\ \ Bitcoin (abbreviation: BTC; sign: ) is the first decentralized cryptocurrency. Based on a free-market ideology, bitcoin was invented in 2008 when an unknown entity published a white paper under the pseudonym of Satoshi Nakamoto.[5] Use of bitcoin as a currency began in 2009,[6] with the release of its open-source implementation.[7]: ch. 1 In 2021, El Salvador adopted it as legal tender.[4] It is mostly seen as an investment and has been described by some scholars as an economic bubble.[8] As bitcoin is pseudonymous, its use by criminals has attracted the attention of regulators, leading to its ban by several countries as of 2021[update].[9]\ \ Bitcoin works through the collaboration of computers, each of which acts as a node in the peer-to-peer bitcoin network. Each node maintains an independent copy of a public distributed ledger of transactions, called a blockchain, without central oversight. Transactions are validated through the use of cryptography, preventing one person from spending another person's bitcoin, as long as the owner of the bitcoin keeps certain sensitive data secret.[7]: ch. 5\ \ Consensus between nodes about the content of the blockchain is achieved using a computationally intensive process based on proof of work, called mining, which is performed by purpose-built computers.[7]: ch. 12