In recent years, cryptocurrencies based on blockchain technology have emerged and attracted widespread attention from various parties. Compared with traditional payment methods, cryptocurrency has the characteristics of decentralization and supports transaction anonymity. However, anonymity also provides hidden conveniences for illegal activities, making cryptocurrency increasingly a payment medium for money laundering, extortion, and other illicit activities. Therefore, cryptocurrency anonymity protection and countermeasures technology are a hot issue for current research. The research on the anonymity of cryptocurrency transactions mainly focuses on protecting transaction anonymity and the confrontation of anonymity. In terms of transaction anonymity protection, researchers mostly use zero-knowledge proof, Tor network, and other anonymity technologies to ensure the anonymity of users in the entire transaction process from different perspectives. Regarding the confrontation of anonymity, the researchers mainly combined the transaction information disclosed in the blockchain ledger and the dissemination data in the network layer to analyze the transaction traces left by users in the transaction process from different perspectives. In this paper, we first give an in-depth analysis of transaction anonymity's connotation and summarize it into three aspects: (1) Unidentifiability. For a given transaction, the observer cannot identify the true identity of the transaction participants in the physical world. (2) Unlinkability. For a given two transactions (at most, one of which is sent by the observer), the observer cannot determine whether they are paid to the same user. Also, the observer cannot determine whether the same user-initiated them. (3) Untraceability. For a given transaction, the observer cannot trace the flow of funds between the address to which the transaction was sent and the address to which the transaction was received. Then, we group the existing anonymity protection schemes into three categories according to the different focuses: (1) Unidentifiability protection schemes represented by Tor network, which mainly prevent observers from associating transactions with real-life user identities by hiding node information; (2) Unlinkability protection schemes represented by zero-knowledge proof technology, which mainly prevent observers from associating transactions with real-life user entities by hiding transaction information in the ledger to prevent observers from associating transactions with user entities; (3) Untraceability protection schemes represented by coin mixing technology, which mainly prevent observers from tracking the flow of funds based on the links between transaction participants by severing the relationships between them. At the same time, we also summarize the anonymity analysis methods of digital currencies from three aspects: (1) Unidentifiability confrontation, the observer uses the traffic information generated at each stage of cryptocurrency transactions, combined with the IP address, geographic location, organization's identity, and other information belonging to the P2P node, to identify the real identity information target transaction. (2) Unlinkability confrontation, where the observer discovers the correlation between user addresses and transactions by observing the transaction records in the public ledger based on attributes such as transaction amount, fund flow, and transaction time. (3) Untraceability confrontation, where the observer combines the ledger data and on-chain information to track the fund flow of the transaction. Finally, the paper summarizes the cryptocurrency anonymity research institute's challenges and future development trends. © 2023, Science Press. All right reserved.
