How to enable fairness for e-commerce applications has attracted years of research. Recent research has proposed employing blockchain smart contract as an efficient trusted third party (TTP) to enable fair data trading. However, the state-of-the-art schemes suffer from two issues, i.e., they either fail to work for situations where data validity cannot be encoded as an oracle function in the smart contract, or leak data to attackers for free. To resolve these issues, this paper proposes a two-stage approach for blockchain-based fair data trading. The main idea is to employ a lightweight off-chain TTP and an on-chain smart contract to handle dispute issues. Both the TTP and smart contract only require a logarithmic complexity for making arbitration in case of disputes; moreover, they are not invoked when there is no dispute. The rationale is that although the off-chain TTP cannot be eliminated, it is only needed in a minimal sense to judge the validity of the traded data. The proposed approach designs a new cryptographic protocol that combines sampling, commitment schemes, and encryption schemes to achieve this logarithmic efficiency. The proposed approach also features privacy protection. Experimental evaluation of the public Ethereum blockchain confirms that the proposed approach is practically usable. Specifically, for a dataset of 15GB, the off-chain computation for each trading party costs approximately 80 seconds while on-chain computation costs around 30 seconds; the additional storage cost is around 9MB; the gas cost is approximately 2.23 million GWei.

中文翻译：

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基于区块链的公平数据交易的两阶段方法


如何为电子商务应用程序实现公平性，吸引了多年的研究。最近的研究建议将区块链智能合约用作高效的可信第三方 （TTP） 来实现公平的数据交易。然而，最先进的方案存在两个问题，即它们要么无法在智能合约中将数据有效性编码为预言机函数的情况下工作，要么免费将数据泄露给攻击者。为了解决这些问题，本文为基于区块链的公平数据交易提出了一种两阶段的方法。主要思想是采用轻量级的链下 TTP 和链上智能合约来处理争议问题。TTP 和智能合约都只需要对数复杂度，以便在发生争议时进行仲裁;此外，在没有争议的情况下，不会调用它们。其基本原理是，虽然链下 TTP 无法消除，但只需要在最低限度的意义上判断交易数据的有效性。所提出的方法设计了一种新的加密协议，该协议结合了采样、承诺方案和加密方案来实现这种对数效率。所建议的方法还具有隐私保护功能。对公共以太坊区块链的实验评估证实，所提出的方法实际上是可用的。具体来说，对于 15GB 的数据集，每个交易方的链下计算成本约为 80 秒，而链上计算成本约为 30 秒;额外的存储成本约为 9MB;天然气成本约为 223 万 GWei。