Bitcoin is a worldwide cryptocurrency and digital payment system[13]:3 called the first decentralized digital currency, since the system works without a central repository or single administrator.[13]:1[14] It was invented by an unknown programmer, or a group of programmers, under the name Satoshi Nakamoto[15] and released as open-source software in 2009.[16] 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 a blockchain.