Bitcoin Core is an implementation of bitcoin. Initially, the software was published by Satoshi Nakamoto under the name "Bitcoin", and later renamed to "Bitcoin Core" to distinguish it from the network. For this reason, it is also known as the Satoshi client. As of 2018, Bitcoin Core repositories are maintained by a team of maintainers, with Wladimir J. van der Laan leading the release process.
The MIT Digital Currency Initiative funds some of the development of Bitcoin Core. The project also maintains the cryptography library libsecp256k1.
Bitcoin Core includes a transaction verification engine and connects to the bitcoin network as a full node. Moreover, a cryptocurrency wallet, which can be used to transfer funds, is included by default. The wallet allows for the sending and receiving of bitcoins. It does not facilitate the buying or selling of bitcoin. It allows users to generate QR codes to receive payment.
The software validates the entire blockchain, which includes all bitcoin transactions ever. This distributed ledger which has reached more than 155 gigabytes in size must be downloaded or synchronised before full participation of the client may occur. Although the complete blockchain is not needed all at once, since it is possible to run in pruning mode. A command line-based daemon with a JSON-RPC interface, bitcoind, is bundled with Bitcoin Core. It also provides access to testnet, a global testing environment that imitates the bitcoin main network using an alternative blockchain where valueless "test bitcoins" are used. Regtest or Regression Test Mode creates a private blockchain which is used as a local testing environment. Finally, bitcoin-cli, a simple program which allows users to send RPC commands to bitcoind, is also included.
Checkpoints which have been hard coded into the client are used only to prevent Denial of Service attacks against nodes which are initially syncing the chain. For this reason the checkpoints included are only as of several years ago. A one megabyte block size limit was added in 2010 by Satoshi Nakamoto. This limited the maximum network capacity to about three transactions per second. Since then, network capacity has been improved incrementally both through block size increases and improved wallet behavior. A network alert system was included by Satoshi Nakamoto as a way of informing users of important news regarding bitcoin. In November 2016 it was retired. It had become obsolete as news on bitcoin is now widely disseminated.
Bitcoin Core includes a scripting language inspired by Forth that can define transactions and specify parameters. ScriptPubKey is used to "lock" transactions based on a set of future conditions. scriptSig is used to meet these conditions or "unlock" a transaction. Operations on the data are performed by various OP_Codes. Two stacks are used - main and alt. Looping is forbidden.
The original creator of the bitcoin client has described their approach to the software's authorship as it being written first to prove to themselves that the concept of purely peer-to-peer electronic cash was valid and that a paper with solutions could be written. While the majority of peers on the network may use Bitcoin Core, the developers' influence on bitcoin is limited by the choice of which implementation people voluntarily decide to use. The lead developer is Wladimir J. van der Laan, who took over the role on 8 April 2014. Gavin Andresen was the former lead maintainer for the software client. Andresen left the role of lead developer for bitcoin to work on the strategic development of its technology. He left because he didn't want to get involved with trivial decision-making.
The code was originally stored at Sourceforge before being available on GitHub. Because there is no formal structure, development is based around Bitcoin Improvement Proposals or BIPs, which are similar to Request for Comments. Public mailing lists are used to vet initial expressions of ideas. If enough support is displayed a BIP document is written. This is the standard for sharing ideas and gaining community feedback on improving bitcoin and was initiated by Amir Taaki in 2011.
Bitcoin 0.1 was released on 9 January 2009 by Satoshi Nakamoto with only Windows supported. This was followed by some minor bug fixing versions. On 16 December 2009 Bitcoin 0.2 was released. It included a Linux version for the first time and made use of multi-core processors for mining. In version 0.3.2 Nakamoto included checkpoints as a safeguard. After the release of version 0.3.9 Satoshi Nakamoto left the project and shortly after stopped communicating on online forums. By this time development of the software was being undertaken by a wide group of independent developers which is referred to as a community, many of whom had various ideas on how to improve bitcoin.
Between 2011 and 2013 new versions of the software were released at Bitcoin.org. Developers wanted to differentiate themselves as creators of software rather than advocates for bitcoin and so now maintain bitcoincore.org for just the software.
Bitcoin-Qt version 0.5.0 was released on 1 November 2011. It introduced a front end that uses the Qt user interface toolkit. The software previously used Berkeley DB for database management. Developers switched to LevelDB in release 0.8 in order to reduce blockchain synchronization time. The update to this release resulted in a minor blockchain fork on the 11 March 2013. The fork was resolved shortly afterwards. Seeding nodes through IRC was discontinued in version 0.8.2. In this release transaction fees, also known as relay fees, were reduced from 50,000 satoshis to 10,000 satoshis. From version 0.9.0 the software was renamed to Bitcoin Core. Transaction fees were reduced again by a factor of ten as a means to encourage microtransactions. Although Bitcoin Core does not use OpenSSL for the operation of the network, the software did use OpenSSL for remote procedure calls. Version 0.9.1 was released to remove the network's vulnerability to the Heartbleed bug.
Release 0.10 was made public on 16 February 2015. It introduced a consensus library which gave programmers easy access to the rules governing consensus on the network. In version 0.11.2 developers added a new feature which allowed transactions to be made unspendable until a specific time in the future. Bitcoin Core 0.12.1 was released on April 15, 2016 and enabled multiple soft forks to occur concurrently. Around 100 contributors worked on Bitcoin Core 0.13.0 which was released on 23 August 2016. It introduced more than ten significant changes.
In July 2016, the CheckSequenceVerify soft fork activated.
In October 2016, Bitcoin Core’s 0.13.1 release featured the "Segwit" soft fork that included a scaling improvement aiming to optimize the bitcoin blocksize. The patch which was originally finalised in April, and 35 developers were engaged to deploy it. This release featured Segregated Witness (SegWit) which aimed to place downward pressure on transaction fees as well as increase the maximum transaction capacity of the network. The 0.13.1 release endured extensive testing and research leading to some delays in its release date. SegWit prevents various forms of transaction malleability. SegWit was activated by miners on 24 August 2017, at block 481,824.
Launched in February 2018, version 0.16.0 supports segregated witness as the native address format, also called bech32 addresses, which were originally developed by Peter Wuille and Greg Maxwell.
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