Cloud storage is an effective way for data owners to outsource their data by remotely storing them in the cloud and enjoy on-demand high quality services. In traditional cloud storage systems, cloud data integrity verification relies on centralized entities and data is stored in a small number of storage servicers. However, these centralized entities and storage servicers may be untrustworthy, and malicious servicers may even refuse to perform a user’s query or update request. Though a few blockchain-based themes have been proposed to address some of these problems, they do not achieve decentralization, accountability, flexibility and practicability simultaneously. In this paper, we present Themis, an accountable P2P cloud storage scheme with smart contracts on Ethereum. Our scheme has the following advantages: First, cloud data integrity verification is decentralized and implemented by miners on blockchain without any trusted third party. Second, by carefully setting up the reward and punishment mechanism within a smart storage contract, all rational nodes will participate in the storage service following an accountable rule. Third, based on reliable information published on the blockchain, users are free to choose appropriate storage servicers who want to share idle storage, making storage service decentralized and flexible. Fourth, compared with the existing related systems, by adopting a payment at maturity method, the malicious behavior of breaking the contract after the servicer obtains some revenue is prevented, and the availability of user data within the specified period is enhanced. Fourthermore, we implement a prototype of Themis on Rinkeby, an Ethereum test network. Extensive experimental results demonstrate that our scheme is able to support a PB-level data storage in a single P2P storage service at a low cost and is feasible for practical deployment. And the entire decentralized arbitration process takes only 40 to 110 seconds.

云存储是数据所有者通过将数据远程存储在云中并享受按需高质量服务的一种有效方式来外包数据。在传统的云存储系统中，云数据完整性验证依赖于集中式实体，并且数据存储在少量的存储服务商中。但是，这些集中式实体和存储服务商可能是不可信的，恶意服务商甚至可能拒绝执行用户的查询或更新请求。尽管已经提出了一些基于区块链的主题来解决其中的一些问题，但是它们并不能同时实现去中心化，问责制，灵活性和实用性。在本文中，我们介绍了Themis，这是一种在以太坊上具有智能合约的负责任的P2P云存储方案。我们的方案具有以下优点：首先，矿工无需任何可信任的第三方就可以在区块链上分散和实施云数据完整性验证。其次，通过在智能存储合同中精心设置奖励和惩罚机制，所有有理节点将遵循一个负责任的规则参与存储服务。第三，基于发布在区块链上的可靠信息，用户可以自由选择想要共享空闲存储的合适的存储服务商，从而使存储服务分散且灵活。第四，与现有的相关系统相比，通过采用到期付款的方式，可以防止服务商获得一定收入后破坏合同的恶意行为，提高了指定时期内用户数据的可用性。第四，我们在Rinkeby上实现Themis的原型，以太坊测试网络。大量的实验结果表明，我们的方案能够以较低的成本在单个P2P存储服务中支持PB级数据存储，并且对于实际部署是可行的。整个分散式仲裁过程仅需40到110秒。