Currentstatus,classificationandopenissues
FranCasinoa,ThomasK.Dasaklisb,ConstantinosPatsakisa,⁎
a DepartmentofInformatics,UniversityofPiraeus,80Karaoli&Dimitrioustr.,18534Piraeus,Greece
b DepartmentofIndustrialManagementandTechnology,UniversityofPiraeus,80Karaoli&Dimitrioustr.,18534Piraeus,Greece
ARTICLEINFO
Keywords: Blockchain Classification Applications
ABSTRACT
Thisworkprovidesasystematicliteraturereviewofblockchain-basedapplicationsacrossmultipledomains.Theaimistoinvestigatethecurrentstateofblockchaintechnologyanditsapplicationsandtohighlighthowspecificcharacteristicsofthisdisruptivetechnologycanrevolutionise“business-as-usual”practices.Tothisend,thetheoreticalunderpinningsofnumerous researchpaperspublishedinhighrankedscientificjournalsduringthelastdecade,alongwith severalreportsfromgreyliteratureasameansofstreamliningourassessmentandcapturingthe continuouslyexpandingblockchaindomain,areincludedinthisreview.Basedonastructured, systematicreviewandthematiccontentanalysisofthediscoveredliterature,wepresenta comprehensiveclassificationofblockchain-enabledapplicationsacrossdiversesectorssuchas supplychain,business,healthcare,IoT,privacy,anddatamanagement,andweestablishkey themes,trendsandemergingareasforresearch.Wealsopointtotheshortcomingsidentifiedin therelevantliterature,particularlylimitationstheblockchaintechnologypresentsandhowthese limitationsspawnacrossdifferentsectorsandindustries.Buildingonthesefindings,weidentify variousresearchgapsandfutureexploratorydirectionsthatareanticipatedtobeofsignificant valuebothforacademicsandpractitioners.
1.Introduction
AlmostadecadeagoSatoshiNakamoto,theunknownperson/groupbehindBitcoin,describedhowtheblockchaintechnology,a distributedpeer-to-peerlinked-structure,couldbeusedtosolvetheproblemofmaintainingtheorderoftransactionsandtoavoidthe double-spendingproblem(Nakamoto,2008).Bitcoinorderstransactionsandgroupstheminaconstrained-sizestructurenamed blocks sharingthesametimestamp.Thenodesofthenetwork(miners)areresponsibleforlinkingtheblockstoeachotherin chronologicalorder,witheveryblockcontainingthehashofthepreviousblocktocreateablockchain(Crosbyetal.,2016).Thus,the blockchainstructuremanagestocontainarobustandauditableregistryofalltransactions.
Blockchainsintroducedseriousdisruptionstothetraditionalbusinessprocessessincetheapplicationsandtransactions,which neededcentralisedarchitecturesortrustedthirdpartiestoverifythem,cannowoperateinadecentralisedwaywiththesamelevelof certainty.Theinherentcharacteristicsofblockchainarchitectureanddesignprovidepropertiesliketransparency,robustness,auditability,andsecurity(Greenspan,2015a;ChristidisandDevetsikiotis,2016).Ablockchaincanbeconsideredadistributeddatabase thatisorganisedasalistoforderedblocks,wherethecommittedblocksareimmutable.Onecanseethatthisisidealinthebanking sectorasbankscancooperateunderthesameblockchainandpushtheircustomers’transactions.Thisway,beyondtransparency, https://doi.org/10.1016/j.tele.2018.11.006
⁎ Correspondingauthor.
E-mailaddresses: francasino@unipi.gr (F.Casino), dasaklis@unipi.gr (T.K.Dasaklis), kpatsak@unipi.gr (C.Patsakis).
Received30May2018;Receivedinrevisedform16November2018;Accepted17November2018
Availableonline22November2018
0736-5853/©2018TheAuthors.PublishedbyElsevierLtd.ThisisanopenaccessarticleundertheCCBY-NC-NDlicense (http://creativecommons.org/licenses/BY-NC-ND/4.0/).
blockchainfacilitatestransactions’auditing.Companiesinvestinthistechnologyastheyseethepotentialofmakingtheirarchitecturesdecentralisedandminimisingtheirtransactioncostsastheybecomeinherentlysafer,transparentandinsomecasesfaster. Therefore,blockchainsarenotjustahype.
ThenumberofcryptocurrenciesillustratesBlockchain’simportance,currentlyexceeding1900andgrowing(CoinMarketCap, 2017).Suchagrowthpacecouldsooncreateinteroperabilityproblemsduetotheheterogeneityofcryptocurrencyapplications (TschorschandScheuermann,2016;HaferkornandQuintanaDiaz,2015).Furthermore,thelandscapeisrapidlyevolvingas blockchainisbeingusedinotherfieldsbeyondcryptocurrencies,with SmartContracts(SCs) playingacentralrole.SCsdefinedin 1994bySzaboas: “acomputerisedtransactionprotocolthatexecutesthetermsofacontract” (Szabo,1994),allowustotranslate contractualclausesintoembeddablecode(Szabo,1997)thusminimizingexternalparticipationandrisks.So,aSCisanagreement betweenpartieswhich,althoughtheydonottrusteachother,theagreedtermsareautomaticallyenforced.Therefore,withinthe blockchaincontext,SCsarescriptsrunninginadecentralisedmannerandstoredintheblockchain(ChristidisandDevetsikiotis, 2016)withoutrelyingonanytrustedauthority.Inparticular,blockchain-basedsystemssupportingSCsenablemorecomplexprocessesandinteractionssotheyestablishanewparadigmwithpracticallylimitlessapplications.
Asaresult,Blockchaintechnologyisbecomingincreasinglyrelevant(Zhaoetal.,2016).Almost1000(33%)ofC-suiteexecutives declarethattheyareconsideringorhavealreadybeenactivelyengagedwithblockchains(IBM,2017).Researchersanddevelopers arealreadyawareofthecapabilitiesofthenewtechnologyandexplorevariousapplicationsacrossavastarrayofsectors(Christidis andDevetsikiotis,2016).Basedontheintendedaudience,threegenerationsofblockchainscanbedistinguished(Zhaoetal.,2016): Blockchain1.0whichincludesapplicationsenablingdigitalcryptocurrencytransactions;Blockchain2.0whichincludesSCsandaset ofapplicationsextendingbeyondcryptocurrencytransactions;andBlockchain3.0whichincludesapplicationsinareasbeyondthe previoustwoversions,suchasgovernment,health,scienceandIoT.
Whilethereareseveralreviewsregardingblockchaintechnology(Tamaetal.,2017;Brandãoetal.,2018),wearguethatthe state-of-the-artofblockchain-enabledapplicationshasreceivedlimitedattention.Evenin Zhengetal.(2016) theapplicationsof blockchainsarenotcoveredtotheirfullextentnorapplicability.Thereareindeedsomereviewsfocusedontheparticularroleof blockchainincludingthedevelopmentofdecentralisedanddata-intensiveapplicationsfortheIoT(Conoscentietal.,2016;Christidis andDevetsikiotis,2016),andmanagingbigdatainadecentralisedfashion(KarafiloskiandMishev,2017a).Otherreviewsfocuson securityissuesoftheblockchain(KhanandSalah,2017;Lietal.,2017a;Mengetal.,2018)andonitspotentialtoenabletrustand decentralisationinservicesystems(Seebacheretal.,2017)andP2Pplatforms(Hawlitscheketal.,2018).Sometechnicalaspectsof theblockchaindesignsuchasitsconsensusprotocol(Sankaretal.,2017),thevulnerabilitiesofSCs(Atzeietal.,2017)andother technicalcharacteristicslikeitssizeandbandwidth,usability,dataintegrity,andscalabilityhavealsobeenstudiedin Yli-Huumo etal.(2016)andKoteskaetal.(2017).Moreover,thereareothersurveyssuchas Bonneauetal.(2015),Tsukerman(2015), Mukhopadhyayetal.(2016),KhalilovandLevi(2018)andContietal.(2018) whicharemorefocusedonthecurrencyaspectof blockchainsandtheofferedsecurityandprivacy.
Evidently,theliteraturelacksaconcreteandsystematicreviewofthecurrentblockchain-enabledstate-of-the-artapplications,a limitationwhichwastheprimarydriverforconductingthisresearch.Inparticular,wetrytoaddressthisbyansweringthefollowing threequestions:(i)Howblockchain-basedapplicationsdevelopovertime?(ii)Howcertaintechnicallimitationsoftheblockchain architectureaffectprocedures/processesinparticulardomains?Whicharetheselimitations?(iii)Whatisthesuitabilityofblockchain technologyacrossdifferentdomainsandthematicareas?
Ourworkcontributestowardsathoroughunderstandingoftheblockchainfeaturesandprovidesasnapshotofcurrentblockchain-enabledapplicationsacrosssectors.Basedonacontentanalysisapproach,wehighlightthegrowinginterestfromtheacademic communityandidentifythreekeyresearchstreams:(i)classificationoftherangeofblockchain-basedapplicationsacrossavastarray ofsectors(ii)suitabilityoftheblockchaintechnologytocreatevalueinthesesectorstakingintoaccountthevariouslimitationsthis technologypresents,and(iii)guidingresearchersbyprovidingaroadmapofpromisingresearchavenues,challengesandopportunitiesforwhichfurtherresearchisneeded.Itisworthnotingthatthisreviewcannotbyanymeansbeconsideredasexhaustive sinceblockchaintechnologyiscontinuouslygrowingataveryfastpace.
Theremainderofthisworkisorganizedasfollows.InSection 2 abriefoverviewofblockchainarchitectureispresented.The methodfollowedtoconductthesystematicliteraturereviewisoutlinedinSection 3.Thedescriptiveanalysisoftheretrieved literatureispresentedinSection 4 whileinSection 5 ataxonomyoftheblockchain-basedapplicationsispresented.Relevantopen issues,trends,andfurtherresearchlinesarediscussedinSection 6.
2.Blockchainoverview
Inprinciple,ablockchainshouldbeconsideredasa distributedappend-onlytimestampeddatastructure.Blockchainsallowusto haveadistributedpeer-to-peernetworkwherenon-trustingmemberscanverifiablyinteractwitheachwithouttheneedforatrusted authority(ChristidisandDevetsikiotis,2016).Toachievethisonecanconsiderblockchainasasetofinterconnectedmechanisms whichprovidespecificfeaturestotheinfrastructure,asillustratedin Fig.1.Atthelowestlevelofthisinfrastructure,wehavethe signed transactions betweenpeers.Thesetransactionsdenoteanagreementbetweentwoparticipants,whichmayinvolvethetransfer ofphysicalordigitalassets,thecompletionofatask,etc.Atleastoneparticipantsignsthistransaction,anditisdisseminatedtoits neighbours.Typically,anyentitywhichconnectstotheblockchainiscalleda node.However,nodesthatverifyalltheblockchain rulesarecalled fullnodes.Thesenodesgroupthetransactionsinto blocks andtheyareresponsibletodeterminewhetherthe transactionsarevalid,andshouldbekeptintheblockchain,andwhicharenot.
Avalidtransactionmeans,forinstance,thatBobreceivedonebitcoinfromAlice.However,Alicemayhavetriedtotransferthe
samebitcoin,asitisadigitalasset,toCarol.Therefore,nodesmustreachtoanagreementonwhichtransactionsmustbekeptinthe blockchaintoguaranteethattherewillbenocorruptbranchesanddivergences(Vukolić,2015;ChristidisandDevetsikiotis,2016). Thisisactuallythegoalofthesecond Consensus layer.Dependingontheblockchaintype,differentConsensusmechanismsexist (Mingxiaoetal.,2017).Themostwell-knownisthe Proof-of-work(PoW).PoWrequiressolvingacomplicatedcomputationalprocess, likefindinghasheswithspecificpatterns,e.g.aleadingnumberofzeroes(Antonopoulos,2014),toensureauthenticationand verifiability.Insteadofsplittingblocksacrossproportionallytotherelativehashratesofminers(i.e.,theirminingpower), Proof-ofStake(PoS) protocolssplitstakeblocksproportionallytothecurrentwealthofminers(Pilkington,2016).Thisway,theselectionis fairerandpreventsthewealthiestparticipantfromdominatingthenetwork.Manyblockchains,suchasEthereum(Dannen,2017), aregraduallyshiftingtoPoSduetothesignificantdecreaseinpowerconsumptionandimprovedscalability.Otherconsensusapproachesinclude ByzantineFaultTolerance(BFT) (CastroandLiskov,2002)anditsvariants(Zhengetal.,2016).
