Themis: An accountable blockchain-based P2P cloud storage scheme

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.