A Two-Stage Approach for Fair Data Trading Based on Blockchain

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.