What is Blockchain Technology?

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The bitcoin network is a peer-to-peer payment network that operates on a cryptographic protocol. Users send and receive bitcoinsthe units of currency, by broadcasting digitally signed messages to the network using bitcoin cryptocurrency wallet software.

Transactions are recorded bitcoin technology wiki a distributed, replicated public database known as the blockchainwith consensus achieved by a proof-of-work system called mining. Satoshi Nakamotothe designer of bitcoin claimed that design and coding of bitcoin begun in The network requires minimal structure to share transactions. An ad hoc decentralized network of volunteers is sufficient. Messages are broadcast on a best effort basis, and nodes can leave and rejoin the network at bitcoin technology wiki. Upon reconnection, a node downloads and verifies new blocks from other nodes to complete its local copy of the blockchain.

A bitcoin is defined by a sequence of digitally signed transactions that began with the bitcoin's creation, as a block reward. The owner of a bitcoin transfers it by digitally signing it over to the next owner using a bitcoin transaction, much like endorsing a traditional bank check.

A payee can examine each previous transaction to verify the chain of ownership. Bitcoin technology wiki traditional check endorsements, bitcoin transactions are irreversible, which eliminates risk of chargeback fraud.

Although it is possible to handle bitcoins individually, it would be unwieldy to require a separate transaction for every bitcoin in a transaction. Common transactions will have either a single input from a larger previous transaction or multiple inputs combining smaller amounts, and one or two outputs: Any difference between the total input and output amounts of a transaction goes to miners as a transaction fee.

To form a distributed timestamp server as a peer-to-peer network, bitcoin uses a proof-of-work system. The signature is discovered rather than provided by knowledge. Requiring a proof of work to provide the signature bitcoin technology wiki the blockchain was Satoshi Nakamoto's key innovation. While the average work required increases in inverse proportion to the difficulty target, a hash can always be verified by executing a single round of double SHA For the bitcoin timestamp network, a valid proof of bitcoin technology wiki is found by incrementing a bitcoin technology wiki until a value is found that gives the block's hash the required number of leading zero bits.

Once the hashing has produced a valid result, the block cannot be bitcoin technology wiki without redoing the work. As later blocks are chained after it, the work to change the block would include redoing the work for each subsequent block. Majority consensus in bitcoin is represented by the longest chain, which required the greatest amount of effort to produce. If a majority of computing power is controlled by honest nodes, the honest chain will grow fastest and outpace any competing chains.

To modify a past block, an attacker would have to redo the proof-of-work of that block and all blocks after it and then surpass the work of the honest nodes. The probability of a slower attacker catching up diminishes exponentially as subsequent blocks are added. To compensate for increasing hardware speed and varying interest in running nodes bitcoin technology wiki time, the difficulty of finding a valid hash is adjusted roughly every two weeks.

If blocks are generated too quickly, the difficulty increases and more hashes are required to make a block and to generate new bitcoins. Bitcoin mining is a competitive endeavor. An " arms race " has been observed through the various hashing technologies that have been used to mine bitcoins: Computing power is often bundled together or "pooled" to reduce variance in miner income.

Individual mining rigs often have bitcoin technology wiki wait for long periods to confirm a block of transactions and receive payment. In a pool, all participating miners get paid every time a participating server solves a block. This payment depends on the amount of work an individual miner contributed to help find that block. Bitcoin data centers prefer to keep a low profile, are dispersed around the world and tend to cluster around the availability of cheap electricity.

InMark Gimein estimated electricity consumption to be about To lower the costs, bitcoin miners have set up in places like Iceland where geothermal energy is cheap and cooling Arctic air is free. A rough overview of the process to mine bitcoins is: By convention, the first transaction in a block is a special transaction that produces new bitcoins owned by the creator of the block.

This is the incentive for nodes to support the network. The reward for mining halves everyblocks. It started at 50 bitcoin, dropped to 25 in late and to Various potential attacks on the bitcoin network and its use as a payment system, bitcoin technology wiki or theoretical, bitcoin technology wiki been considered.

The bitcoin protocol includes several features that protect it against some of those attacks, such as unauthorized spending, double spending, forging bitcoins, and tampering with the blockchain. Other attacks, such as theft of private keys, require due care by users. Unauthorized spending is mitigated by bitcoin's implementation of public-private key cryptography.

