Data owners upload large files to cloud storage servers, but malicious servers may potentially tamper data. To check integrity of remote data, Proof of Retrievability (PoR) schemes were introduced. Existing PoR protocols assume that data owners and third-party auditors are honest and audit only the potentially malicious cloud server to check integrity of stored data. In this paper we consider a system where any party may attempt to cheat others and consider collusion cases. We design a protocol that is secure under such adversarial assumptions and use blockchain smart contracts to act as mediator in case of dispute and payment settlement. We use state channels to reduce blockchain interactions in order to build a practical audit solution. We implement and evaluate a prototype using Ethereum as the blockchain platform and show that our scheme has comparable performance.

翻译：数据所有者将大文件上传至云存储服务器，但恶意服务器可能篡改数据。为验证远程数据的完整性，可检索性证明方案被提出。现有PoR协议假设数据所有者和第三方审计者诚实可信，仅对可能存在恶意的云服务器进行数据完整性审计。本文考虑系统中任意一方可能试图欺骗其他方的情形，并探讨共谋场景。我们设计了一种在此类对抗性假设下仍保持安全性的协议，利用区块链智能合约在发生争议时充当调解者并处理支付结算。为构建实用审计方案，我们采用状态通道减少区块链交互。基于以太坊区块链平台的原型实现与评估表明，该方案具有可比的性能表现。