Bitcoin Core is free and open-source software that serves as a bitcoin node (the set of which form the bitcoin network) and provides a bitcoin wallet which fully verifies payments. It is considered to be bitcoin's reference implementation.[326] Initially, the software was published by Satoshi Nakamoto under the name "Bitcoin", and later renamed to "Bitcoin Core" to distinguish it from the network.[327] It is also known as the Satoshi client.[328]
The MIT Digital Currency Initiative funds some of the development of Bitcoin Core.[329] The project also maintains the cryptography library libsecp256k1.[330]
Bitcoin Core includes a transaction verification engine and connects to the bitcoin network as a full node.[328] Moreover, a cryptocurrency wallet, which can be used to transfer funds, is included by default.[330] 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 235 gigabytes in size as of Jan 2019, must be downloaded or synchronized before full participation of the client may occur.[328] 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.[331] 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.[332][333][failed verification] 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.[334] 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.[335] 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.[336] 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.
Bitcoin Core uses OpenTimestamps to timestamp merge commits.[337]
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. The lead developer is Wladimir J. van der Laan, who took over the role on 8 April 2014.[338] 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.[338] Bitcoin Core in 2015 was central to a dispute with Bitcoin XT, a competing client that sought to increase the blocksize.[339] Over a dozen different companies and industry groups fund the development of Bitcoin Core.
Hodl (/ˈhɒdəl/ HOD-əl; often written HODL) is slang in the cryptocurrency community for holding a cryptocurrency rather than selling it. A person who does this is known as a Hodler. It originated in a December 2013 post on the Bitcoin Forum message board by an apparently inebriated user who posted with a typo in the subject, "I AM HODLING."[340] It is often humorously suggested to be a backronym to "hold on for dear life".[341] In 2017, Quartz listed it as one of the essential slang terms in bitcoin culture, and described it as a stance, "to stay invested in bitcoin and not to capitulate in the face of plunging prices."[342] TheStreet.com referred to it as the "favorite mantra" of bitcoin holders.[343] Bloomberg News referred to it as a mantra for holders during market routs.[344]
In Charles Stross's 2013 science fiction novel Neptune's Brood, the universal interstellar payment system is known as "bitcoin" and operates using cryptography.[345] Stross later blogged that the reference was intentional, saying "I wrote Neptune's Brood in 2011. Bitcoin was obscure back then, and I figured had just enough name recognition to be a useful term for an interstellar currency: it'd clue people in that it was a networked digital currency."[346]
The 2014 documentary The Rise and Rise of Bitcoin portrays the diversity of motives behind the use of bitcoin by interviewing people who use it. These include a computer programmer and a drug dealer.[347] The 2016 documentary Banking on Bitcoin is an introduction to the beginnings of bitcoin and the ideas behind cryptocurrency today.[348]
In September 2015, the establishment of the peer-reviewed academic journal Ledger (ISSN 2379-5980) was announced. It covers studies of cryptocurrencies and related technologies, and is published by the University of Pittsburgh.[349] 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.