Bitcoin is both a cryptocurrency and an electronic payment system:3 invented by an unidentified programmer, or group of programmers, under the name of Satoshi Nakamoto. Bitcoin was introduced on 31 October 2008 to a cryptography mailing list, and released as open-source software in 2009. The identity of Nakamoto remains unknown, though many have claimed to know it. The system is peer-to-peer, and transactions take place between users directly, without an intermediary.:4 These transactions are verified by network nodes and recorded in a public distributed ledger called the blockchain, which uses bitcoin as its unit of account. Since the system works without a central repository or single administrator, the U.S. Treasury categorizes bitcoin as a decentralized virtual currency. Bitcoin is often called the first cryptocurrency, although prior systems existed,[note 4] and it is more correctly described as the first decentralized digital currency. Bitcoin is the largest of its kind in terms of total market value.