Bitcoin is a cryptocurrency and a digital payment system invented by an unknown programmer, or a group of programmers, under the pseudonym Satoshi Nakamoto. It was released as open-source software back in 2009. The system is P2P, and all transactions take place between users directly. They are verified by network nodes and recorded in a blockchain. Since it requires no central authority to operate, bitcoin is called the first decentralized digital currency. Besides being created as a reward for mining, bitcoin can be exchanged for other currencies, products, and services. The maximum and total amount of bitcoins that can ever exist is 21 million.