Anadditionallayer,the ComputeInterface,allowsblockchainstooffermorefunctionality.Practically,ablockchainstoresastate whichconsistse.g.ofallthetransactionsthathavebeenmadebytheusers,therebyallowingthecalculationofeachuser’sbalance. However,formoreadvancedapplicationsweneedtostorecomplexstateswhichareupdateddynamicallyusingdistributedcomputing,e.g.statesthatshiftfromonetoanotheroncespecificcriteriaaremet.ThisrequirementhasgivenrisetoSCswhichusenodes oftheblockchaintoexecutethetermsofacontract.
Finally,the Governance layerextendstheblockchainarchitecturetocoverthehumaninteractionstakingplaceinthephysical world.Indeed,althoughblockchainsprotocolsarewelldefined,theyarealsoaffectedbyinputsfromdiversegroupsofpeoplewho integratenewmethods,improvetheblockchainprotocolsandpatchthesystem.Whilethesepartsarenecessaryforthegrowthof eachblockchain,theyconstituteoff-chainsocialprocesses.Therefore,blockchaingovernancedealswithhowthesediverseactors cometogethertoproduce,maintain,orchangetheinputsthatmakeupablockchain.1
Currentliteraturecategorisesblockchainnetworksinseveralways(Buterin,2015;Zhengetal.,2016;ErisIndustries,2016; ChristidisandDevetsikiotis,2016;Kravchenko,2016;Wood,2016).Thesecategoriesareformedaccordingtothenetwork’smanagementandpermissionsas public, private and federated.Inpublicblockchains(permissionless)anyonecanjoinasanewuserornode miner.Moreover,allparticipantscanperformoperationssuchastransactionsorcontracts.Inprivateblockchains;whichalongwith thefederatedbelongtothe permissioned blockchaincategory,usually,awhitelistofallowedusersisdefinedwithparticularcharacteristicsandpermissionsoverthenetworkoperations.SincetheriskofSybilattacksisalmostnegligiblethere(Swanson,2015), privateblockchainnetworkscanavoidexpensivePoWmechanisms.Instead,awiderrangeofconsensusprotocolsbasedondisincentivescouldbeadopted.Afederatedblockchainisahybridcombinationofpublicandprivateblockchains(Buterin,2015;Zheng etal.,2016).Althoughitsharessimilarscalabilityandprivacyprotectionlevelwithprivateblockchain,theirmaindifferenceisthata setofnodes,named leader nodes,isselectedinsteadofasingleentitytoverifythetransactionprocesses.Thisenablesapartially decentraliseddesignwhereleadernodescangrantpermissionstootherusers.Inthisarticle,weprovideamorefine-grained blockchainnetworkclassificationthancurrentthestate-of-the-art(Buterin,2015;Zhengetal.,2016;ChristidisandDevetsikiotis, 2016;Kravchenko,2016)because,inadditiontoclassicalfeaturessuchastheownershipandmanagementoftheinformationshared intheblockchain,weconsiderfeaturessuchastransactionapprovaltime,orsecurityaspectssuchasanonymity. Table1 summarises themaincharacteristicsofeachblockchainnetworkregardingefficiency,securityandconsensusmechanisms.
Well-knownimplementationsofpublicblockchainsincludeBitcoin,Ethereum,Litecoinand,ingeneral,mostcryptocurrencies (Nakamoto,2008;HaferkornandQuintanaDiaz,2015).Oneoftheirmainadvantagesisthelackofinfrastructurecosts:thenetwork isself-sustainedandcapableofmaintainingitself,drasticallyreducingmanagementoverheads.Inprivateblockchains,themain applicationsaredatabasemanagement,auditingand,ingeneral,performancedemandingsolutions(Zhengetal.,2016).Multichain (Greenspan,2015b)isanexampleofanopenplatformforbuildinganddeployingprivateblockchains.Finally,federatedblockchains aremostlyusedinthebankingandindustrysectors(R3,2015).ThisisthecaseoftheHyperledgerproject(HyperledgerProject, 2015)whichdevelopscross-industrypermission-basedblockchainframeworks.Recently,Ethereumhasalsoprovidedtoolsfor buildingfederatedblockchains.Otherprojectssuchas Cardano(2018) areratherambitioustryingtoprovidemorefunctionality.For moreonblockchaincategorisation,theinterestedreadermayreferto Walport(2016)andSwanson(2015)
1 https://www.oii.ox.ac.uk/blog/understanding-public-blockchain-governance/.
Fig.1. Anoverviewofblockchainarchitecture.
Table1
Classificationandmaincharacteristicsofblockchainnetworks.
Property Public Private Federated
Consensus
Mechanism
Identity
Anonymity
ProtocolEfficiency&
Consumption
Immutability
Ownership&
•CostlyPoW
•Allminers
•(Pseudo)Anonymous
•Malicious?
•Lowefficiency
•Highenergy
•Almostimpossible
•Public
•LightPoW
•Centralisedorganisation
•Identifiedusers
•Trusted
•Highefficiency
•Lowenergy
•Collusionattacks
•Centralised
•LightPoW
•Leadernodeset
•Identifiedusers
•Trusted
•Highefficiency
•Lowenergy
•Collusionattacks
•Semi-Centralised Management
TransactionApproval
3.Researchmethodology
•Permissionless
•Orderofminutes
•Permissionedwhitelist
•Orderofmilliseconds
•Permissionednodes
•Orderofmilliseconds
Toprovideatransparent,reproducibleandscientificliteraturereviewofblockchain-basedapplications,theprocesssuggestedby BrinerandDenyer(2012) aswellassomefeaturesofthePRISMAstatement(Moheretal.,2009)havebeenadopted.Theoverall methodologicalapproachincludesthefollowingsteps:
1.Identifytheneedforthereview,prepareaproposalforthereview,anddevelopthereviewprotocol.
2.Identifytheresearch,selectthestudies,assessthequality,takenotesandextractdata,synthesisethedata.
3.Reporttheresultsofthereview.
3.1.Locatingstudies
Toaddressourprimaryresearchquestion,asystematicliteraturesearchwascarriedoutduringJanuary2018withouttimeframe restrictionsandtheresultsweresubsequentlyupdatedduringApril2018.Scopuswasusedasthemainscientificdatabaseinwhich theterm“blockchain”wassearchedinallarticles’titles.Additionalsearchesusingthereferencedworksofrelevantarticleswerealso conducted(snowballeffect).Relevant“greyliterature”,includingunpublishedresearchcommissionedbygovernmentsorprivate/ publicinstitutionswasalsoidentifiedthroughelectronicsearches.Toidentifythepublishedgreyliterature,weevaluatedthefirst 200hitsfromGoogle.Alternatetermsfor“blockchain”and“application”wereusedduringthesearch.Thehand-searchreferencelist inseveralreportsresultedinadditionalgreyliterature,particularlyresearchandcommitteereportsorpolicybriefsfrombothprivate andpublicsectorinstitutions/organizations.Aflowchartofthestrategyimplementedispresentedin Fig.2.Inaddition,several refinementfeaturesofScopuswereextensivelyused(multiplerefinementsofresultsfollowingthecontextofspecificarticles,related documentssearch,etc.).Whentheabstractofaparticularstudywasnotavailable,thefullarticlewasretrievedandassessedfor relevance.Allpotentiallyrelevantarticleswereretrievedinfulltext.
3.2.Studyselectionandevaluation
Theeligibilityoftheretrievedliteraturewasevaluatedindependentlybytheauthorsbasedonasetofpredefinedexclusionand inclusioncriteria(see Table2).Someexclusioncriteriawereusedbeforeintroducingtheliteratureinthebibliographicmanager (language,subjectareaanddocumenttyperestrictions).Initially,theabstractsofallresearchpapersandintroductorysectionsof greyliteraturewereassessed.Articlesmeetingoneoftheexclusioncriteriawereexcludedandsortedbyreasonofexclusion. Afterwards,afull-textreviewalsotookplace,andsomeadditionalarticleswereexcludedfromthestudydocumentingthereasonsfor exclusion.Anydiscrepancywithrespecttotherelevanceofreviewedarticleswasresolvedthroughdiscussionuntilconsensuswas reached.Overall,severalstudieswereexcludedbecausetheywerefocusedprimarilyonthetechnicalaspectsofblockchaintechnologyand/orblockchainarchitecture.Articlesnotfittingtheinclusioncriteriaweresetasideandconsequentlyusedintheintroductionofthisarticle.
3.3.Analysisandsynthesis
Allarticlesandreportsmeetingtheinclusioncriteriawereenteredintoaqualitativeanalysissoftware(MAXQDA11),anddata wereanalysedinemergingthemes.Thereviewersindependentlycarriedoutthethematiccontentanalysis.Afterwards,thethree clustersofcodedsegmentswerecompared(rateofconsensuswasapproximately75%),agreeduponforallarticlesandsummarised inonesetofthemesandsub-themes.
Table2
Inclusionandexclusioncriteria.
Selectioncriteria
Scientificdatabase
Inclusion Peer-reviewedresearcharticles(includingarticlesinpress),conferenceproceedings papers,bookchapters,reviewpapers,shortsurveys,serialsetc.
Withouttime-framerestrictions
ExclusionPriortoimportationto bibliographicmanager
Duringtitlescreening
NonEnglisharticles,articleswithmissing abstracts,notes,editorials
Genericarticlesrelatedtotheblockchain technologyand/orblockchainarchitecture
Duringabstractscreening Software-orientedarticlesrelatedtothe blockchaintechnology
Duringfull-textscreening Articlesaddressingtechnicalaspectsof blockchaintechnology
4.Descriptiveanalysis
Greyliterature
Englishreports
Withouttime-framerestrictions
Genericreportsrelatedtotheblockchain technologywithoutdescribingspecific applications.
Thestudyanalyzes260researchpaperspublishedbetween2014andApril2018(forconformity,greyliteraturehasbeenexcludedfromthedescriptiveanalysis).Thepurposeofthedescriptiveanalysisisthreefold:(i)itprovidesinterestinginsightsregarding currentresearchtrendsinblockchaintechnology,anditsapplications(ii)ithelpstovisualisethemultidisciplinaryresearchapproachesdevelopedsofarinthescientificliterature,and(iii)itfurthersupportstheclassificationstructurepresentedinSection 5 Forclassifyingtheavailableliterature,thedescriptiveanalysisisbasedontwokey-criteria:(i)distributionofpublicationsovertime andthematicareaand(ii)distributionoftypeofpublicationovertime.
Ayear-wiseanalysisoftheselectedpapersisillustratedin Fig.3.Itisworthnotingthatduring2017thenumberofpublications hassky-rocketed.Until2016justalittlemoreof40publicationsexistedrelatedtoblockchain-enabledapplicationswhereasduring 2017theirnumberreachedalmost180.Therefore,researchhasslowly,yetsignificantly,pickedupintheareaofblockchain-enabled applicationsduringthelastcoupleofyears.Thisupwardtrendhighlightstheemergingandgrowingnatureoftheblockchain technologyandthegrowingacademicinterest.EventhoughblockchaintechnologywasfirstintroducedwithBitcoinasitscore underlyingtechnology,ittookseveralyearstotheresearchcommunitytobecomefullyawareofblockchain’spotentialandtotake advantageofitspossibleapplications.Unsurprisingly,duringitsfirstyears,blockchainwasconsideredasynonymoustoBitcoin,and
Fig.2. Flowchartofthesearchstrategy.
