With the massive amount of digital data generated everyday, transactions of digital goods become a trend. One of the essential requirements for such transactions is fairness, which is defined as that both of the seller and the buyer get what they want, or neither. Current fair trade protocols generally involve a trusted third-party (TTP), which achieves fairness by heavily relying on the TTP's behaviors and the two parties' trust in the TTP. With the emergence of Blockchain, its decentralization and transparency make it a very good candidate to replace the TTP. In this work, we attempt to design a secure and fair protocol for digital goods transactions through smart contracts on Blockchain. To ensure security of the digital goods, we propose an advanced passive proxy re-encryption (PRE) scheme, which enables smart contracts to transfer the decryption right to a buyer after receiving his/her payment. Furthermore, based on smart contracts and the proposed passive PRE scheme, a fair trade protocol for digital goods transactions is proposed, whose fairness is guaranteed by the arbitration protocol. The proposed protocol supports Ciphertext publicity and repeatable sale, while involving less number of interactions. Comprehensive experiment results validate the feasibility and effectiveness of the proposed protocol.

翻译：随着每天产生海量数字数据，数字商品的交易已成为一种趋势。此类交易的基本要求之一是公平性，即买卖双方要么各得其所，要么双方均无所获。现有的公平交易协议通常涉及可信第三方（TTP），通过高度依赖TTP的行为及双方对TTP的信任来实现公平性。区块链的出现及其去中心化与透明性使其成为替代TTP的理想候选方案。本文尝试通过区块链智能合约设计一种安全公平的数字商品交易协议。为确保数字商品的安全性，我们提出一种先进的被动代理重加密（PRE）方案，使智能合约在收到买家付款后能够将解密权转移给买家。此外，基于智能合约与所提出的被动PRE方案，本文设计了一种数字商品公平交易协议，其公平性通过仲裁协议得以保证。该协议支持密文公开性与重复销售，且交互次数更少。全面的实验验证了所提协议的可行性与有效性。