Abstract: Block chain has the advantages of openness,non-tampering and distributed sharing of global accounts,but at the same time,these characteristics also bring about the privacy disclosure of transaction data,which seriously affects its application in many business areas,especially in the field of enterprise alliance chain.With the continuous development of block chain,how to protect the privacy of transaction data on block chain platform is a very worthwhile problem to study.To this end,firstly,the exi-sting methods of data privacy protection in block chain transactions were studied and their shortcomings were pointed out.Se-condly,the requirements of data privacy protection in block chain transactions were qualitatively analyzed.Each transaction data was divided into sensitive data and basic data.A demand analysis matrix was established to obtain the essential and implicit needs of transaction privacy protection and possible application scenarios.Then,combining the characteristics of symmetric encryption and asymmetric encryption and the consensus of intelligent contract,a privacy protection method of block chain transaction data based on double encryption was designed.The method mainly includes three modules:encrypting and storing transaction data by private data provider,using and decrypting private data to read transaction data,and sharing transaction data by private data accessible party.The workflow of each module was discussed in detail.Finally,the method was validated on the Mychain Platform,which combines with the actual business of multi-party participation in international trade.The evaluation results show that the proposed method can achieve fine-grained transaction data privacy protection at the field level,and can efficiently and steadily share private data on the chain and complete the full-link operation of private data.More than 1 million transaction tests have been completed on the block chain platform constructed by four nodes,and the TPS has reached 800.Compared with the original transaction performance without privacy protection,there is no significant reduction in performance.Compared with Bitcoin,Ethereum and other block chain platforms,the performance of the proposed method is improved dozens of times.

Key words: Block chain, Data sharing, Double encryption, Global ledger, Privacy protection, Smartcontract