Fig.3. Year-wiseanalysisoftheselectedliteraturepertypeofpublication.
inprinciple,researchersweretryingtocreatetheinfrastructureratherthanusethisnewtechnologyforapplicationpurposes. Therefore,journal-orientedcontentrelatedtoblockchain-enabledapplicationshasbeennotablypublishedfrom2016onwards.From Fig.3 itisalsoevidentthatalargevolumeofscientificliteraturehasbeenpublishedinconferenceproceedings,withasteadyupward trend.
Thedomain-specificdistributionofthe260researchitemsovertimemaybeseenin
Fig.4.Elevendomainsofblockchain-based applicationshavebeenidentifiedfromtheanalysis.Business-orientedapplicationsrepresentalargeportionofallavailableapplications(58outofthe260researchitems)followedbyGovernance,IoT,andDatamanagementapplications.Health-orientedapplicationsalsoreceivemuchattentionfromthescientificcommunityduringthelastcoupleofyears. Fig.4 showsthatalthough blockchainseemedtohave,atleastatitsveryearlystages,apivotalroletoplayinfinance,theresearchcommunityisyettoproduce asubstantialamountoffinancial-orientedapplications.Moreover,therelativelylargenumberofmiscellaneousapplications(applicationsthatfalloutsidethecategoriesdescribedabove)alsohighlightstheinterdisciplinarypotentialoftheblockchaintechnology.
5.Taxonomyofblockchain-basedapplications
Mostauthorsclassifyblockchainapplicationsintofinancialandnon-financialones(Crosbyetal.,2016)sincecryptocurrencies representaconsiderablepercentageoftheexistingblockchainnetworks.Othersclassifythemaccordingtoblockchainversions(i.e., 1.0,2.0and3.0)(Swan,2015;Zhaoetal.,2016).Inthiswork,weproposeanapplication-orientedclassification,similartotheone proposedin Zhengetal.(2016).Ourapproach,however,differsfromothersimilarworksinthatitusesarigorousstatistical methodologybasedontheliterature(seeSections 3and4),andthusitfitsbettertocurrentblockchaindevelopmentsandillustrates withhighfidelitythefutureblockchaintrends.Therefore,takingintoaccounttheactualandforthcomingheterogeneityofblockchainsolutions,wepresentamorecomprehensiveandin-depthclassificationofblockchain-basedapplications,whichisgraphically representedin Fig.5.Inthefollowingsubsectionsweprovideasoundclassificationoftheavailableblockchain-enabledapplications basedontheanalysisoftheavailableliterature.
5.1.Financialapplications
Currently,blockchaintechnologyisappliedtoawidevarietyoffinancialfields,includingbusinessservices,settlementoffinancialassets,predictionmarketsandeconomictransactions(HaferkornandQuintanaDiaz,2015).Blockchainisexpectedtoplayan essentialroleinthesustainabledevelopmentoftheglobaleconomy,bringingbenefitstoconsumers,tothecurrentbankingsystem andthewholesocietyingeneral(Nguyen,2016).
Theglobalfinancialsystemisexploringwaysofusingblockchain-enabledapplicationsforfinancialassets,suchassecurities,fiat money,andderivativecontracts(PetersandPanayi,2016;FanningandCenters,2016;Nijeholtetal.,2017;Paech,2017).For example,blockchaintechnologyoffersamassivechangetocapitalmarketsandamoreefficientwayforperformingoperationslike securitiesandderivativestransaction(VandeVeldeetal.,2016;WuandLiang,2017),digitalpayments(Papadopoulosetal.,2015; Becketal.,2016;Minetal.,2016;Yamadaetal.,2017;EnglishandNezhadian,2017;Lundqvistetal.,2017;Gaoetal.,2018),loan
Fig.4. Distributionofresearchitemsaccordingtothethematicareaidentified.
managementschemes(Gazalietal.,2017),generalbankingservices(Coccoetal.,2017),financialauditing(DaiandVasarhelyi, 2017)orcryptocurrencypaymentandexchange(i.e.,e-wallets)(Cawrey,2014;Rizzo,2014).Notably,asetoftheworld’sbiggest banks,includingBarclaysandGoldmanSachshavejoinedforceswithR3(R3,2015)toestablishanoperatingblockchain-based frameworkforthefinancialmarket(Crosbyetal.,2016).AnotherexampleofbankcooperationistheGlobalPaymentsSteering
Fig.5. Mindmapabstractionofthedifferenttypesofblockchainapplications.
Group(GPSG)(Ripple,2016),whosemembersincludeSantander,BankofAmericaandUniCredit,amongothers.Thecryptocurrency behindGPSGisXRP,createdbyRipple(Brittoetal.,2012)whichimplementsaninteroperableandscalableopen-sourceinfrastructureenablingglobalpaymentsandcurrencyexchanges.
Predictionmarketplacesystems(PMS),whichserveasoraclesorinformationproviders,arealsoaninterestingfieldwhichmay impactbusinessesandcryptocurrencies.Blockchain-basedP2PimplementationsofPMScanbefoundin Viacoin(2014),anopen sourcecryptocurrencythatfeaturesScryptMergedmining,atypeofPoWthatpermitsmuchfastertransactionsthanBitcoin. Augur (2014) isadecentralisedPMSthatallowsuserstotradesharesbeforetheoccurrenceofaneventundertheparadigmofthewisdomof thecrowds.Usersarerewardedforcorrectlypredictingfuturereal-worldevents. Bitshares(2014) aredigitaltokensstoredinthe blockchainthatreferencespecificassetssuchascurrenciesorproducts.Thetokenholdersmayearninterestonmarketproducts,such asgold,oil,gasandalsooncurrencies.BitShares2.0offersastackoffinancialservicesincludingcurrencyexchangeorbanking operationsinadecentralisedblockchain-basedfashion.TheNasdaq-Citiplatform(Rizzo,2017)isaplatformthatenablesfunctionalitiessuchasrelationshipmanagementandinvestmentsforprivatecompanies. Ventures(2014) iscoinedintheblockchain2.0 platformandusestheCounterpartyprotocol,whichimplementsfinancialinstrumentsasSCs,tocreateanovelstockmarket.Another exampleisCoinsetter,aNYC-basedForextradingplatformforbitcoins(Coinsetter,2012).Plasma(PoonandButerin,2017)isanSC frameworkwhichenablestheuseofSCstoprocessfinancialactivity,aswellastoconstructeconomicincentivesforglobally persistentdataservices.
Otherfinancial-orientedareasmayincludecommercialpropertyandcasualtyclaimsprocessing,syndicatedloanscontingent convertiblebonds,automatedcompliance,proxyvoting,assetrehypothecation,andover-the-countermarket(Deloitte,2016a;F.R. Ltd,2016;InfosysConsulting,2016;McWatersetal.,2016).Finally,blockchainadoptionbythefinancialsectorwilleventuallylead tocostsavingsinareaslikecentralfinancereporting,compliance,centralisedoperations,andbusinessoperations(Accenture, 2017a).
5.2.Integrityverification
Oneofthemostemergingblockchain-relatedfieldsisintegrityverification(BhowmikandFeng,2017;Dupont,2017;Xuetal., 2017a;JamthagenandHell,2016;Zikratovetal.,2017).Blockchainintegrityverificationapplicationsstoreinformationand transactionsrelatedtothecreationandlifetimeofproductsorservices.Thepossibleapplicationsare:(i)provenanceandcounterfeit, (ii)insurance;and(iii)intellectualproperty(IP)management.
AnintegrityverificationsubsetofblockchainapplicationsarethoseorientedtoIPprotection(Kishigamietal.,2015;Kitahara etal.,2014;Fujimuraetal.,2015;DeLaRosaetal.,2017).Asstatedin Swan(2015),thetermdigitalartreferstoIPandnotjustto onlineartworks,soblockchaintechnologiescanbeconsideredtocoverallsuchscenarios(O’DairandBeaven,2017;McConaghy etal.,2017;Zeilinger,2018).MaturesolutionslikeAscribe(Ascribe,2014)andMediachain(Labs,2016)useBitcoinblockchainto linkdigitalcontentwiththeircreators.Ascribeusesittotransferownershipandloandigitalassets,whileMediachaintriestostore metadataontheblockchaintoallowmediarecoveryandquerying.MonetisationapproachessuchasMonegraph(MonegraphInc., 2014)enablesharingofrevenueacrossthevaluechainofmediadistributionforonlinebroadcasts,videoclips,imagereels,andother licensedorbrand-sponsoredcontent,previouslyverifiedintheblockchain.Factom(Snowetal.,2015)isanotherblockchainsolution forstorageandvalidationofdigitalassets.SilentNotary(SilentNotary,2017)isablockchain-basedserviceforconfirmationofevent existence,recordedinadigitalformatsuchascommunicationinmessenger,image,videofile,ande-mail.Kodakcoin(Kodaketal., 2018),isanovelapaymentmethodusedtoacquirephotolicensesandimagerightsfromathekodakOneplatform,whichstoresthe worksofregisteredphotographers.Anotherexampleofnetworkmedia’sdigitalrightsmanagementcanbefoundin Xuetal.(2017b) HerbautandNegru(2017) proposeauser-centricapproachthathelpsthenecessaryreshapingofthecontentdeliveryecosystem.
Theworkpresentedin KimandLaskowski(2016) describesanontologytostoreandinterpretdatainanautomatedway,inthe contextofdataprovenanceandintegrity.AuthorsclaimthatSCsarecloselyrelatedwithontologiesandthatsuchsystemscanbe adapteddependingonthetopic.CounterfeitsolutionssuchasEverledger(Lomas,2015)andBlockverify(Blockverify,2015)use blockchainandSCstoavoidfraudforbanksandinsurancesandtointroducetransparencytosupplychains,respectively.Further examplesondataintegritycanbefoundin Xunetal.(2017),whereauthorsimplementtherelevantprotocolsandthefollowing prototypesystemofablockchain-basedframeworkfordataintegrityserviceandin Jaagetal.(2016),whereauthorsshowhow blockchainsmaybeusedforsupplychainmanagement,identityservicesordevicemanagementinabusinesssetting.
Blockchaintechnologyisrecentlyreceivinganever-increasingattentionfromtheinsuranceindustryinavarietyofareas,includingsales,underwriting,customeronboarding,claimsprocessing,payments,assettransfers,andreinsurance(Cognizant,2017; Lambertietal.,2017;KPMGInternational,2017).Forinstance,European-basedinsurershaverecentlylaunchedtheB3i-ablockchain industryinitiativeforexploringhowblockchaincanbeusedtodevelopprocessesandstandardsforindustry-wideusageandto accelerateefficiencygainsintheinsurancesector(Cognizant,2017).SCsenabledbyblockchainleadtotheautomationofseveral processesintheinsurancesector,resultingtherebytosubstantiallyreducedcosts,increasedefficiency,andprocessingspeed (Cognizant,2017).Healthinsurance-specificpotentialimplicationsoftheblockchaintechnologymayincludetheestablishmentof moresecuredatarepositoriesformedicalandwellnessinformation,fortriggeringalertstotakeprescriptionsormakeregulardoctor visitsordiagnostictests,forfacilitatingcontinuousunderwritingandpricingassessments,forestablishinglessarbitrary,moreup-todateriskpooling,andforallowingformorepersonalisationandindividualisedcoverage(Deloitte,2016b).Moreexamplesof blockchain-basedinsuranceapplicationscanbefoundin(McKinseyCompany,2016;Nath,2016;Voetal.,2017).