For example; when Alice sends a bitcoin to Bob, Bob becomes the new owner of the bitcoin. Eve observing the transaction might want to spend the bitcoin Bob just received, but she cannot sign the transaction without the knowledge of Bob's private key.

A specific problem that an internet payment system must bitcoin technology wiki is double-spendingwhereby a user pays the same coin to two or more different recipients. An example of such a problem would be if Eve sent a bitcoin to Alice and later sent the same bitcoin to Bob. The bitcoin network guards against double-spending by recording all bitcoin transfers in a ledger the blockchain that is visible to all users, and ensuring for bitcoin technology wiki transferred bitcoins that they haven't been previously spent.

If Eve offers to pay Alice a bitcoin in exchange for goods and signs a corresponding transaction, it is still possible that she also creates a different transaction at the same time sending the same bitcoin to Bob.

By the rules, the network accepts only one of the transactions. This is called a race attacksince there is a race which transaction will be bitcoin technology wiki first. Alice can reduce the risk of race bitcoin technology wiki stipulating that she will not deliver the goods until Eve's payment to Alice appears in the blockchain.

A variant race attack which has been called a Finney attack by reference to Hal Finney requires the participation of a miner. Instead of sending both bitcoin technology wiki requests to pay Bob and Alice with the same coins to the network, Eve issues only Alice's payment request to the network, while the bitcoin technology wiki tries to mine a block that includes the payment to Bob instead of Alice.

There is a positive probability that the rogue miner will succeed before the bitcoin technology wiki, in which case the payment to Alice will be rejected. As with the plain race attack, Alice can reduce the risk of a Finney attack by waiting for the payment to be included in the blockchain. Each block that is added to the blockchain, starting with the block containing a given transaction, is called bitcoin technology wiki confirmation of that transaction.

Ideally, merchants and services that receive payment in bitcoin should wait for at least one confirmation to be distributed over the network, before assuming that the payment was done. Deanonymisation is a strategy in data mining in which anonymous data is cross-referenced with other sources of data to re-identify the anonymous data source. Along with transaction graph analysis, which may reveal connections between bitcoin addresses pseudonyms[20] [25] bitcoin technology wiki is a possible attack [26] which links a user's pseudonym to its IP address.

If the peer is using Torthe attack includes a method to separate the peer from the Tor network, forcing them to use their real IP address for any further transactions. The attack makes use of bitcoin mechanisms of relaying peer addresses and anti- DoS protection. Each miner can choose which transactions are included in or exempted from a block. Upon bitcoin technology wiki a new transaction a node must validate it: To carry out that check the node needs to access the blockchain.

Any bitcoin technology wiki who does not trust his network neighbors, should keep a full local copy of the blockchain, so that any input can be verified. As noted in Nakamoto's whitepaper, it is possible to verify bitcoin payments bitcoin technology wiki running a full network node simplified payment verification, SPV. A user only needs a copy of the block headers of the longest chain, which are available by querying network nodes until it is bitcoin technology wiki that the longest chain has been obtained.

Then, get bitcoin technology wiki Merkle branch bitcoin technology wiki the transaction to its block. Linking the transaction to a place in the chain demonstrates that a network node has accepted it, and blocks added after it further establish the confirmation.

While it is possible to store any digital file in the blockchain, the larger the transaction size, the larger any associated fees become. The use of bitcoin by criminals has attracted the attention of financial regulators, legislative bodies, law enforcement, and the media.

Senate held a hearing on virtual currencies in November Several news outlets have asserted that the popularity of bitcoins hinges on the ability to use them to purchase illegal goods.

A CMU researcher estimated that in4. Bitcoin technology wiki to the anonymous nature and the lack of central control on these markets, it is hard to know whether the services are real or just trying to take the bitcoins.

Several deep web black markets have been shut by authorities. In October Silk Road was shut down bitcoin technology wiki U.

Some black market sites may seek to steal bitcoins from customers. The bitcoin community branded one site, Sheep Marketplace, as a scam when it prevented withdrawals and shut down after an alleged bitcoins theft. According to the Internet Watch Foundationa UK-based charity, bitcoin is used to purchase child pornography, and almost such websites accept it as payment.

