1. Features

Bitcoin Core includes a transaction verification engine and connects to the bitcoin network as a full node.^[1] Moreover, a cryptocurrency wallet, which can be used to transfer funds, is included by default.^[2] 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.^[1] 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.^[3] 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.^[4][5] 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.^[6] 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.^[7] 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.^[8] 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.^[9]

2. Development

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.^[10] 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.^[10]

3. Version History

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.

Between 2011 and 2013 new versions of the software were released at Bitcoin.org.^[11] 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.^[12] 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. 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.^[13] Bitcoin Core 0.12.1 was released on April 15, 2016 and enabled multiple soft forks to occur concurrently.^[14] Around 100 contributors worked on Bitcoin Core 0.13.0 which was released on 23 August 2016.

In July 2016, the CheckSequenceVerify soft fork activated.^[15]

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.^[16] 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.^[17]

In September 2018, a Bitcoin Cash developer discovered the vulnerability CVE-2018-17144 in the Bitcoin Core software that could allow an attacker to crash vulnerable Bitcoin Core nodes and exceed the 21 million coin limit.^[18]

4. Bitcoin Improvement Proposals

A Bitcoin Improvement Proposal (BIP) is a design document, typically describing a new feature for Bitcoin with a concise technical specification of the feature and the rationale for it. This is broadly similar to the way in which Internet "Request for Comments" (RFCs) and the Python computer language's "Python Enhancement Proposals" (PEPs) are used.

The process itself is documented in BIP 2, and BIP 123 provides a categorization.^[19]

2 BIP process, revised

BIP 2 specifies the BIP process. BIP numbers are awarded liberally. As of February 2017, 152 BIP numbers have been assigned, but only 27 BIP's have reached the active/final stages.

9 Version bits with timeout and delay

BIP specifies a state machine for determining 95% miner consensus of soft forks. There has been one successful BIP 9 soft fork, and one, Segregated Witness, is, as of 2017, open for voting.^[20]

16 Pay to script hash

Allows transactions to be sent to a script hash (address starting with 3) instead of a public key hash (addresses starting with 1). To spend bitcoins sent via P2SH, the recipient must provide a script matching the script hash, and data which makes the script evaluate to true. The recipient might need the signatures of several people to spend these bitcoins, or a password might be required, or the requirements could be completely unique.

32 Defines HD wallets

These HD ('Hierarchical Deterministic") wallets can be shared partially or entirely with different systems.^[21]

39 Mnemonic code or sentences

For the generation of deterministic wallets.^[22]

43 Adds a "Purpose Field" for use HD wallets

To determine the further structure; for example, the scheme described in BIP44 should use the value 44' as the "purpose".^[23]

44 Logical hierarchy for deterministic wallets

based on the algorithm described in BIP32 and "purpose" scheme described in BIP43.^[23]

65 CHECKLOCKTIMEVERIFY

CLTV allows a transaction output to be unspendable until some specific point of time in the future.^[24]

112 CHECKSEQUENCEVERIFY

CSV enables making an address (starting with 3) which can't spend bitcoin received, for a specified amount of time after receiving. One can have a 2-of-3 multisig address, which times out to a backup rule, unless there is 2-of-3 consensus.

141, 143, 144

See SegWit

152 Compact Blocks

Merged on 22 June 2016, Compact Blocks enables faster block propagation,^[25] and was used on 97% of nodes in November 2017.

158 Compact Block Filters for Light Clients

Merged in August 2018, these Golomb-coded set-based filters allows for clients to probe whether a block contains a transaction without telling anyone which transactions they are interested in.^[26]