
"broadcasting",
"it",
"before",
"issuing",
"a",
"transaction",
"that",
"double",
"-",
"spends",
"the",
"same",
"coin",
"to",
"a",
"merchant",
".",
"When",
"the",
"goods",
"are",
"delivered",
"in",
"exchange",
"of",
"the",
"coins",
",",
"the",
"attacker",
"broadcasts",
"its",
"block",
"to",
"over",
"##ride",
"the",
"payment",
"to",
"the",
"merchant",
".",
"The",
"vector",
"##7",
"##6",
"attack",
"[",
"32",
"]",
"consists",
"of",
"an",
"attacker",
"solo",
"-",
"mining",
"after",
"block",
"b",
"##0",
"a",
"new",
"block",
"b",
"##1",
"containing",
"a",
"transaction",
"to",
"a",
"merchant",
"to",
"purchase",
"goods",
".",
"Once",
"another",
"block",
"b",
"′",
"1",
"is",
"mined",
"after",
"b",
"##0",
",",
"the",
"attacker",
"quickly",
"sends",
"b",
"##1",
"to",
"the",
"merchant",
"for",
"an",
"external",
"action",
"to",
"be",
"taken",
".",
"If",
"b",
"##1",
"is",
"accepted",
"by",
"the",
"system",
",",
"the",
"attacker",
"can",
"issue",
"another",
"transaction",
"with",
"the",
"coins",
"spent",
"in"
] | 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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"to",
"expand",
"its",
"local",
"block",
"##cha",
"##in",
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"than",
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".",
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"a",
"result",
",",
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"E"
] | 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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".",
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".",
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"the",
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"only",
"a",
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"##co",
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".",
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"extended",
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"bound",
"on",
"message",
"delivery",
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"unknown",
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"showed",
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"[",
"36",
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".",
"This",
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"of",
"work",
"reveal",
"an",
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"limitation",
"of",
"Bit",
"##co",
"##in",
":",
"delay",
"##ing",
"propagation",
"of",
"blocks",
"can",
"waste",
"the",
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"of",
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"processes",
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"letting",
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"mine",
"blocks",
"un",
"##ne",
"##cess",
"##arily",
"at",
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"same",
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"of",
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".",
"In",
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",",
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"need",
"more",
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",",
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"##cha",
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"[",
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"E"
] | 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
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"the",
"blocks"
] | 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 |
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"the"
] | 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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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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".",
"This",
"difference",
"is",
"possible",
"thanks",
"to",
"the",
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"of",
"the",
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"pre",
"##dicate",
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".",
"Without",
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"pre",
"##dicate",
",",
"the",
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"Second",
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".",
"This",
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"a",
"number",
"of",
"transactions",
"that",
"grows",
"potentially",
"with",
"the",
"number",
"of",
"participants",
".",
"To",
"solve",
"the",
"classic",
"Byzantine",
"consensus",
",",
"only",
"one",
"of",
"the",
"proposed",
"block",
"could",
"be",
"decided",
",",
"hence",
"limiting",
"the",
"number",
"of",
"decided",
"block",
"to",
"1",
"out",
"of",
"n",
"−",
"t",
"blocks",
"of",
"transactions",
"proposed",
"by",
"correct",
"participants",
".",
"To",
"solve",
"the",
"Block",
"##cha",
"##in",
"consensus",
",",
"however",
",",
"the",
"decided",
"block",
"could",
"represent",
"the",
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"all",
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"t",
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".",
"SEC",
"##TI",
"##ON",
":",
"R",
"##EF",
"##IN",
"##ING",
"THE",
"B",
"##L",
"##OC",
"##K",
"##CH",
"##A",
"##IN",
"M",
"##OD",
"##EL",
"F",
"##OR",
"CO",
"##NS",
"##OR",
"##TI",
"##UM",
"##S",
"As",
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"previously",
",",
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"violation",
"of",
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"##cha",
"##in",
"systems",
"stems",
"from",
"the",
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"##oss",
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"de",
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"##mini",
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"##ally",
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"case",
",",
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"the",
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"Byzantine",
"consensus",
"(",
"De",
"##fin",
"##i",
"-",
"t",
"##ion",
"4",
")",
".",
"There",
"are",
"however",
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"that",
"consist",
"of",
"restrict",
"##ing",
"the",
"model",
"by",
"listing",
"additional",
"assumptions",
"under",
"which",
"an",
"alter",
"-",
"native",
"block",
"##cha",
"##in"
] | 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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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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".",
"Each"
] | 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 | 799 |