5.3.Governance
Governmentsthroughoutyearsareentrustedwithmanagingandholdingofficialrecordsofbothcitizensand/orenterprises. Blockchain-enabledapplicationsmightchangethewaygovernmentsatlocalorstateleveloperatebydisintermediatingtransactions andrecordkeeping(Reijersetal.,2016;Hou,2017).Theaccountability,automation,andsafetythatblockchainoffersforhandling publicrecordscouldeventuallyobstructcorruptionandmakegovernmentservicesmoreefficient.Inparticular,blockchaincould serveasasecurecommunicationplatformforintegratingphysical,social,andbusinessinfrastructuresinasmartcitycontext(Ibba etal.,2017;Jaffeetal.,2017;BiswasandMuthukkumarasamy,2016;Sharmaetal.,2017).Blockchaingovernanceaimsatproviding thesameservicesthatareofferedbythestateanditscorrespondingpublicauthoritiesinadecentralisedandefficientwaywhile maintainingthesamevalidity.Examplesofsuchservicesincluderegistrationorlegaldocuments,attestation,identification,marriage contracts,taxesandvoting(Swan,2015).
TheWorldCitizenproject(McMillan,2014)isanexampleofadecentralisedpassportservicetoidentifycitizensalloverthe world.Blockchainscanalsobeusedtootherpublicservicessuchasmarriageregistration,patentmanagement,andincometaxation systems(Akinsetal.,2013).Otherprojectsfocusonideassuchasdelegativedemocracy,wheredelegates(insteadofparliamentary representatives)takethevotingpower(Swan,2015).Similarly,Holacracy(Robertson,2015)isacustomisableself-management practicefororganisationswhereauthorityanddecision-makingaredistributedthroughoutself-organisingteamsinsteadofrelyingon atypicalhierarchicalorganisationsetting.
5.3.1.Citizenshipanduserservices
Theintegrationofdigitaltechnologiesineverydayliferequiresmechanismsabletodetermineaccuratelywhotheusersare(Lee, 2018)andcertifytheirbasicattributeslikename,address,creditrecord,aswellasotherpersonalcharacteristics(Lemieux,2016; LeidingandNorta,2017;Augotetal.,2017;Buchmannetal.,2017).Therefore,digitalidentityhasbecomeacrucialsecurity measure(Riveraetal.,2017).In PaulDunphy(2018) theauthorsanalysethreedecentralisedidentitymanagementapproaches,
namelyuPort,ShoCardandSovrinandassesstheirbenefitsandshortcomings.Moreover,accordingto Roberts(2017),one-sixthof theworld’spopulationlackdocumentedproofoftheirexistence.Thissituationaffectsimmigrantsandrefugees,sincetheircountries mayoftenrefusetohandoverthedocumentsif,forinstance,theybelongtotheopposition.Therefore,blockchainbecomesan instrumenttoreinforceequalityandopportunitiestoworldwidecitizens.Formoreondigitalidentityandblockchain,onemayrefer to Riveraetal.(2017).
TheemergenceofTheInternetofAgreements(IoA)(Summit,2017),whichestablishestheconnectionbetweendigitalcontents (theInternet)andreal-worlddeals,contractsorregulations,enablesthenextgenerationofdigitalcommerce.Therefore,blockchain applicationsthatimplementSCstoverifymultipletypesofoperations,suchasindividualproperties,areusedtostatethecontractual relationshipsbetweentheInternetactors,beingthemcompaniesorindividuals(ChenandZhu,2017;Ishmaev,2017;Governatori etal.,2018;Herian,2017).Forinstance,Pavilion.io(Duhamel,2014)isablockchain-basedcompanythatprovidesanAPItoenablea verificationinterfacethateliminatestheneedfore-commercebuyerstoplacetrustinsellersorthird-partyproviders.Mattereum (Mattereum:SmartContracts,RealProperty,2017)isanIoAprojecttomanagelegalrightsoverphysicalandIPontheblockchain. Stampery(Stampery,2015)isacertificationcompanythatusesblockchaintocreateastampofemailsordocuments.Thissystem providesproof-of-existence(PoE),proof-of-ownership(PoO),proof-of-integrity(PoI)aswellasproofofreceiptbystoringthe transaction’sinformationinthepublicledger.IntheProofofExistence(ProofofExistence,2017)project,authorsuseblockchainto ensuretheexistenceofadocumentanditscreationdatewithoutrevealingitscontents.Likewise,VirtualNotary,Bitnotar,Blocksing, btcluck,andChronobituseblockchaintocertifythecontentsofdocumentssecurelyandverifiably(Swan,2015).Thus,wemayuse thesystemsabovetostoreproofsoftransactionsandoperationsmadebetweenindividualsand/orcompanies.Inthisregard,the increaseinonlinetransactions,suchasine-commerce,havecausedanupsurgeofdisputes.Duetotheubiquitousnatureofonline disputes,efficientconflictmanagementmustbeprovided,thatovercomescross-borderandinstitutionaloverheads.Themethodsand projectsmentionedabovepermitthecreationofefficientdisputeresolutionmethodssincetheinformationstoredintheblockchain canbeverifiedandaudited.Othermethodsandprojectsthatimplementfunctionaldisputeresolutionmechanismscanbefoundin Koulu(2016)andSwan,2015
5.3.2.Publicsector
Inthecaseofpublicservices,weconsiderthatvirtualnotary,PoE,PoO,PoI,reputationanddisputeresolutionaretypesof servicesthatcanbedevotedtocitizenswithouttheparticipationofofficialinstitutions.NotethatPoE,PoOandPoIarecloselyrelated andeasilyverifiableinablockchain.Governmentagenciesaroundtheworldarelookingforopportunitiesrelatedtotheadoptionof blockchaintechnologyinthepublicsector(DeloitteDevelopmentLLC,2017;Chiangetal.,2018),particularlyforutilisingthesecure, distributed,open,andinexpensivedatabasetechnologytoreducecostandbureaucracy,increaseefficiencyandforauthenticating manytypesofpersistentdocuments(Ølnes,2016;Nordrum,2017;ØlnesandJansen,2017;Ølnesetal.,2017).Otherblockchain applicationsinthepublicsectormayincludedocumentverification,e-residencyapproaches(SullivanandBurger,2017)thedevelopmentofmorereliableandtransparenttaxationmechanisms(Pokrovskaia,2017;Wijayaetal.,2017),thedevelopmentofmore robustregulatorycomplianceframeworks(FilippiandHassan,2016;Gerstl,2016;Engelenburgetal.,2017)andlandmanagement (Pichel,2016).
5.3.3.Voting
Forseveralyearse-votinghasbeenconsideredapromisingandinevitabledevelopmentwhichcouldspeedupvotingprocesses, simplifyandreducethecostofelections,andthedevelopmentofstrongerdemocracies(Boucher,2016).However,existingelectronic votingsystemsrelyonproprietaryandcentraliseddesignbyasingleentity,characteristicsthatharmthetrustandconfidencevoters havetothevotingprocess(MouraandGomes,2017).DecentralisedvotingsystemssuchasBitCongress(Deitz,2014)andLiquid Democracy(Schiener,2014)proposeframeworkstoenforcedistributeddecisionmaking.Futarchy(Hanson,2013)isavotingsystem whereparticipantsproposetopicsandpossiblestrategiestoachievetheminatwo-stepfashion.Moreconcretely,participantssupport policiesdependingonwhethertheprediction/bettingmarketsoptimisethegeneralrevenueforthem(e.g.benefitsincaseofaprivate companyorGDPinthecaseofacountry).Ingeneral,blockchaintechnologyoffersanopen-source,peer-to-peer,decentralisedand independentlyverifiablenetworktogaintheconfidencerequiredbyvotersandelectionorganisers(Noizat,2015;Kubjas,2017; Meter,2017;Hsiaoetal.,2018)whilebeingconsistentwithdomesticlegislation(SchulzandSchafer,2017).
5.4.Internetofthings
Around90%ofthedataintheworldtodayhasbeencreatedinthepasttwoyearsalone(IBM,2017).Suchgrowthpacewill increaseduetoa)theadventoftheInternetofThings(IoT),b)tothepopulationgrowth(Stats,2017).WhiletheexpansionpossibilitiesoftheblockchainandIoTtechnologiesarealreadyvastontheirown,thesymbioticrelationshipofthesetwofieldsarises myriadmore.Forinstance,thedistributedwirelesssensornetworks,whichdespitetheirdrawbacks(Pietroetal.,2014;Linetal., 2017)areoneofthepillarsoftechnologicalandhumanevolution,demonstratethatblockchainarchitecturemayenhanceIoTby minimisingitsdeficienciesandmaximisingitspotential(Kshetri,2017;Liaoetal.,2017;Buccafurrietal.,2017a;Fabiano,2017; ÖzyilmazandYurdakul,2017).
TheincreasingattentionandinvestmentsforimplementingdecentralisedIoTplatforms(SamaniegoandDeters,2016a;Novo, 2018;ZhangandWen,2017)aremainlydrivenbytheblockchaintechnologyanditsinherentcapabilities(Christidisand Devetsikiotis,2016).Themainideaistoprovidesecureandauditabledataexchangeinheterogeneouscontext-awarescenarios (Casinoetal.,2017)withplentyofinterconnectedsmartdevices(Crosbyetal.,2016).Moreover,operatinginanautomatedand
decentralisedfashionenablesthenetwork’shighscalabilityandefficientmanagement(Sharmaetal.,2018;LiandZhang,2017; Sakakibaraetal.,2017).
Blockchaininteroperabilityenablesindependentandsecurereal-timepaymentservices,enhancingtraditionalcommerce,ecommerceorpublicandprivatetransportationsystems(ChristidisandDevetsikiotis,2016).ThereareseveralexamplesofapplicationsthatagglomeratethesecharacteristicssuchastheFilecoin(Benet,2014),whichisamemorystorageprovider,orthe EtherAPIs(EtherAPIs,2016),whichenablesAPIcalls’monetisation.Inthefuture,IoTdevicescouldbedirectlylinkedwiththeir cryptocurrency-basedbankaccount(ChristidisandDevetsikiotis,2016)sothatmicrotransactions(Passetal.,2015)couldbeperformedinexchangeforservices(Huckleetal.,2016;Hwangetal.,2017),whilesimilarapproachesmayalsobeappliedtothesmartgriddomainforallowingtheenergysale(Rutkin,2016;Lietal.,2017b).Inthecaseofprovenanceorsupplychains,distributed networksofRFIDsensorsenabletheautomatedprocessingofproductsinmultiplecontexts,suchasinfoodsupplychains,transportationservicesorinventorymanagement(Liuetal.,2017;Shafaghetal.,2017).Inthesecontexts,theinformationmonitoredby thedevicescouldbestoredintheformofSCsortransactionsintotheblockchain.AnexampleofaP2PdistributedIoTsystemcanbe foundin(FoundationsfortheNextEconomicRevolution,2016).Theimplementationofblockchain-basedIoTsolutionscouldsolve severalissues,suchasthehighmaintenancecostofcentralisedapproaches(ChristidisandDevetsikiotis,2016;Bottaetal.,2016). Moreover,adecentralisedandsecureP2PmodelcouldincreasethesecurityofIoTandwirelesssensornetworks(Pietroetal.,2014; Dazaetal.,2017;Ouaddahetal.,2016;Ouaddahetal.,2017),enablingahighercontrolofIoTdevicesforkeepingsystemsup-todate(LeeandLee,2017;SamaniegoandDeters,2016b;Boudguigaetal.,2017;Samaniegoetal.,2017;SamaniegoandDeters,2016).
Undoubtedly,therearesomeissues,suchaslowcomputationalpowerandstoragecapabilitiesofIoTdevices,thatmaylimitthe useofBlockchain.In Buccafurrietal.(2017a) theauthorsproposeanalternativewaytoimplementapublicledgerovercomingthese drawbacksandthusenhancingIoTapplications.Otherefficientarchitecturesarepresentedin Dorrietal.(2017a)andDorrietal. (2017b) whereauthorsproposeasecurelightweightblockchain-basedarchitectureforIoTindifferentapplicationcontexts.
