Bitcoin is a cryptocurrency and a digital payment system [13] :3 invented by an unknown programmer, or a group of programmers, under the name Satoshi Nakamoto . [14] It was released as open-source software in 2009. [15] The system is peer-to-peer , and transactions take place between users directly, without an intermediary. [13] :4 These transactions are verified by network nodes and recorded in a public distributed ledger called the blockchain . Since the system works without a central repository or single administrator, bitcoin is called the first decentralized digital currency . [13] :1 [16] Besides being obtained by mining , bitcoins can be exchanged for other currencies, [17] products, and services (legal or illegal ones). [18] [19] As of February 2015, over 100,000 merchants and vendors accept bitcoin as payment. [20] According to a research produced by Cambridge University in 2017, there are 2.9 to 5.8 million unique users actively using a cryptocurr