Smart Reputation-Based Counter-Collusion Contracts for Cloud Verification

To resist collusion between two cloud providers (CPs) in cross-verifiable outsourced computation, several smart contract-based schemes have been proposed. Their key idea is to incentivize a cloud to secretly betray collusion. In multi-round task outsourcing, collusion profits increase, and the trust built between CPs fosters collusion, making existing deposit-based anti-collusion mechanisms ineffective. To solve these problems, we propose a new solution. It differs from existing works in the following ways: (1) By introducing a reputation certificate system, it fundamentally changes the existing solutions by increasing deposits in the cat-and-mouse game. The new Outsourcing contract automatically updates a CP’s reputation certificate based on its behavior, which influences further client’s willingness to delegate tasks to the CP. As the increasing profits obtained by multiple collusions are much less than the corresponding delegation profit, a rational CP will not take the risk to lose future computation tasks. (2) We introduce a public Report contract. This contract is uniquely designed to report collusion attempts before a colluder’s contract is signed. Since no binding colluder’s contract has been signed, the reporter will not face any penalties. Consequently, our approach resolves the secrecy dilemma present in existing schemes. A further feasibility study is done by executing the contracts on an Ethereum network. Then, we conducted comparisons with state-of-the-art schemes. Both theoretical and experimental comparisons show that our scheme is the most effective in combating collusion.