ThereexistotherexamplesofIoTapplications,suchastheAutonomousDecentralisedP2PTelemetry(ADEPT)(IIBM,2015) systemdevelopedbytheIBMwhichusesblockchaintobuildadistributednetworkofdevices.Filament(FoundationsfortheNext EconomicRevolution,2016)ensuressecureeconomicexchangeamongautonomousdevices.Moreover,theauthorsassignaunique identitytoeachdevicethroughablockhain-basedIoTsoftware.Inthesameline,Huhetal.(Huhetal.,2017)proposestheuseofthe EthereumplatformtoperformsecurekeymanagementinIoTcontexts.FormoreontheIoTandblockchain,wereferthereaderto (ChristidisandDevetsikiotis,2016;KhanandSalah,2017;Conoscentietal.,2016;KravitzandCooper,2017).
5.5.Healthcaremanagement
Blockchaintechnologycouldplayapivotalroleinthehealthcareindustrywithseveralapplicationsinareaslikepublichealthcare management,longitudinalhealthcarerecords,automatedhealthclaimsadjudication,onlinepatientaccess,sharingpatients’medical data,user-orientedmedicalresearch,drugcounterfeiting,clinicaltrial,andprecisionmedicine(Mettleretal.,2016;Petersonetal., 2016;Chamberofdigitalcommerce,2016;Ahrametal.,2017;AlOmaretal.,2017;Capgemini,2017;EmrifyInc.,2017;Freed Associates,2017;Shaeetal.,2017;Zhaoetal.,2017;Mamoshina,2018;Mytis-Gkomethetal.,2017;BorioliandCouturier,2018;Lee andYang,2018;Xiaetal.,2017a,2017c;Yueetal.,2016;Patel,2018;Junejaetal.,2018).Inparticular,blockchaintechnologyand theuseofSCscouldsolveproblemsofscientificcredibilityoffindings(missingdata,endpointswitching,datadredging,andselective publication)inclinicaltrials(Nugentetal.,2016)aswellasissuesofpatients’informedconsent(BenchoufiandRavaud,2017; Benchoufietal.,2017).
Managingpatients’ElectronicHealthcareRecords(EHRs)isprobablytheareawiththehighestpotentialgrowth(Liu,2016; Angraaletal.,2017;Hoy,2017;Kuoetal.,2017;Baxendale,2016).AnEHRcontainsapatient’sshortmedicalhistory,aspartofher medicalrecord,aswellasdata,predictions,andinformationofanykindrelatingtotheconditionsandtheclinicalprogressofa patientthroughoutthecourseofatreatment.AblockchainsystemforEHRscouldbeseenasaprotocolthroughwhichusersmay accessandmaintaintheirhealthdatathatsimultaneouslyguaranteessecurityandprivacy(Azariaetal.,2016;Young,2016;BurstIQ, 2017;Dubovitskayaetal.,2017;Sullivan,2017;Medicalchain,2017;Center,2017;Xiaetal.,2017b).ThebenefitsofablockchainbasedsystemforEHRsaremanifold:recordsarestoredinadistributedway(theyarepublicandeasilyverifiableacrossnon-affiliated providerorganisations),thereisnocentralisedownerorhubforahackertocorruptorbreach,dataisupdatedandalwaysavailable whereasdatafromdisparatesourcesisbroughttogetherinasingleandunifieddatarepository(GreyHealthcareGroup,2017).
5.6.Privacyandsecurity
Centralisedorganisations–bothpublicandprivate–amasslargequantitiesofpersonalandsensitiveinformation.Althoughthe GDPR(Parliament,2016)aimstoregulatetheprocessingofthisdata,thereisstillabiggaptocover(Politouetal.,2018).Blockchain isconsideredasanopportunityforenhancingthesecurityaspectsofbigdata(Puthaletal.,2018;Kshetri,2017;Cohenetal.,2017) anditsscalabilitywhencombinedwithotherefficientstoragesystemsthatimplementdataminingmethods(Bozicetal.,2016). Therefore,privacyandsecurityorientedapplicationsthatrelyonblockchaintechnologycanbefoundintheliterature(DiFrancesco Maesaetal.,2017;Dorrietal.,2017c;HariandLakshman,2016;Leeetal.,2017;Tangetal.,2018;Chansonetal.,2017;Anjum etal.,2017).
Namecoin(HaferkornandQuintanaDiaz,2015)isanopen-sourceblockchaintechnologythatimplementsadecentralisedversion ofDNS.ThemainbenefitsofadecentralisedDNSapproacharesecurity,censorshipresistance,efficiency,andprivacy.Alexandria (TheDecentralizedLibraryofAlexandria,2015)isanopen-sourceblockchain-basedprojectthatprovidesasecureanddecentralised
libraryofanykindofmediawhileallowingthefreedomofspeech.Bothsystemsmaybeenhancedutilisingdigitalidentityservices whichcanconfirmanindividual’sidentities(e.g.usingpseudonyms),enablingsecurityandanonymityinastandardisedverification model(Swan,2015;Zhangetal.,2017).In Zyskindetal.(2015a) theauthorsproposeadecentralisedP2Pblockchain-basedplatform thatcomprisesthreetypesofentities:(i)users,whichinteractwiththeapplications;(ii)services,whichprovidesuchapplicationsand processusers’personaldataforoperationalandbusiness-relatedreasons;and(iii)nodes,entitiesthatreceiverewardsinexchangefor maintainingtheblockchain.Sinceonlyhashpointersarestored,usershavecontrolovertheirdata.
Blockchaintechnologymayalsobeusedtoenhancesecurityandreliabilityindistributednetworksthroughhardwareand softwaresolutions(Fanetal.,2018;Chaetal.,2017;SuzukiandMurai,2017).Forinstance,SIRINLABS(Labs,2014)developedthe firstblockchain-basedsmartphone,capableofprovidingfast,fee-lessandsecuretransactions.BitAvisanantimalwareblockchainbasedsolution(Noyes,2018)thatenhancesviruspatterndistribution.In Axon(2015),theauthorsimplementaprivacy-awarepublic keyinfrastructure,whichenhancessecurityagainstasinglepointoffailureormaliciousattacks. Liangetal.(2018) proposetheuseof adistributedblockchain-basedprotectionframeworktoenhancethesecurityofmodernpowersystemsagainstcyber-attacks.In Xu etal.(2017c),authorsrecalltheuseofDockercontainers(Docker,2013)inIoTandtheirbenefits. Rodriguesetal.(2017) proposea novelarchitecture,whichcombinesblockchainandSCtechnologies,introducingtherebynewopportunitiesforflexibleandefficient DDoSmitigationsolutionsacrossmultipledomains,withparticularregardoninsecureportableandstationarydevices. Toshetal. (2017) discussvulnerabilitiesinblockchaincloudanditscapabilitytoenabledataprovenance.Blockchaincouldalsobeusedasa verificationprotocolforenabling,securingandauthenticatingspectrumsharingincognitiveradionetworks(KotobiandBilen,2017; Rajuetal.,2017;Niuetal.,2017).
Transactionalprivacyisoneofthemostchallengingproblemsofblockchaintechnologies.Therefore,severalmethodshavebeen proposedtoimproveanonymityofblockchains(Zhengetal.,2016)suchasmixingservices(Möseretal.,2013)orzero-knowledge proofs.Inthecaseofmixingservices,theaimistoprovidetransactionalprivacybytransferringfundsfrom N inputaddressesto M outputaddresses,sothatusersavoidalwaysusingthesameaddress.ExamplesimplementingsuchtechniqueareMixcoin(Bonneau etal.,2014),whichisalsoabletodetectdishonesttransactionbehaviours,andCoinjoin(Maxwelletal.,2013)orCoinShuffle (Ruffingetal.,2014),thatusesathirdpartytoshuffleoutputaddresses.InthecaseofZerocoin(Miersetal.,2013),transactionsand theoriginofcoinsareunlined,whileminersusezero-knowledgeproofstovalidateoperations.Animprovedversionwithstronger privacyguaranteeshidesbothtransactionamountsandtheoriginofcoins(Miersetal.,2013).
5.7.Businessandindustrialapplications
Blockchainhasthepotentialtobecomeasignificantsourceofdisruptiveinnovationsinbusinessandmanagementthrough improving,optimising,andautomatingbusinessprocesses(TapscottandTapscott,2017;Bogneretal.,2016;Yingetal.,2018).Many e-businessmodelsbasedonIoTandblockchainareemerging.Oneexamplecanbefoundin ZhangandWen(2015) whereauthors proposeabusinessmodelinwhichtransactionsbetweendevicesareperformedusingSCsonablockchain-baseddistributeddatabase. In HardjonoandSmith(2016) theauthorsproposeaprivacy-preservingsystemthatusesanIoTnetworkandblockchaintoprove provenancemanufacturingwithoutthethirdpartyauthentication.
Blockchainapplicationsappeartoofferconsiderableperformanceenhancementandcommercialisationopportunities(White, 2017;Klemsetal.,2017;Kogureetal.,2017),improvingcredibilityine-commerceandenablingIoTcompaniestooptimisetheir operations(Xuetal.,2017b;YooandWon,2018)whilesavingtimeandcost(IBMCorporation,2016).Blockchain-basedapplications couldserveasdecentralisedbusinessprocessmanagementsystemsforseveralenterprises.Insuchcases,eachbusinessprocess instancemaybemaintainedontheblockchain,andtheworkflowroutingcouldbeperformedbySCs,therebystreamliningand automatingintra-organisationalprocessesandreducingcost(Weberetal.,2016;López-Pintadoetal.,2017;Prybila,2017;Rimba etal.,2017;Mendlingetal.,2018).
5.7.1.Supplychainmanagement
Blockchaintechnologyisexpectedtoincreasetransparencyandaccountabilityinsupplychainnetworks,thusenablingmore flexiblevaluechains(Ahrametal.,2017;Kshetri,2017;Kshetri,2018;O’Leary,2017).Inparticular,blockchain-basedapplications havethepotentialtogeneratebreakthroughsinthreeareasinsupplychains:visibility,optimisation,anddemand(IBMCorporation, 2016).Blockchaincanbeusedinlogistics,identifyingcounterfeitproducts,decreasingpaperloadprocessing,facilitatingorigin tracking(HackiusandPetersen,2017;Kennedyetal.,2017;LeeandPilkington,2017;Toyodaetal.,2017;Tanetal.,2018)and enablingbuyersandsellerstotransactdirectlywithoutmanipulationbyintermediaries(Subramanian,2017).Moreover,ithasbeen demonstratedthattheusageofblockchain-basedapplicationsinsupplychainnetworkscansafeguardsecurity(Dorrietal.,2017a), leadtomorerobustcontractmanagementmechanismsbetweenthirdandfourthpartylogistics(3PL,4PL)forcombatinginformation asymmetry(Polimetal.,2017),enhancetrackingmechanismsandtraceabilityassurance(ApteandPetrovsky,2016;Tianetal., 2016;DüdderandRoss,2017;HeberandGroll,2017;LuandXu,2017;Tian,2017),providebetterinformationmanagementacross theentiresupplychain(InfosysLimited,2017;O’Learyetal.,2017;TurkandKlinc,2017),foodsafety(AhmedandBroek,2017), enhanceIPprotection(HerbertandLitchfield,2015;Hollandetal.,2017;Tsaietal.,2017),offerbettercustomerservicethrough advanceddataanalytics(i.e.encryptedcustomerdata)andnovelrecommendersystems(Freyetal.,2016a;Freyetal.,2016b), improveinventoryandperformancemanagementacrosscomplexsupplychains(MadhwalandPanfilov,2017),andfinally,itcan improvesmarttransportationsystems(YuanandWang,2016;Leietal.,2017;Leidingetal.,2016)andoffernewdecentralised manufacturingarchitectures(SyncFab,2018).
