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A Blockchain is a growing list of records, called blocks, that are linked together using cryptography. Each block contains a cryptographic hash of the previous block, a timestamp, and transaction data (generally represented as a Merkel Tree). The timestamp proves that the transaction data existed when the block was published to get into its hash. As blocks each contain information about the block previous to it, they form a chain, with each additional block reinforcing the ones before it. Therefore, blockchains are resistant to modification of their data because once recorded, the data in any given block cannot be altered retroactively without altering all subsequent blocks.

Blockchains are typically managed by a peer-to-peer network for use as a publicly distributed ledger, where nodes collectively adhere to a protocol to communicate and validate new blocks. Although blockchain records are not unalterable as forks are possible, blockchains may be considered secure by design and exemplify a distributed computing system with high Byzantine fault tolerance.

The blockchain was popularized by a person (or group of people) using the name Satoshi Nakamoto in 2008 to serve as the public transaction ledger of the cryptocurrency bitcoin, based on work by Stuart Haber, W. Scott Stornetta, and Dave Bayer. The identity of Satoshi Nakamoto remains unknown to date. The implementation of the blockchain within bitcoin made it the first digital currency to solve the double-spending problem without the need for a trusted authority or central server. The bitcoin design has inspired other applications and blockchains that are readable by the public and are widely used by cryptocurrencies. The blockchain is considered a type of payment rail.

Private blockchains have been proposed for business use. Computerworld called the marketing of such privatized blockchains without a proper security model "snake oil", however, others have argued that permissioned blockchains, if carefully designed, maybe more decentralized and therefore more secure in practice than permissionless ones.

Cryptographer David Chaum first proposed a blockchain-like protocol in his 1982 dissertation "Computer Systems Established, Maintained, and Trusted by Mutually Suspicious Groups". Further work on a cryptographically secured chain of blocks was described in 1991 by Stuart Haber and W. Scott Stornetta. They wanted to implement a system wherein document timestamps could not be tampered with. In 1992, Haber, Stornetta, and Dave Bayer incorporated Merkel Trees into the design, which improved its efficiency by allowing several document certificates to be collected into one block. Under their company Surety, their document certificate hashes have been published in The New York Times every week since 1995.

The first decentralized blockchain was conceptualized by a person (or group of people) known as Satoshi Nakamoto in 2008. Nakamoto significantly improved the design using a Hashcash-like method to timestamp blocks without requiring them to be signed by a trusted party and introducing a difficulty parameter to stabilize the rate at which blocks are added to the chain. the design 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.

In August 2014, the bitcoin blockchain file size, containing records of all transactions that have occurred on the network, reached 20 GB. In January 2015, the size had grown to almost 30 GB, and from January 2916 to January 2017, the bitcoin blockchain grew from 50 GB to 100 GB in size. The ledger size has exceeded 200 GB by early 2020.

The words block and chain were used separately in Satoshi Nakamoto's original paper but were eventually popularized as a single word, blockchain by 2016.

Blockchain technology can be integrated into multiple areas. The primary use of blockchains is as a distributed ledger for cryptocurrencies such as bitcoin; there were also a few operational products that had matured from proof of concept by late 2016. Some of the major uses are as follows;

  • Cryptocurrencies
    Most cryptocurrencies use blockchain technology to record transactions. For example, the bitcoin network and Ethereum network are both based on blockchain. on 8 May 2018 Facebook confirmed that it would open a new blockchain group which would be headed by David Marcus, who previously was in charge of Messenger. Facebook's planned cryptocurrency platform, Libra (now known as Diem), was formally announced on June 18, 2019.

    The criminal enterprise Silk Road, which operated on Tor, utilized cryptocurrency for payments, some of which the US federal government has seized through research on the blockchain and forfeiture.

    Governments have mixed policies on the legality of their citizens or banks owning cryptocurrencies. China implements blockchain technology in several industries including a national digital currency which launched in 2020. To strengthen their respective currencies. Western governments including the European Union and the United States have initiated similar projects.
    • Games
      Blockchain technology, such as cryptocurrencies and non-fungible tokens (NFTs), has been used in video games for monetization. Many live-service games offer in-game customization options, such as character skins or other in-game items, which the players can earn and trade with other players using in-game currency.

      Some games also allow for trading of virtual items using real-world currency, but this may be illegal in some countries where video games are seen as akin to gambling and have led to gray market issues such as skin gambling, and thus publishers typically have shied away from allowing players to earn real-world funds from games. Blockchain games typically allow players to trade these in-game items for cryptocurrency, which can be exchanged for money while avoiding problems associated with gray markets due to blockchain's accountability.
    • Energy Trading
      Blockchain is also being used in peer-to-peer energy trading.
    • Supply Chain
      Blockchain technology has been used for tracking the origins of gemstones and other precious commodities. 

      Blockchain technology has been used to allow retailers and consumers to track the provenance of meat and other food products from their origins to stores and restaurants. As of 2018, Walmart and IBM were running a trial to use a blockchain-backed system for supply chain monitoring for lettuce and spinach - all nodes of the blockchain were administered by Walmart and were located on the IBM cloud. One cited benefit is that the system could enable rapid tracing of contaminated produce. Some analysts are less convinced that most consumers will be that interested in this capability.
    • Anti-counterfeiting
      Blockchain could be used in detecting counterfeits by associating unique identifiers to products, documents and shipments, and storing records associated with transactions that cannot be forged or altered. It is however argued that blockchain technology needs to be supplemented with technologies that provide a strong binding between physical objects and blockchain systems.
    • Healthcare
      In response to the 2020 COVID-19 Pandemic, The Wall Street Journal reported that Ernst & Young was working on a blockchain to help employers, governments, airlines, and others keep track of people who have had antibody tests and could be immune to the virus. Hospitals and vendors also utilized a blockchain for needed medical equipment. Additionally, blockchain technology was being used in China to speed up the time it takes for health insurance payments to be paid to healthcare providers and patients.
    • Elections
      Several countries are trying to use blockchain technology for elections and voting, which will enable people from around the nation to vote without leaving their homes. This will ensure more participation of people, as they will not be required to leave their home, or where ever they are located.

      This will also ensure that people can vote for their parties without going to hometowns, as most of the people are in different cities, and cannot go home for votes, which is one of the most important works to be done by a citizen.


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