Enhanced Mixing Protocol For Privacy Protection In Blockchain

Bitcoin is a decentralized virtual encrypted digital currency,and its security and anonymity are two important reasons for the rise of Bitcoin.The mixing protocol is proposed to solve the problem of Bitcoin’s pseudoanonymity,and it is a privacy protection method designed to hide the relationship between input and output addresses of transactions.However,there are two major problems with mixing protocols.Firstly,the mixing protocol only focuses on the confusion of addresses,but has no corresponding protection method for transaction amounts.Secondly,most of centralized mixing protocols are based on full trust in the third party,with all transaction information exposed to it,which causes security risks.Based on the two problems,we research and enhance the existing mixing protocols,and achieve results as follows.Firstly,based on TumbleBit,the centralized coin mixing protocol with an untrusted third-party,we propose TumbleBit++ to achieve protection of transaction amounts.Without destroying the anonymity and the third-party untrustworthiness of original TumbleBit,TumbleBit++ uses homomorphic commitments to hide transaction amounts to achieve complete privacy.The specific methods are as follows.First,the commitment values are hidden by blinding,so that the third party cannot leak the association between the input and output addresses by comparing the commitment values,which maintains the anonymity.Second,the verification step is added to the third party to guarantee the verifiability of the third party’s input and output amounts of the same transaction without revealing the transaction amounts.Third,zero-knowledge proof is used to ensure that the payer and payee will not commit fraud to the third party,so that the mutual non credibility between the third party and traders is protected.In addition,TumbleBit++ redefines the 2-of-2 escrow smart contract of TumbleBit,allowing multiple bitcoins to be packaged in one transaction,which greatly improves the flexibility and efficiency of TumbleBit.Secondly,based on Obscuro,a centralized mixing protocol,we propose Obscuro++,which redefines the trust model of the third party and reduces the dependence of the mixing protocol on the trust of the third party.Based on the curious-but-honest trust model,output addresses of transactions are hidden by encryption,making addresses invisible to the third party.The specific methods are as follows.First,we rebuild the system entities,split one centralized coin mixer into multiple distributed coin mixers,and perform public-key cryptosystem with distributed decryption based on Bresson cryptosystem.Second,based on the homomorphism of Paillier cryptosystem,we mix addresses in the form of ciphertext to construct a list of output addresses on the shared encryption domain.Third,a second round of shuffle algorithm is added to ensure that the address is invisible to every mixer.