5.7.2.Energysector
Thepotentialapplicationsofblockchainintheenergysectorarefar-reachingandmayhaveanenormousimpactbothintermsof processesaswellasplatforms(Bilaletal.,2014).Forexample,blockchainmayreducecostsandenablenewbusinessmodelsand marketplaces,canbettermanagecomplexity,datasecurity,andownershipalonggrids,canengageprosumersintheenergymarket actingasenablerforthecreationofenergycommunities(Mengelkampetal.,2018;Wuetal.,2017;Danzietal.,2017),canenhance thetransparencyandtrustoftheenergymarketsystem,canguaranteeaccountabilitywhilepreservingprivacyrequirements,can enhancedirectpeer-to-peertradingtosupportthesmoothoperationofthepowergrid,andcanbetterhandledemandresponseand provideaframeworkformoreefficientutilitybillingprocessesandtransactiveenergyoperations(DeutscheEnergie-AgenturGmbH, 2016;Ioannisetal.,2017;PricewaterhouseCoopersandWirtschaftsprungsgesellschaft,2017;EnergyWebFoundation,2018; KyriakarakosandPapadakis,2018).Blockchaintechnologymayalsobeusedforissuingcertificatesoforigin,particularlyforgreen energyproductionandrenewableenergysources(Castellanosetal.,2017;Tanakaetal.,2017;Houetal.,2018;Parketal.,2018; Patiletal.,2018),fordevelopingpeer-to-peerenergytransactionsschemes(Chengetal.,2017;Imbaultetal.,2017;Mylreaand Gourisetti,2017a;MylreaandGourisetti,2017b;Sikorskietal.,2017;Popetal.,2018)andforestablishingenergymanagement schemesforelectricvehicles(Kimetal.,2017;Knirschetal.,2018;Knirschetal.,2017;Huangetal.,2018).Itisalsoworth mentioningthatblockchainisconsideredanenablerforthedecarbonisationoftheenergysectorfacilitatingitsmovetowardsmore decentralisedenergysources(WorldEnergyCouncilandPricewaterhouseCoopers,2018).
5.8.Education
Blockchaincansolveissuesofvulnerability,security,andprivacyinthecaseofubiquitouslearningenvironments(Bdiwietal., 2017)andcanbeusedforstoringeducationalrecordsrelatedtoreputationalrewards(SharplesandDomingue,2016a;Turkanović etal.,2018). SharplesandDomingue(2016b) proposetheuseofablockchain-baseddistributedsystemforeducationalrecordand reputation.Similarreputationsystemsareshownin Carboni(2015)andDennisandOwen,2015.In Devine(2015),teachersadd blocksintotheblockchainstoringthelearningachievementsofstudents.Educationalcertificatemanagementcanalsobeenhanced byblockchainimprovingdatasecurityandtrustindigitalinfrastructures(Xuetal.,2017d),andforcreditmanagement(forinstance, relevanttotheEuropeanCreditTransferandAccumulationSystem-ECTS)(Turkanovićetal.,2018).Moreover,blockchain-based applicationscouldenhancethedigitalaccreditationofpersonalandacademiclearning(Grechetal.,2017).Blockchain-enabled schoolinformationhubscouldalsobeestablishedforcollecting,reporting,andanalysingdataaboutschoolsystemsforsupporting decision-making(Boreetal.,2017).Finally,inthecaseofscholarlypublishing,blockchaincanbeusedeitherforbetterhandling manuscriptsubmissionsandforconductingsuitablereviewsinatimelyfashion(Spearpoint,2017)orformanuscriptverification (Gippetal.,2017).
5.9.Datamanagement
Datamanagementisoneofthemostindisputablepropertiesoftheblockchain.Implementationsandapplicationsbasedonthis technologyhavenotonlyenhanceddatamanagement(AsharafandAdarsh,2017)buthavealsofacilitatedbydefaultauditability (SuttonandSamavi,2017;Neisseetal.,2017)sincealloftheiroperationsareverifiable.Inthislastblockchain-basedapplications sectionweciterelevantliteraturethataimsatefficient,secureandverifiabledatamanagement(Zhang,2016;Jinetal.,2017).
Althoughcross-organisationalmanagementhasnotyetreachedalevelthatenablesfullinteroperabilitybetweenparties,several examplesofcross-organisationaldatamanagementcanbefoundintheliterature.In Fridgen(2018) theauthors,inajointeffortwith aGermanBank,followtheDesignScienceResearchapproach(HevnerandChatterjee,2010)todesign,implement,andevaluatea blockchainprototypeforcross-organisationalworkflowmanagement.TheresultsareencouraginganddemonstratethatBlockchain hasthepotentialtoserveasaninfrastructureforcross-organisationalworkflowmanagement.Hawk(Kosbaetal.,2016)isaframeworkforbuildingprivacy-preservingSCsthatenablesprivacy-awareintermediatecomputationstoavoidorminimiseseveral typesofdisclosures,suchastransactionalprivacy.Authorsalsoprovideanalgorithmicframeworktoenablecodingfunctionsthat willbeparsedintroprivateandblockchaincompliantprotocols.
Blockchainalsodisruptsthehumanresourcearea(Ahmed,2018;O’Learyetal.,2017;Wangetal.,2017),byenhancingdata storage(Wangetal.,2018)andselectionprocesses,e.g.auditablecandidateselectionandverifiableparticipants’data.
Inthecaseofsecuredatadistributionandmanagementsolutions, García-Barriocanaletal.(2017) proposetheuseofadecentralisedblockchain-basedsolutionformetadatasupportingkeyfunctionsanddiscussitsimplicationstowardsmanagementand sustainabilityofdigitalarchives. Yangetal.(2018) stresstheimportanceoftrustinthebigdataareaandpresentacrediblebigdata sharingmodelbasedonblockchaintechnologyandSCtoensurethesafecirculationofdataresources. DoandNg(2017) proposea systemthatenablessecureanddistributedclientdatamanagementusingcryptographicprimitivesaswellasakeywordsearch service.Besides,thedataownercangrantsearchandreadpermissionoftheirdatatothirdparties.Similarly,Searchain(Jiangetal., 2017)isablockchain-basedkeywordsearchsystemthatenablesefficientoblivioussearch(theuserknowsthechosenkeywordand thecorrespondingciphertext,buttheyareunknowntothedatasupplier)overanauthorisedkeywordsetinthedecentralisedstorage. Moreexamplescanbefoundin Zyskindetal.(2015a)andAzariaetal.(2016) inwhichsystemsthatenableblockchain-based decentralisedsensitivedatadistributionwithPoOaredescribed.Othersecuredatasharingapproachescanbefoundin Hull(2017), Fukumitsuetal.(2017),Kiyomotoetal.(2017),Hasnainetal.(2017)andKarafiloskiandMishev(2017b).Notethataccesscontrol andauthenticationmechanismsmayalsobeusedtoensureprivacyandsecurityindatadistribution(Kalraetal.,2017;Lietal., 2017c).
Cloud-baseddecentralisedandefficientsolutionsthatuseblockchaintechnologycanalsobefoundintheliterature(Shettyetal., 2017;Gaetanietal.,2017;Liangetal.,2017).Suchsystemsaimatovercomingbigdatachallenges(KarafiloskiandMishev,2017b; Yueetal.,2017)toenabletheanalysisoflargevolumesoftransactions(Abdullahetal.,2017;Xuetal.,2018;ChenandXue,2017).
5.10.Miscellaneousapplications
Thissubsectionreferstoresearchdescribingblockchain-basedapplicationsthatfalloutsidethedomainsmentionedabove.For example,crowd-fundingplatformsarestartingtouseblockchain(BracamonteandOkada,2017;Buccafurrietal.,2017b;Zhuand Zhou,2016).Swarm,Lighthouse,andbitFylerareexamplesofcryptocurrencycrowdfundingplatforms(Swan,2015;Lietal.,2017). Blockchainapplicationsmayalsobefoundinthehumanitariansectorandphilanthropy(Mazet,2017),particularlyasameansof fightingpoverty(Kewelletal.,2017;Kshetri,2017;Pilkingtonetal.,2017;Zhouetal.,2017;Larios-Hernández,2017;Jayasinghe etal.,2017;Accenture,2017b;KoandVerity,2016).Blockchaincanalsobeusedtobuildintelligent,secure,distributedand autonomoustransportsystemsinsmartcitiescontexts(Marsal-Llacuna,2017;Sharmaetal.,2018;Adametal.,2017)ortomanage eventticketssecurely(Tackmann,2017).Blockchainisexpectedtoplayapivotalroleinenvironmentalmanagement(Saberietal., 2018;Khaqqietal.,2018).Forinstance,blockchaincouldbeusedasanovel“emissionlink”systemwithinEmissionTrading Schemes(Fuetal.,2018).Anotherinterestingapplicationmaybefoundinthecontextofsocialmedia(FuandLiri,2016;Sarretal., 2015;deSoto,2017).Inparticular,user-centricblockchainapplicationscouldenableend-userstocontrol,traceandclaimownership ofeverypieceofcontenttheyshare(ChakravortyandRong,2017).OfparticularinterestaresomeIT-orientedblockchainapplicationslike,forexample,edgecomputingandtheestablishmentofcomputationalresourcesharingsystems(Hongetal.,2017; Stanciu,2017),gridcomputing(GattermayerandTvrdik,2017),cloudcomputing(Xiangetal.,2017),andtheuseofblockchainasa softwareconnector(Xuetal.,2016;TeslyaandSmirnov,2018).Finally,blockchaintechnologymayalsoimprovesocialsharing dynamics(Pazaitisetal.,2017).
6.Openissuesandfuturetrends
Fromtheanalysisoftheselectedliterature,aseriesofinsightscanbederivedconcerningthelimitationsoftheblockchain technologyanditsusabilityacrossawideareaofdomains.AsdescribedinSection 5,blockchainisnowadaysadoptedinmany researchfieldsandbusinessareas,providinglimitlessopportunitiesforexploration.However,likeanyotheremergingtechnology, issuesandchallengesarise.Inthissection,wediscusscertainlimitationsoftheblockchaintechnology,andwedevelopseveral avenuesoffruitfulareasforfurtherresearchdirections.
6.1.Suitabilityofblockchain
Companiesacrossdifferentsectorsareexcitedaboutblockchaintechnologyanditspotentialtodrivetheirdigitaltransformation whilesolvingreal-lifeproblems(Umeh,2016).Nevertheless,whileseveralITspecialistsenvisagetheusageofblockchaininalmost everyproject,theydonotquiteunderstandthefundamentalreasonsforusingit,particularlyfromadatamanagementperspective. Forinstance,ifnodataneedstobeeverstored,blockchainwillnotaddanyvaluetoalreadyestablishedtechnicalsolutions.Similarly,ifonlyonewriterinagivensystemisforeseen,ablockchainwillnotprovideadditionalguaranteescomparedtoaregular databasewhichwouldmostprobablybeamoreappropriatechoice,particularlyfromaperformanceperspective(transactionsspeed) (Greenspan,2015c).Ontheotherhand,blockchainissuitablewhenonerequiresatransactionbetweentrustlesssourcesora permanenthistoricalrecord.Forinstance,ifthereisaneedformultiplemutuallymistrustingentitiestointeractandchangethestate ofasystem,thenblockchainmaybeaviablesolution(WüstandGervais,2017).
