Virtual currency, cryptocurrency, or crypto-currency is a collection of binary data designed to act as a medium of exchange. Individual coin ownership records are stored in a digital ledger, a computerized database using robust cryptography to secure transaction records, control the creation of additional coins, and verify coin transfers. Cryptocurrencies are usually fiat currencies, as they have no support or cannot be converted into commodities. Some crypto schemes use validators to maintain cryptocurrency. In the proof-of-stake model, the owners keep their tokens as collateral. In return, they are entitled to a token in proportion to the amount they paid. Typically these token stackers receive additional ownership of the token through a network fee, newly created tokens, or another such reward mechanism. Cryptocurrency does not exist in physical form (like paper money) and is not usually issued by a central authority. Cryptocurrencies typically use decentralized controls against the central bank's digital currency.