blockchain for other uses see block chain disambiguation distributed data store for digital transactions blockchain formation the main chain black consists of the longest series of blocks from the genesis block green to the current block orphan blocks purple exist outside of the main chain bitcoin network data a blockchain a timestamp and transaction data generally represented as a merkle tree root hash by design a blockchain is resistant to modification of the data it is an open distributed ledger that can record transactions between two parties efficiently and in a verifiable and permanent way blockchain was invented by satoshi nakamoto in 2008 to serve as the public transaction ledger of the cryptocurrency bitcoin history bitcoin transactions january 2009 september 2017 the first work on a cryptographically secured chain of blocks was described in 1991 by stuart haber and w scott stornetta the first blockchain was conceptualized by a person or group of people known as satoshi nakamoto in 2008 nakamoto improved the design in an important way using a hashcashlike method to add blocks to the chain without requiring them to be signed by a trusted party in august 2014 the bitcoin blockchain file size containing records of all transactions that have occurred on the network reached 20gb gigabytes in january 2015 the size had grown to almost 30gb and from january 2016 to january 2017 the bitcoin blockchain grew from 50gb to 100gb in size the words block and chain were used separately in satoshi nakamotos original paper but were eventually popularized as a single word blockchain by 2016 the term blockchain 20 refers to new applications of the distributed blockchain database first emerging in 2014 as of 2016 ibm opened a blockchain innovation research center in singapore in july 2016 a working group for the world economic forum met in november 2016 to discuss the development of governance models related to blockchain according to accenture an application of the diffusion of innovations theory suggests that blockchains attained a 135 adoption rate within financial services in 2016 therefore reaching the early adopters phase industry trade groups joined to create the global blockchain forum in 2016 an initiative of the chamber of digital commerce in may 2018 gartner found that only 1 of cios indicated any kind of blockchain adoption within their organisations and only 8 of cios were in the shortterm planning or structure a blockchain is a decentralized distributed and public digital ledger that is used to record transactions across many computers so that the record cannot be altered retroactively without the alteration of all subsequent blocks and the consensus of the network a blockchain can assign title rights because when properly set up to detail the exchange agreement it provides a record that compels offer and acceptance blocks blocks hold batches of valid transactions that are hashed and encoded into a merkle tree sometimes separate blocks can be produced concurrently creating a temporary fork in addition to a secure hashbased history any blockchain has a specified algorithm for scoring different versions of the history so that one with a higher value can be selected over others blocks not selected for inclusion in the chain are called orphan blocks block time the block time is the average time it takes for the network to generate one extra block in the blockchain hard forks this section is transcluded from fork blockchain edit history a hard fork is a rule change such that the software validating according to the old rules will see the blocks produced according to the new rules as invalid in case of a hard fork all nodes meant to work in accordance with the new rules need to upgrade their software if one group of nodes continues to use the old software while the other nodes use the new software a split can occur for example ethereum has hardforked to make whole the investors in the dao which had been hacked by exploiting a vulnerability in its code alternatively to prevent a permanent split a majority of nodes using the new software may return to the old rules as was the case of bitcoin split on 12 march 2013 decentralization by storing data across its peertopeer network the blockchain eliminates a number of risks that come with data being held centrally the decentralized blockchain may use adhoc message passing and distributed networking peertopeer blockchain networks lack centralized points of vulnerability that computer crackers can exploit likewise it has no central point of failure blockchain security methods include the use of publickey cryptography every node in a decentralized system has a copy of the blockchain data quality is maintained by massive database replication openness open blockchains are more userfriendly than some traditional ownership records which while open to the public still require physical access to view because all early blockchains were permissionless controversy has arisen over the blockchain definition an issue in this ongoing debate is whether a private system with verifiers tasked and authorized permissioned by a central authority should be considered a blockchain permissionless the great advantage to an open permissionless or public blockchain network is that guarding against bad actors is not required and no access control is needed bitcoin and other cryptocurrencies currently secure their blockchain by requiring new entries to include a proof of work to prolong the blockchain bitcoin uses hashcash puzzles while hashcash was designed in 1997 by adam back the original idea was first proposed by cynthia dwork and moni naor and eli ponyatovski in their 1992 paper pricing via processing or combatting junk mail financial companies have not prioritised decentralized blockchains in 2016 venture capital investment for blockchainrelated projects was weakening in the usa but increasing in china bitcoin has the highest market capitalization permissioned private blockchain distributed ledger permissioned blockchains use an access control layer to govern who has access