Therefore,beforeadoptingblockchain-enabledsolutionsoneshouldexaminethesuitabilityoftheblockchaintechnologyagainst theusecasesrequirements(Loetal.,2017).Alimitednumberofframeworkshavebeendevelopedinthescientificliteraturefor assessingthesuitabilityofblockchain-enabledapplications.Forexample,in Loetal.(2017) theauthorsproposeanevaluation frameworkforblockchain-enabledapplicationsinspecificindustrialdomainslikesupplychain,EHRs,identitymanagement,andthe stockmarket.In WüstandGervais(2017) ananalysisisprovidedrelatedtothepropertiesofdifferentblockchaintypes(i.e.,permissionedandpermissionless)andamethodologicalframeworkisdevelopedforidentifyingthesuitabilityofblockchain-enabled applicationsacrossseveraldomains.
Databasesarebytheirverynature mutable whereapredefinedsetofentitieshaveaccessandmayinsertorupdatedata.These entitiesmayhavespecificroles,buttheiridentitiesareknown.However,thereareadministrativeroleswhichmaycompletelyalter thecontentsandstructureofthehostedinformationregardlessofwhethertheyarecentralisedornot.
Basedonthefindingsofourresearch,wehighlighttherequirementsofeachsector,see Table3,andwedevelopedaframework (Table4)toevaluatethesuitabilityofblockchain-basedsolutions.Moreconcretely,weevaluatethepotentialofblockchainagainst traditionaldatabasesinfourmaindomainareas:requiredtrustassumptions,contextrequirements,performancecharacteristicsand requiredconsensusmechanisms.Anintuitivethree-levelscale(i.e.,low,mediumandhigh)isusedtomeasuretherelevanceofeach prerequisite.Theframeworkactsasacomprehensivetoolforpractitionersaspiringtoevaluatewhethertheirsystemswillbeenhancedbyblockchainornot.Intermsoftrustness,blockchainavoidstheuseoftrustedthirdparties,onwhichdatabasesrelyon,and thus,enhancesreliabilityandverifiabilityofcontents.Blockchainisalsosuitablewhentransactionsandoperationsneedtobetraced (sequentialchainofevents)orwhenoperationsrequirestrongsecurityandprivacy(centraliseddatastructuresaremorevulnerable tomaliciousattacksthandecentralisedstructures(Zyskindetal.,2015b)).Regardingmaintenance,blockchainmayprovidea
Table3
Characteristics/requirementsthatenable/requireeachfamilyofblockchainapplications.Check(✓)denotesthatthisrequirementismandatory while denotesthatitdependsonthecase.
Table4
Analysisofattributesandprerequisitesofblockchainversustraditionaldatabases.
AttributesPrerequisites&determinants
significantcostreductionsinceitdoesnotrequirehosting.Finally,consensusmechanismsimplementedinblockchainnetworks (NguyenandKim,2018)enablemultiplewriterstomodifythedatabaseandprovideanauthoritativetransactionloginwhichall nodesprovablyagree.
6.2.Latencyandscalability
Mostcryptocurrencieshavealowtransactions’rate.Forinstance,Bitcointransactions2 cannotbyanychancecomparetosystems likeVISA’screditcardprocessingnetworkthatconstantlyhandlesupthousandsoftransactionspersecond.Undoubtedly,thebroad adoptionofcryptocurrenciesneedstoaddressthislatencyissueaswell(Swan,2015).NotethateachBitcoinblockisprocessedin approximately10minwhich,alongwiththeassociatedsecuritychecks(e.g.toavoidthedouble-spentattackinthesubsequent transactions),resultsineachtransactionconfirmationtolastuptoseveralminutes3.Therefore,blockchainarchitecturesfaceserious latencyissueswhichmaybeprovedmoresignificantastheyevolve.Privateblockchains,ontheotherhand,althoughtheyareindeed farmoreefficient,theyhavenotreachedtherequiredstandards.
Datastorageoptimisationexamplesmayalsobefoundintheliterature.In Bruce(2013),authorsproposeaschemewhereold transactionrecordsareremovedbythenetworkandatree-structureddatabasebalancesallnon-emptyaddresses.Hence,thenumber oftransactionsstoredbythenodesisdecreased,therebyimprovingthetransactionvalidationstep.In Eyaletal.(2016),theauthors proposedBitcoin-NextGenerationwherethecoreideaistodecoupleablockintotwoparts:thekeyblockforleaderelectionand
2 https://blockchain.info/charts/transactions-persecond
3 https://blockchain.info/charts/avg-confirmation-time.
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Dorat, Jean, 72, 163
Drama, 12, 87, 133-154, 190
Dramaturgy, 6, 134, 137
Dryden, 13, 132
Du Bellay, seeBellay
Duetrattati(Camillo), 55
Eclogues, 81 ff.
EducationofChildren(Montaigne), 233
Egger, Émile, 21n, 23n, 202
ElizabethanCriticalEssays(Smith), 180
Elizabethan drama, 12, 133, 144 ff., 152, 190
Elocutio, 54
Elyot, Sir Thomas, 227, 231
Empedocles, 184
Encomium, 55, 72, 75, 90, 111, 112, 131, 188
Endymion(Lyly), 152
English language, 36-38
English lyric, 66
Epithalamion(Spenser), 74, 77
Erasmus, 17, 40n, 42, 49, 54, 57, 63, 83, 98, 173, 204, 226, 231
EssayonSatire(Dryden), 132
Essays, 223-39
Estrif, 42
Étienne (printers), 9
Étienne, Henri, 21, 23, 34
Etymologiae(Isidore of Seville), 10
Euphues(Lyly), 200
Euphuism, 198, 200, 222
Euripides, 53, 61, 138, 139, 144, 165
Euryalus, 157
Eusebius, 39
Everaerts, Jan, seeSecundus
Exempla, 131, 194, 196, 231, 232
Exordium, 60
Fabliau, 109, 120, 134, 190, 194, 196
Fabri, Pierre, 68
Fabricadelmondo,Della(Alunno), 31
FaerieQueene(Spenser), 77, 123, 128-32, 152
Fenton, 198
Fiammetta(Boccaccio), 60
Fichet, Guillaume, 7
FonsBandusiae, 72
Fracastoro, Girolamo, 162, 163, 170, 179
FrameoftheWorld,The(Alunno), 31
French language, 31-36
Gaguin, Robert, 7
Gallus, Cornelius, 87
Garnier, Robert, 53, 139-44, 150, 151, 153
Gascoigne, George, 180
GawainandtheGreenKnight, 96
Genealogiadeorumgentilium(Boccaccio), 23
Geoffrey of Monmouth, 9
George of Trebizond, 53
Georgics(Vergil), 165
Gerlandia, Johannes de, 15
Gerusalemmeliberata(Tasso), 7, 53, 123, 124-27
GestesensemblelaviedupreulxchevalierBayard(Champier), 93
Giovanni di Virgilio, 82
Giraldi Cinthio (Giovan-Battista Giraldi), 195-97
Golden Age, 87, 89
GoldenAss(Apuleius), 93
GoldenLegend, 9
Googe, 181
Governour,The(Elyot), 227
Grandeetvraieartdepleinerhétorique(Fabri), 68
Gray, Thomas, 74
Greek dramaturgy, 6, 134, 137
Greek exiles in Italy, 20
Greek language, revival, 5 ff., 19-27, 35
Greek Literature, 7, 78, 79, 90, 137, 145, 179, 188
Greek oratory, 40
Greene, Robert, 201
Grotesque, 101, 106
Gryphius, 9, 13
Guarini, 148 ff.
Guarino da Verona, 20, 210
Guicciardini, 228
Gyroneilcortese(Luigi Alamanni), 96
Harvey, Gabriel, 183
Hecatommithi(Giraldi Cinthio), 195-97
Heinsius, 138
Heliodorus, 79, 188
HenryV(Shakspere), 146
Heptameron(Marguerite de Navarre), 14, 194
Hermes Trismegistus, 20
Hermogenes, 21, 55, 63, 79, 160, 170, 171, 188
Herodotus, 92, 214
Hesiod, 181
Hippocrates, Aphorisms, 211
Hippolyte(Garnier), 141
Historiarumflorentini(Bruni), 214-17
History, 213-22
History plays, 144-46
HistoryofKingRichardtheThirde,The(More), 217-20, 226
Homer, 7, 20, 22, 58, 96, 156, 157, 160, 165, 168, 173, 184, 185
Hooker, Richard, 52, 220
Horace, 65, 71, 72, 73, 86, 87, 114, 131, 156, 160, 161, 166, 168, 169, 170, 174, 179, 180, 181, 182, 189; “Ars poetica,” 10, 15, 133, 155, 158, 161, 163, 164, 165, 171, 186, 188
Howard, Henry, Earl of Surrey, 13, 37, 67, 180, 181
Hugh of St. Victor, 9
Humanism, 14, 17-19, 30, 39, 71, 82, 137, 138
Humanistic Latin tragedy, 139
Humbert, Louis, 34n
Hypnerotomachia(Colonna), 25, 202, 209
Ideadeltheatro,L’(Camillo), 56
Ideas(Hermogenes), 55
IdyllsoftheKing(Tennyson), 93
Imaginary addresses, 40
ImitatioChristi, 52
Imitation, 19, 39-90, 187
Imitationepoetica,Della(Partenio), 169-71
Inventio, 53, 54, 60, 63, 176
Isidore of Seville, 10
Isocolon, 200
Isocrates, 21
Isopet, 10, 81
Istoriefiorentine(Macchiavelli), 220-22, 224
Italian language, 27-31, 35
Italian orations, 40
Jardindeplaisance,Le, 10
Jean d’ Arras, 95
Jephthes(Buchanan), 138, 139
JerusalemDelivered(Tasso), 7, 53, 123, 124-27
John of Salisbury, 8, 42, 45
Johnson, Samuel, 42, 212
Jonson, Ben, 89
Juifves,Les(Garnier), 143
Junta, 9
Kelso, Ruth, 162
Labé, Louise, 13, 71
Ladislaus, King, 39
Lancaster, H. C., 142n
Landriani, Gherardo, 44
Latin language, 6, 7, 14, 17, 26, 27, 46, 155, 216, 232
Latin literature, 5, 17-19, 39, 41, 45, 46, 65, 79, 134, 138, 139
Lemaire, Jean, 31, 65, 68, 69, 71
Letters, 39, 41, 42
Libanius, 21
LiberTheoduli, 82
Livy, 21, 63, 213, 214
Lodge, 179n
Longueil, Christophe de (Longolius), 46, 50, 57, 59
Longus, 23, 79
Loyola, 210
Lucan, 10, 79, 174
Lucian, 93, 226
Lucretius, 26, 58, 87, 236
Lumby, J. R., 217
Lydgate, John, 67
Lyly, John, 38, 152, 153, 181, 200
Macchiavelli, 10, 12, 178, 213, 216, 220-24, 229, 231, 232
Macrobius, 188
Maggio, Vicentio, 159
Mair, G. H., 62
Malory, Sir Thomas, 11, 36, 37, 91-99, 103, 105, 107, 130
Manilius, 58
Mantuan (Baptista Spagnolo), 82, 88, 89, 180
Manuals, 15, 53
Marc-Antoine(Garnier), 141
Marguerite de Navarre, 14, 194
Marionettes, Carlomagno, 100
Marlowe, Christopher, 137, 146
Marot, Clement, 65, 67, 70, 75, 90
Martial, 70
Martianus Capella, 10
Matthieu de Vendôme, 10
Medea(Euripides), 138, 139
Medici, Cosimo dei, 12
Medici, Lorenzo de’, 147n
Melanchthon, 179, 228
Melodrama, 12, 144
Mélusine(Jean d’ Arras), 9, 95
Memoria, 54
Menander, 170
MerchantofVenice(Shakspere), 146
Metamorphoses(Ovid), 185
Methodusadfacilemhistoriarumcognitionem(Bodin), 228
Metric, 180
Michael Angelo, 29
MidsummerNight’sDream(Shakspere), 153
Migne, 42n
Mignonne(Ronsard), 77
Milton, 132, 144
Minturno, 13, 15, 44n, 164-69
Miracle, 133, 134, 135
Miracle plays, 7
Montaigne, 18, 23, 178, 232-39
Montano, Cola, 222
Montemayor, 186n
More, Sir Thomas, 17, 37, 217-20, 226
Morgantemaggiore(Pulci), 100-102
Morted’Arthur(Malory), 92, 98-99
Mortier, Alfred, 152n
Muret, Marc Antoine, (Muretus), 41, 50, 179
Muzio, 161-62
Mystère, 133, 134, 135
Mythologiae(Conti), 23
Mythology, 23, 77, 87, 147, 198
Naenia(Pontano), 66
Narratio, 60, 64, 220
Narrative, prose, 190-222
Nativity(York), 137
Navagero, 67, 170
Nemesianus, 87
Neo-Platonists, 7, 20
Nepos, Cornelius, 93
Nicoli, Nicolao, 12
“Nizolian paper books,” 46
Novella, 14
Novelle(Bandello), 190-94, 198-99
ObservationsonPetrarch(Alunno), 31
ObservationsontheVernacular(Dolce), 30
OdetoMicheldel’Hospital(Ronsard), 73
OdetotheKingonthePeace(Ronsard), 73
Odes, Pindaric, 72 ff.