Bitcoin isn't the sole way to purchase child pornography online, as Troels Oertling, head of the cybercrime unit at Europolstates, "Ukash and Paysafecard Bitcoins bitcoin technology wiki not be ideal for money laundering, because all transactions are public. In earlyan operator of a U. Securities and Exchange Commission charged the company and its founder in "with defrauding investors in a Ponzi scheme involving bitcoin".

From Wikipedia, the free encyclopedia. For a broader coverage related to this topic, see Bitcoin. Information technology portal Cryptography portal.

Archived from the original on 3 November Retrieved 2 November Retrieved 30 January Retrieved 20 December Financial Cryptography and Data Security.

Retrieved 21 August Retrieved 3 October Retrieved 9 January

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A blockchain , [1] [2] [3] originally block chain , [4] [5] is a continuously growing list of records , called blocks , which are linked and secured using cryptography. It is "an open, distributed ledger that can record transactions between two parties efficiently and in a verifiable and permanent way".

Once recorded, the data in any given block cannot be altered retroactively without the alteration of all subsequent blocks, which requires collusion of the network majority.

Blockchains are secure by design and exemplify a distributed computing system with high Byzantine fault tolerance. Decentralized consensus has therefore been achieved with a blockchain.

Blockchain was invented by Satoshi Nakamoto in for use in the cryptocurrency bitcoin , as its public transaction ledger.

The bitcoin design has been the inspiration for other applications. The first work on a cryptographically secured chain of blocks was described in by Stuart Haber and W. The first blockchain was conceptualized by a person or group of people known as Satoshi Nakamoto in It was implemented the following year by Nakamoto as a core component of the cryptocurrency bitcoin, where it serves as the public ledger for all transactions on the network.

The words block and chain were used separately in Satoshi Nakamoto's original paper, but were eventually popularized as a single word, blockchain, by The term blockchain 2.

Second-generation blockchain technology makes it possible to store an individual's "persistent digital ID and persona" and provides an avenue to help solve the problem of social inequality by "potentially changing the way wealth is distributed". In , the central securities depository of the Russian Federation NSD announced a pilot project, based on the Nxt blockchain 2. 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 collusion of the network.

They are authenticated by mass collaboration powered by collective self-interests. The use of a blockchain removes the characteristic of infinite reproducibility from a digital asset. It confirms that each unit of value was transferred only once, solving the long-standing problem of double spending. Blockchains have been described as a value -exchange protocol.

Blocks hold batches of valid transactions that are hashed and encoded into a Merkle tree. The linked blocks form a chain. Sometimes separate blocks can be produced concurrently, creating a temporary fork.

In addition to a secure hash-based 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. They keep only the highest-scoring version of the database known to them. Whenever a peer receives a higher-scoring version usually the old version with a single new block added they extend or overwrite their own database and retransmit the improvement to their peers.

There is never an absolute guarantee that any particular entry will remain in the best version of the history forever. Because blockchains are typically built to add the score of new blocks onto old blocks and because there are incentives to work only on extending with new blocks rather than overwriting old blocks, the probability of an entry becoming superseded goes down exponentially [34] as more blocks are built on top of it, eventually becoming very low.

There are a number of methods that can be used to demonstrate a sufficient level of computation. Within a blockchain the computation is carried out redundantly rather than in the traditional segregated and parallel manner.

The block time is the average time it takes for the network to generate one extra block in the blockchain. In cryptocurrency, this is practically when the money transaction takes place, so a shorter block time means faster transactions. The block time for Ethereum is set to between 14 and 15 seconds, while for bitcoin it is 10 minutes.

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 hard-forked to "make whole" the investors in The DAO , which had been hacked by exploiting a vulnerability in its code.

In the Nxt community was asked to consider a hard fork that would have led to a rollback of the blockchain records to mitigate the effects of a theft of 50 million NXT from a major cryptocurrency exchange. The hard fork proposal was rejected, and some of the funds were recovered after negotiations and ransom payment. 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 By storing data across its peer-to-peer network, the blockchain eliminates a number of risks that come with data being held centrally.

Peer-to-peer 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 public-key cryptography. Value tokens sent across the network are recorded as belonging to that address. A private key is like a password that gives its owner access to their digital assets or the means to otherwise interact with the various capabilities that blockchains now support.

Data stored on the blockchain is generally considered incorruptible. While centralized data is more easily controlled, information and data manipulation are possible. By decentralizing data on an accessible ledger, public blockchains make block-level data transparent to everyone involved. Every node in a decentralized system has a copy of the blockchain.