to the network the new york times noted in both 2016 and 2017 that many corporations are using blockchain networks with private blockchains independent of the public system disadvantages of private blockchain nikolai hampton pointed out in computerworld that there is also no need for a 51 percent attack on a private blockchain as the private blockchain most likely already controls 100 percent of all block creation resources if you could attack or damage the blockchain creation tools on a private corporate server you could effectively control 100 percent of their network and alter transactions however you wished uses blockchain technology can be integrated into multiple areas the primary use of blockchains today is as a distributed ledger for cryptocurrencies most notably bitcoin there are a few operational products maturing from proof of concept by late 2016 as of 2016 cryptocurrencies cryptocurrency most cryptocurrencies use blockchain technology to record transactions for example the bitcoin network and ethereum network are blockchainbased smart contracts smart contract blockchainbased smart contracts are proposed contracts that could be partially or fully executed or enforced without human interaction banks major portions of the financial industry are implementing distributed ledgers for use in banking banks are interested in this technology because it has potential to speed up back office settlement systems banks such as ubs are opening new research labs dedicated to blockchain technology in order to explore how blockchain can be used in financial services to increase efficiency and reduce costs berenberg a german bank believes that blockchain is an overhyped technology that has had a large number of proofs of concept but still has major challenges and very few success stories other uses blockchain technology can be used to create a permanent public transparent ledger system for compiling data on sales tracking digital use and payments to content creators such as wireless users new distribution methods are available for the insurance industry such as peertopeer insurance parametric insurance and microinsurance following the adoption of blockchain blockchains can be used to catalog game assets digital assets as with cryptokitties other designs include hyperledger is a crossindustry collaborative effort from the linux foundation to support blockchainbased distributed ledgers with projects under this initiative including hyperledger burrow by monax and hyperledger fabric spearheaded by ibm quorum a permissionable private blockchain by jpmorgan chase with private storage used for contract applications tezos decentralized voting94 proof of existence is an online service that verifies the existence of computer files as of a specific time on may 8 2018 facebook confirmed that it is opening a new blockchain group in september 2018 ibm and a startup humanityco launched a blockchainbased app that let patients sell anonymized data to pharmaceutical companies within the video game industry while blockchain use is seen as part of a marketplace mechanism such as with robot cache blockchain is also a way seen to share video game assets between various games the blockchain game alliance was formed in september 2018 to explore alternative uses of blockchains in video gaming with support of ubisoft and fig among others types of blockchains currently there are three types of blockchain networks public blockchains private blockchains and consortium blockchains public blockchains a public blockchain has absolutely no access restrictions anyone with an internet connection can send transactions usually such networks offer economic incentives for those who secure them and utilize some type of a proof of stake or proof of work algorithm some of the largest most known public blockchains are bitcoin and ethereum private blockchains a private blockchain is permissioned one cannot join it unless invited by the network administrators participant and validator access is restricted this type of blockchains can be considered a middleground for companies that are interested in the blockchain technology in general but are not comfortable with a level of control offered by public networks typically they seek to incorporate blockchain into their accounting and recordkeeping procedures without sacrificing autonomy and running the risk of exposing sensitive data to the public internet consortium blockchains a consortium blockchain is often said to be semidecentralized it too is permissioned but instead of a single organization controlling it a number of companies might each operate a node on such a network the administrators of a consortium chain restrict users reading rights as they see fit and only allow a limited set of trusted nodes to execute a consensus protocol academic research blockchain panel discussion at the first ieee computer society techignite conference in october 2014 the mit bitcoin club with funding from mit alumni provided undergraduate students at the massachusetts institute of technology access to 100 of bitcoin the adoption rates as studied by catalini and tucker 2016 revealed that when people who typically adopt technologies early are given delayed access they tend to reject the technology energy use of proofofwork blockchains external video cryptocurrencies looking beyond the hype hyun song shin bank for international settlements 248 the bank for international settlements has criticized the public proofofwork blockchains for high energy consumption nicholas weaver of the international computer science institute at the university of california berkeley examines blockchains online security and the energy efficiency of proofofwork public blockchains and in both cases finds it grossly inadequate journals ledger journal in september 2015 the first peerreviewed academic journal dedicated to cryptocurrency and blockchain technology research ledger was announced the inaugural issue was published in december 2016 the journal encourages authors to digitally sign a file hash of submitted papers which will then be