Bitcoin is a digital store of value and payment system invented by Satoshi Nakamoto, who published the invention in 2008 and released it as open-source software in 2009. The system is peer-to-peer; users can transact directly without needing an intermediary.:4 Transactions are verified by network nodes and recorded in a public distributed ledger called the block chain. The ledger uses its own unit of account, also called bitcoin.[note 7] The system works without a central repository or single administrator, which has led the US Treasury to categorize it as a decentralized virtual currency. Bitcoin is often called the first cryptocurrency, although prior systems existed.[note 8] Bitcoin is more correctly described as the first decentralized digital currency. It is the largest of its kind in terms of total market value.