Data quality is maintained by massive database replication [9] and computational trust. No centralized "official" copy exists and no user is "trusted" more than any other. Messages are delivered on a best-effort basis.

Mining nodes validate transactions, [33] add them to the block they are building, and then broadcast the completed block to other nodes. Open blockchains are more user-friendly 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.

These blockchains serve as a distributed version of multiversion concurrency control MVCC in databases. 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. Financial companies have not prioritised decentralized blockchains. Permissioned blockchains use an access control layer to govern who has access to the network.

They do not rely on anonymous nodes to validate transactions nor do they benefit from the network effect. The New York Times noted in both and that many corporations are using blockchain networks "with private blockchains, independent of the public system. 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 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 percent of their network and alter transactions however you wished.

It's unlikely that any private blockchain will try to protect records using gigawatts of computing power—it's time consuming and expensive.

This means that many in-house blockchain solutions will be nothing more than cumbersome databases. Data interchange between participants in a blockchain is a technical challenge that could inhibit blockchain's adoption and use.

This has not yet become an issue because thus far participants in a blockchain have agreed either tacitly or actively on metadata standards. Standardized metadata will be the best approach for permissioned blockchains such as payments and securities trading with high transaction volumes and a limited number of participants.

Such standards reduce the transaction overhead for the blockchain without imposing burdensome mapping and translation requirements on the participants. However, Robert Kugel of Ventana Research points out that general purpose commercial blockchains require a system of self-describing data to permit automated data interchange.

According to Kugel, by enabling universal data interchange, self-describing data can greatly expand the number of participants in permissioned commercial blockchains without having to concentrate control of these blockchains to a limited number of behemoths.

Self-describing data also facilitates the integration of data between disparate blockchains. Blockchain technology can be integrated into multiple areas. The primary use of blockchains today is as a distributed ledger for cryptocurrencies, most notably bitcoin. Blockchain technology has a large potential to transform business operating models in the long term.

Blockchain distributed ledger technology is more a foundational technology —with the potential to create new foundations for global economic and social systems—than a disruptive technology , which typically "attack a traditional business model with a lower-cost solution and overtake incumbent firms quickly". As of [update] , some observers remain skeptical. Steve Wilson, of Constellation Research, believes the technology has been hyped with unrealistic claims.

This means specific blockchain applications may be a disruptive innovation, because substantially lower-cost solutions can be instantiated, which can disrupt existing business models. Blockchains alleviate the need for a trust service provider and are predicted to result in less capital being tied up in disputes. Blockchains have the potential to reduce systemic risk and financial fraud.

They automate processes that were previously time-consuming and done manually, such as the incorporation of businesses. As a distributed ledger, blockchain reduces the costs involved in verifying transactions, and by removing the need for trusted "third-parties" such as banks to complete transactions, the technology also lowers the cost of networking, therefore allowing several applications.

Starting with a strong focus on financial applications, blockchain technology is extending to activities including decentralized applications and collaborative organizations that eliminate a middleman. Frameworks and trials such as the one at the Sweden Land Registry aim to demonstrate the effectiveness of the blockchain at speeding land sale deals.

The Government of India is fighting land fraud with the help of a blockchain. In October , one of the first international property transactions was completed successfully using a blockchain-based smart contract.

Each of the Big Four accounting firms is testing blockchain technologies in various formats. It is important to us that everybody gets on board and prepares themselves for the revolution set to take place in the business world through blockchains, [to] smart contracts and digital currencies. Blockchain-based smart contracts are contracts that can be partially or fully executed or enforced without human interaction.

The IMF believes smart contracts based on blockchain technology could reduce moral hazards and optimize the use of contracts in general. Some blockchain implementations could enable the coding of contracts that will execute when specified conditions are met. A blockchain smart contract would be enabled by extensible programming instructions that define and execute an agreement. Companies have supposedly been suggesting blockchain-based currency solutions in the following two countries:.

Some countries, especially Australia, are providing keynote participation in identifying the various technical issues associated with developing, governing and using blockchains:. Don Tapscott conducted a two-year research project exploring how blockchain technology can securely move and store host "money, titles, deeds, music, art, scientific discoveries, intellectual property, and even votes".