Odyssey(Homer), 185
OldWives’Tale(Peele), 152
OratioinexpositioneHomeri(Poliziano), 22
Orationesclarorumvirorum, 40
Orationesobscurorumvirorum, 40n
Orations, 39, 40, 41, 61
Orator(Cicero), 44, 45
Orfeo(Poliziano), 147
Orlandofurioso(Ariosto), 111-23, 142, 168
Orlandoinnamorato(Boiardo), 93, 102-11
Ovid, 23, 58, 65, 71, 72, 79, 86, 87, 160, 174, 180, 181, 185
Padelford, F. M., 171n
Pagan terms used for Christian religion, 49
Pageantry, 87, 133
PaladinidiFrancia,I, 100
Pantagruel(Rabelais), 207, 211
ParadiseLost(Milton), 132
Parison, 200
ParlementofFoules(Chaucer), 70
Parody, in Rabelais, 204
Paromoion, 200
Partenio, Bernardino, 169-71, 188
Partitionesoratoriae(Brocardo), 63
Pastoral, 78-90, 146-54
Pastorfido,Il(Guarini), 148 ff.
Patrizzi, Francesco, 61, 184
Peele, 152, 153
Peletier, Jacques, 163-64
Pellissier, Georges, 186
Périon, 33
PetitePallaceofPettieHisPleasure,A(Pettie), 199
Petrarch, 5, 6, 13, 20, 27, 28, 29, 30, 31, 32, 37, 60, 65, 66, 67, 68, 69, 71, 82, 87, 114, 161, 162, 168, 170, 177, 179, 188, 222, 237
Petrus Comestor, 10
Pettie, William, 199
Pharmaceutria, 58, 81
Pico, 170
PiersPlowman, 131
Pilgrim’sProgress, 52
Pindar, 71, 72, 79, 168, 179
Pindarics, 72 ff.
Plantin, Christophe, 9
Plato, 7, 20, 25, 26, 43, 44, 62, 63, 165, 178, 179, 226
Platonic dialogue, 43, 61
Platonism, 25, 61
Plattard, J., 210n, 211n, 212n
Plautus, 58, 134, 147, 151
Plays, sacred, 134-37
Pléiade, 32, 37, 65, 69, 70, 71, 74, 119, 163, 175, 183, 186, 204
Pliny, 26, 198
Plutarch, 23, 63, 140, 179, 208, 238
Poemaeroico(Tasso), 176
Poetic, confusion with rhetoric, 15, 188
Poetic(Aristotle), 15, 25, 158, 164, 176, 189, 238
Poetica(Scaliger), 4, 171-75
Poetica(Trissino), 67, 158
Poeticadialogus(Fracastoro), 162
PoeticadiFrancescoPatrici..., 184
PoeticadiJasonDenores, 185
Poetics, sixteenth-century, 155-89
Politics, discussions on, 223-32
Poliziano, 22, 41, 47-49, 50, 57, 87, 166, 226
Pollio(Vergil), 81
Pontano, 58, 66, 72, 170, 179
PontusandtheFairSidoine, 9, 95
Porcari, Stefano, 221
Porcie(Garnier), 139
Praelectio, 41, 47, 50
PreëminenceoftheFrenchlanguage(Étienne), 34
Principe(Macchiavelli), 223
Printers, 9
Printing, effect on literary progress, 5, 9, 30, 231
Priscian, 10
ProjectdulivreentituléDelaprécellencedulangagefrançois (Étienne), 34
Pronuntiatio, 54
Propertius, 87
Prose(Bembo), 28
Prose forms, imitation of, 39-64
Prose narrative, 190-222
Prosopopoeia, 40
Prothalamion(Spenser), 74
Provençal, 28, 36, 66
Proverbia(Alain de Lille), 10
Publishers, 9, 13
Pulci, Luigi, 100-102, 109
Puttenham, 182, 183
Quintilian, 15, 47, 53, 55, 62, 63, 163, 164, 236n, 238
Rabelais, François, 22, 25n, 37, 202-13, 239
Ragionamentidellalinguatoscana(Tomitano), 59
Raphael, 29
Ratiostudiorum, 64
Religion, in Tasso and Spenser, 123, 124
Rerumscoticarumhistoria(Buchanan), 216
Retorica,Della(Patrizzi), 61
Retorica,La(Cavalcanti), 63
Rhetoric, confusion of poetic with, 15;
of Hermogenes, 188; recovery of Cicero’s works on, 44; Renaissance preoccupation with, 41
Rhetoric(Aristotle), 62, 63
Rhetoric(Soarez), 64
Rhetorica(Joannes Caesarius), 55
Rhetorica(George of Trebizond), 53
RhetoricaadHerennium, 10, 45, 63
Rhetorics, 53-64
Rhétoriques, 68
Rhétoriqueurs, 65, 68
Rhythm, English tradition of, 180
Ricci, Bartolomeo, 40, 40n, 50, 57
RichardIII(More), 217-20, 226
RichardIII(Shakspere), 146
RichesoftheVernacular,The(Alunno), 31
RimediversedelMutioIustinopolitano...(Muzio), 161-62
Robinson, Edwin Arlington, 93
Robinson, Ralph, 226
Romance, 91-132;
Arthurian cycle in Malory, 98; Carolingian cycle, 100, 101, 124; Greek, 7, 90, 179, 188; separate romances, 95-98
RomandelaRose, 70
Romantic contrast, 91-95
Ronsard, Pierre de, 10, 21, 23, 34, 35, 65, 67, 71-75, 77, 113, 119, 163, 175, 179, 188, 238
Round Table stories, 98, 108
Rouville, 13
Ruddiman, Thomas, 230n
Ruskin, John, 62
Rustic and pastoral comedy, 146-54
Ruzzante (Angelo Beolco), 150 ff.
Sacred plays, 133-37
Sacrerappresentazioni, 134
Sallust, 18, 58
SamsonAgonistes(Milton), 144
Sannazaro, Jacopo, 83-87, 165, 170, 179
Sansovino, Francesco, 40
Sappho, 87
Satire, of Rabelais, 207
Scaliger, Joseph, 137, 138, 188, 216
Scaliger, Julius Caesar, 4, 23, 171-75, 179
Scève, Maurice, 13
Schemata, 200
Scholars, gentlemen, 187
Scholemaster(Ascham), 38n
Scotorumhistoriae(Boece), 216
SecundaPastorum, 137, 150
Secundus, Joannes, 66, 72
Seneca, 10, 21, 87, 133, 134, 137, 142, 144, 145, 146, 158, 159, 165, 166, 179, 234, 238
Sententiae, 221
Shakspere, 145, 146, 147, 150, 153, 196
Shaksperian sonnet, 68
Shepherd’sCalendar(Spenser), 88-90
Sidney, Sir Philip, 46, 77, 89, 90, 178-80, 181, 188, 189, 202
Sidonius, 10, 188
Silius Italicus, 174, 188
SixLivresdelarépublique,Les, (Bodin), 230
Skelton, 37, 67, 202
Smith, G. Gregory, 180, 182
Soarez, 64
Solerti, Angelo, 87, 176
Sonnet, 66, 68, 77
Sophistic, 21, 40, 53, 55, 188, 200
Sophocles, 20, 21, 61, 142, 144, 165
Spagnolo, Baptista (Mantuan), 82, 88, 89, 180
Speculum(Vincent of Beauvais), 10
Spenser, 9, 13, 14, 24, 37, 73, 74, 75, 77, 88-91, 111, 123, 12732, 152, 180, 181, 183
Speroni, Sperone, 26, 43, 59, 60
Sprague, Arthur C., 183n
Statius, 10, 47, 87, 142, 174, 185, 188
Status, 54
Stephen of Tournay, 42n
Stoer, Jacobus, 228
Story-tellers, street, 100, 101
Story-telling, seeTales
Strowski, F., 233n
Style, literary preoccupation with, 18, 29, 52, 137
Suppositi,I(Ariosto), 147
Surrey, Henry Howard, Earl of, seeHoward, Henry
Tacitus, 41, 50, 213, 214, 223
Tales, 190-202
Tamburlaine(Marlowe), 146
Tasso, 7, 30, 52, 87, 91, 123-27, 148, 150, 153, 176-78, 184, 185, 189, 238
Tatius, seeAchilles Tatius
Tennyson, 93
Terence, 58, 134, 147, 170
Theocritus, 58, 79, 81, 86, 87, 88, 173
Theodulus, 82
Thesaurus, 46
Three styles, 15, 165, 166, 177
Thucydides, 21, 63, 213, 214
Tibullus, 87
Timone(Boiardo), 93
Tobias(Matthieu de Vendôme), 10
Tolomei, Claudio, 40
Tomitano, Bernardino, 59
Tournes, Jean de, 163
Towneley Plays, SecundaPastorum, 137, 150
Tragedy, 133, 134, 137-45
Tragicomedy, 133, 142, 148
Trattatodellaimitatione(Camillo), 50
Trimerone(Patrizzi), 184
Trionfi(Petrarch), 168
Trissino, Giovan Giorgio, 67, 158, 159, 177
Troade,La(Garnier), 141
TroilusandCriseyde(Chaucer), 15, 24, 96, 122
Turnebus, 137
Tuscan, 28, 30
Tusser, 181
UrbiumdictaMaximilianum(Caviceo), 40
Utopia(More), 226
Valla, Lorenzo, 8, 19
Varro, 58
Vauquelin de la Fresnaye, 186, 188, 189
Venatio(Adrian), 24
Vérard, Antoine, 68
Vergil, 18, 26, 30, 53, 56, 58, 63, 65, 71, 72, 79, 80, 82, 84, 86, 87, 88, 96, 98, 110, 114, 122, 123, 155, 156, 157, 160, 162, 163, 165, 166, 170, 173, 174, 177, 179, 181, 236, 238; Aeneid, 102, 104, 114, 121, 126, 127, 165, 185
Vernaculars, 5, 17, 27-38, 68-78
Verse, English discussion of, 180-84
Verse narrative, medieval, 7, 10, 91, 96
Vespasiano, 12
Vida, Marco Girolamo, 155-58, 163, 170, 172
Villon, 10, 68
Vincent of Beauvais, 10
Vinea, Gabriel, 13
Vittorino, 210
Vives, Juan Luis, 54, 210
Vopiscus, 50
Webbe, William, 180, 183
Weston, G. B., 100n
Wiat, Sir Thomas, 13, 67
Wilson, Thomas, 62
Witherspoon, A. M., 144n
Wolff, Samuel Lee, 202n
Wyatt, Sir Thomas, seeWiat, Sir Thomas
Xenophon, 93
York Plays, Nativity, 137
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