Bitcoin, a digital currency created by an anonymous computer developer or software group known as Satoshi Nakamoto in 2009. Bitcoins owners can use various Websites to sell them in natural currencies, such as US dollars or euros, or they can exchange them for goods and Nakamoto was concerned that traditional currencies depended heavily on banks' reliability. Nakamoto has proposed a digital currency, Bitcoin, which can serve as a means of exchange without relying on any financial institutions or governments. The proposal was made in October 2008 in a paper published on the Bitcoin Web site, which was launched in August 2008. Bitcoin relies on a public key hack, where users have a public key available to everyone and the only key known on their computers. In Bitcoin transactions, users who receive Bitcoins send their keys publicly to users who transfer Bitcoins. Users who transfer token coins with their secret keys, and the transaction is transmitted via the Bitcoin network. In order to prevent Bitcoin from being used more than once at a time, the time and amount of each transaction are recorded in the ledger file located in each network node. User details remain anonymous, but everyone can see that certain Bitcoins have been transferred. Transactions are grouped into groups called blocks. Blocks are arranged chronologically called a blockchain. Blocks are placed in a chain using a mathematical process that makes it very difficult for each user to hijack a blockchain. The blockchain technology that supports Bitcoin has attracted a lot of attention, even from critics of Bitcoin, as a basis for allowing reliable record keeping and trading without central authority.