I. Introduction
Since Ethereum was launched in 2015, it has become the world’s leading programmable blockchain, allowing anyone to write Smart Contracts and Decentralized Applications (DApps), on top of the possibility to transfer ether (ETH), Ethereum’s own cryptocurrency. On Ethereum, the execution of a transaction requires some computational resources (called Gas ) which the initiator of the transaction has to pay for. She sets (i) the amount she is willing to pay per unit of gas (i.e., the GasPrice ), and (ii) the maximum units of gas she is willing to pay for (i.e., the GasLimit ); and she pays in advance the amount GasPrice × GasLimit . If the transaction does not exceed the amount of gas set by the initiator, then the latter will get back the ( GasPrice × GasLimit ) – Gas Cost ; while the Gas Cost is paid to the miner. Otherwise, the amount is lost.