Fair payments for verifiable cloud services using smart contracts

Abstract

With the advent of cloud computing, outsourcing a computation has become a common practice. A client, to have greater confidence in computations performed by the cloud, should be able to verify the correctness of the results returned. Two approaches are followed to verify the correctness of the results: (1) Proof-based verifiable computing and (2) Replication-based Verifiable Computing. The first approach uses cryptography techniques, whereas the second approach follows game-theoretic methods. Although both approaches are almost perfect solutions to verify the results, they do not discuss fairness in verifiable computing.In this paper, we consider the problem of fairness in verifiable cloud computing, which means that the cloud gets the payment from a client for an outsourced computation if and only if the client receives the correct output of the computation. Most of the existing solutions rely on a trusted third-party to realise fairness. Motivated by the recent advances in blockchain technology and smart contracts, we propose protocols for different verifiable computing techniques using smart contracts. We first present a smart-contract based solution for fair proof-based verifiable computation and show that the overhead of using smart-contract is almost negligible when both client and cloud are honest. Then we show two different solutions depending upon the number of clouds hired to achieve fairness in replication-based verifiable computing. We have designed our protocols in the Blockchain model of cryptography and proved their security under universally composable theory. We also show the financial and transactional cost analysis of proposed contracts by implementing them in solidity and running them on Ethereum Blockchain.