Abstract: Aiming at the situation that blockchain transaction mapping analysis may leak users' privacy and the third-party mi-xing service providers are not trustworthy,this paper proposes an agent-based bidirectional mixing protocol PBShuffle without the need of a third party.The protocol process does not require the participation of a third-party mixing service provider,and it adopts the method of delivering the output address to the aggregated users through an agent.The agent is randomly selected by the participant among all participants and needs to perform two rounds of mixing to deliver output addresses to two aggregated users respectively.The protocol utilizes double encryption to achieve privacy protection in the process of output address delivery,the agent can only decrypt the encrypted message encrypted with the public key of the aggregated user,and the aggregated user can only know that the message is delivered by the agent,and cannot derive the source participant of the message.The protocol is theoretically analyzed to be highly secure in terms of non-connectivity,verifiability and robustness.Comparison experiments with CoinShuffle show that PBShuffle has higher efficiency and lower overhead in the case of a larger number of participating users,and is more suitable for practical applications.

Key words: Blockchain, Privacy protection, Coin mixing mechanism, Encryption, Anonymit