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One major shortcoming of permissionless blockchains such as Bitcoin andEthereum is that they are unsuitable for running Computationally Intensivesmart Contracts (CICs). This prevents such blockchains from running MachineLearning algorithms, Zero-Knowledge proofs, etc. which may need non-trivialcomputation. In this paper, we present YODA, which is to the best of ourknowledge the first solution for efficient computation of CICs inpermissionless blockchains with guarantees for a threat model with bothByzantine and selfish nodes. YODA selects one or more execution sets (ES) viaSortition to execute a particular CIC off-chain. One key innovation is theMultI-Round Adaptive Consensus using Likelihood Estimation (MIRACLE) algorithmbased on sequential hypothesis testing. M I RACLE allows the execution sets tobe small thus making YODA efficient while ensuring correct CIC execution withhigh probability. It adapts the number of ES sets automatically depending onthe concentration of Byzantine nodes in the system and is optimal in terms ofthe expected number of ES sets used in certain scenarios. Through a suite ofeconomic incentives and technical mechanisms such as the novel RandomnessInserted Contract Execution (RICE) algorithm, we force selfish nodes to behavehonestly. We also prove that the honest behavior of selfish nodes is anapproximate Nash Equilibrium. We present the system design and details of YODAand prove the security properties of MIRACLE and RICE. Our prototypeimplementation built on top of Ethereum demonstrates the ability of YODA to runCICs with orders of magnitude higher gas per unit time as well as total gasrequirements than Ethereum currently supports. It also demonstrates the lowoverheads of RICE.

YODA: Enabling computationally intensive contracts on blockchains with Byzantine and Selfish nodes