IntroductionEthereum has been referred to as a "world computer", a global, decentralized, general-purpose, shared-state machine. However, Ethereum and all other general-purpose blockchains provide a very isolated and computationally limited execution environment. In particular, blockchains are unable to carry out expensive computational tasks (e.g. run a deep neural network), and interoperate with external systems (e.g. connect to the Internet, react to off-chain events and interact with othe...