timestamped into the bitcoin blockchain authors are also asked to include a personal bitcoin address in the first page of their papers information technology portal cryptography portal economics portal computer science portal changelog a record of all notable changes made to a project checklist an informational aid used to reduce failure economics of digitization crosby michael nachiappan pattanayak pradhan verma sanjeev kalyanaraman vignesh 16 october 2015 blockchain technology beyond bitcoin pdf report sutardja center for entrepreneurship technology technical report university of california berkeley retrieved 20170319 kakavand hossein de sevres nicolette kost chilton bart 12 october 2016 the blockchain revolution an analysis of regulation and technology related to distributed ledger technologies report luther systems dla piper ssrn2849251 mazonka oleg 29 december 2016 blockchain simple explanation pdf journal of reference tapscott don tapscott alex 2016 blockchain revolution how the technology behind bitcoin is changing money business and the world london portfolio penguin isbn9780241237854 oclc971395169 saito kenji yamada hiroyuki june 2016 whats so different about blockchain blockchain is a probabilistic state machine ieee 36th international conference on distributed computing systems workshops nara nara japan ieee pp16875 doi101109icdcsw201628 isbn9781509036868 issn23325666 retrieved 20170215 raval siraj 2016 decentralized applications harnessing bitcoins blockchain technology oreilly bashir imran 2017 mastering blockchain packt publishing ltd isbn9781787125445 oclc967373845 d puthal n malik s p mohanty e kougianos and g das everything you wanted to know about the blockchain ieee consumer electronics magazine volume 7 issue 4 july 2018 pp0614 subbiah kannan ferrarini benno maupin julie hinojales marthe guhathakurta rahul kulshrestha sanatan wright danika the age of blockchain a collection of articles indrastra open access 18 march 2018 isbn9781980595816 retrieved 20180623 wikiversity has learning resources about blockchain media related to blockchain at wikimedia commons patent landscape report on blockchain by patseer pro patseer retrieved 20171103 vtecryptocurrenciestechnology atomic swap blockchain cryptocurrency tumbler cryptocurrency exchange cryptocurrency wallet cryptographic hash function distributed ledger fork lightning network smart contract consensus mechanisms proofofauthority proofofspace proofofstake proofofwork proofofwork currenciessha256based bitcoin bitcoin cash counterparty namecoin nxt peercoin steem ethashbased ethereum ethereum classic scryptbased bitcoin gold dogecoin feathercoin litecoin equihashbased zcash zcoin cryptonotebased bytecoin monero x11based dash petro proofofstake currencies ada eosio erc20 tokens aion augur basic attention token binance coin omisego other currencies bitshares filecoin nem neo primecoin ripple stellar tether related topics airdrop anonymous internet banking bitlicense blockchain game complementary currency cryptoanarchism cryptoeconomics digital currency doublespending bitcoin gold double spend attack initial coin offering initiative q stablecoin token money virtual currency category commons list vtebitcoin history economics legal status people gavin andresen andreas antonopoulos wences casares tim draper hal finney mark karpels satoshi nakamoto charlie shrem nick szabo amir taaki roger ver erik voorhees cody wilson winklevoss twins groups list of bitcoin companies list of bitcoin organizations list of people in blockchain technology technologies base58 bitcoin network blockchain coinjoin colored coins cryptocurrency bitcoin atm ecdsa lightning network p2p pow segregated witness sha2 software client bitcoin core forksclient bitcoin xt bitcoin classic bitcoin unlimited currency bitcoin cash bitcoin gold bitcoin private exchanges anx bitcoin center nyc bitfinex bitmex bitstamp bittrex btcc btc markets cexio coinbase gdax coincheck coinfloor coinsph gatecoin gemini huobi kraken localbitcoins okex poloniex quadrigacx shapeshift defunct bitinstant btce buttercoin mt gox book category commons portal vtedatabase management systemstypes objectoriented comparison relational comparison documentoriented graph nosql newsql concepts database acid armstrongs axioms cap theorem crud null candidate key foreign key superkey surrogate key unique key objects relation table column row view transaction transaction log trigger index stored procedure cursor partition components concurrency control data dictionary jdbc xqj odbc query language query optimizer query plan functions administration and automation query optimization replication related topics database models database normalization database storage distributed database federated database system referential integrity relational algebra relational calculus relational database relational model objectrelational database transaction processing vtesemantic webbackground databases hypertext internet ontologies semantic networks world wide web subtopics data web dataspaces hyperdata linked data rulebased systems applications semantic analytics semantic broker semantic computing semantic mapper semantic matching semantic publishing semantic reasoner semantic search semantic serviceoriented architecture semantic wiki related topics collective intelligence description logic folksonomy geotagging information architecture knowledge extraction knowledge management knowledge representation and reasoning library 20 digital library digital humanities metadata mind mapping odbc references topic map web 20 web engineering web science trust standardssyntax and supporting technologies http iri uri rdf triples rdfxml jsonld turtle trig notation3 ntriples trix no w3c standard rrid sparql xml schemas ontologies and rules common logic owl rdfs rule interchange format semantic web rule language alps semantic annotation erdf grddl microdata microformats rdfa sawsdl facebook platform common vocabularies doap dublin core foaf schemaorg sioc skos microformat vocabularies hatom hcalendar hcard hproduct hrecipe hresume hreview 