Ethereum is a decentralized operating system. The platform introduced, in practice, the concept of "decentralized applications". Its founder, Vitalik Buterin, has been engaged in the development of blockchain and Bitcoin since its inception. Today, several cryptocurrencies are issued in ICOs using Ethereum. These tokens are called ERC-20, and they are the foundation for running scripts in the Ethereum Virtual Machine. Through the development of "smart contracts", Ethereum made it possible to sign tokens that have a specific function besides being a medium of exchange and broadened the capabilities of blockchain "transactions". Its key cryptocurrency is the Ether, which is usually used for payments in smart contracts or dApps. In addition, Ethereum is also fueled with GAS, a cryptocurrency associated with operations that require some kind of computation from its nodes. The GAS price is proportional to the type of the computational power required execute a task. Therefore, a fee in GAS is included to pay for the nodes running or authenticating a transaction. Originally, Ethereum used proof-of-work to reach consensus. The Casper protocol, however, gradually shifted Ethereum consensus from PoW to a custom proof-of-stake protocol.