Bitcoin (code: BTC) is a cryptocurrency where the creation and transfer of bitcoins is based on an open-source cryptographic protocol that is independent of any central authority. Bitcoins can be transferred through a computer or smartphone without an intermediate financial institution. The concept was introduced in a 2008 paper by pseudonymous developer Satoshi Nakamoto, who called it a peer-to-peer, electronic cash system.

The processing of Bitcoin transactions is secured by servers called bitcoin miners. These servers communicate over an internet-based network and confirm transactions by adding them to a ledger which is updated and archived periodically using peer-to-peer filesharing technology. In addition to archiving transactions, each new ledger update creates some newly minted bitcoins. The number of new bitcoins created in each update is halved every 4 years until the year 2140 when this number will round down to zero. At that time no more bitcoins will be added into circulation and the total number of bitcoins will have reached a maximum of 21 million bitcoins. To accommodate this limit, each bitcoin is subdivided down to eight decimal places; forming 100 million smaller units called satoshis per bitcoin.

Bitcoin is accepted in trade by merchants and individuals in many parts of the world. Like other currencies, illicit drug and gambling transactions constitute some of its commercial usage. Although the bitcoin is promoted as a digital currency, many commentators have criticized the bitcoin's volatile exchange rate, relatively inflexible supply, and minimal use in trade.

Transactions
Bitcoin wallets & addresses
A bitcoin transaction log showing addresses.

A user can have one or more bitcoin addresses from which bitcoins are sent or received using either a website or downloaded software often called a "wallet" like a digital wallet. Users can obtain new bitcoin addresses as needed. Many bitcoin services provide addresses tied to a user's individual account to hold funds on the user's behalf.

Specifically, a bitcoin address is a cryptographic public key ie human-readable strings of numbers and letters around 33 characters in length, beginning with the digit 1 or 3, as in the example of 175tWpb8K1S7NmH4Zx6rewF9WQrcZv245W. The matching private key is often stored in a digital wallet or mobile device and protected by a password or other means of authentication. Each bitcoin transaction is signed by the private key of the user initiating the transaction.

Various vendors offer banknotes and coins denominated in bitcoins; what is sold is really a bitcoin private key as part of the coin or banknote. Usually, a seal has to be broken to access the key, while the receiving address remains visible on the outside so that the balance can be verified.

Payment network & mining
The Bitcoin network protocol operates to provide solutions to the problems associated with creating a decentralized currency and a peer-to-peer payment network. Key among them is the use of a blockchain to achieve consensus and to solve the double-spending problem.

A bitcoin is defined by a chain of digitally-signed transactions that began with its creation as a block reward through bitcoin mining. Each owner transfers bitcoins to the next by digitally signing them over to the next owner in a Bitcoin transaction. A payee can then verify each previous transaction to verify the chain of ownership.

The network timestamps transactions by including them in blocks that form an ongoing chain called the blockchain. Such blocks cannot be changed without redoing the work that was required to create each block since the modified block. The longest chain serves not only as proof of the sequence of events but also records that this sequence of events was verified by a majority of the Bitcoin network's computing power. As long as a majority of computing power is controlled by nodes that are not cooperating to attack the network, they will generate the longest chain of records and outpace attackers.

The network itself requires minimal structure to share transactions. Messages are broadcast on a best effort basis, and nodes can leave and rejoin the network at will. Upon reconnection, a node will download and verify new blocks from other nodes to complete its local copy of the blockchain.

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