## 2 Related Works
### Indirect Privacy Protection  
Indirect privacy protection methods employ an indirect approach to safeguarding transaction privacy information. These methods mainly rely on techniques such as obfuscation or temporary generation. Common technological solutions include coin mixing, stealth addresses, and ring signatures. 1. Coin mixing technology  
Mixing technology achieves privacy by disrupting the correspondence between the addresses of the transacting parties, ensuring transaction unlinkability. In centralized approaches, an additional third party serves as a mixer, disrupting the relationship between the currency and the transactors during the mixing service. Malicious entities find it difficult to obtain critical transaction information. However, this approach relies on a third party, requires payment for the mixing service, and carries the risk of token theft. A decentralized approach, Coinjoin [15], was introduced to address these issues. It eliminates the need for an anonymous third-party intermediary. The coordinator of the mixing service combines multiple transactions, severing the relationship between the transactions and the transacting parties to blur the linkability of the transactions. However, this approach introduces new challenges. It is difficult to ensure that all participants in the mixing process are honest users, and it cannot eliminate the risk of Sybil attacks or guarantee internal unlinkability. To tackle these limitations, a new decentralized mixing scheme called CoinShuffle [16] was proposed. Users shuffle the transaction relationships using a shuffling protocol, and their address information is protected to ensure the security of the mixing transactions. However, users need to be online throughout the mixing process. Addressing this drawback, the CoinParty scheme [17] was introduced. Unlike CoinShuffle, it does not require users to be online throughout the process. Instead, participating users only need to send encrypted output addresses and corresponding funds to the mixing group before the mixing service begins. Decentralized mixing technologies have improved privacy and convenience, but they still have some limitations, such as susceptibility to DoS attacks and relatively low mixing efficiency. Currently, they are not able to provide fully mature privacy protection. 2. Ring signature technology  
Ring signature is a simplified form of group signature where the core idea is that users (members of the ring set) can utilize a \"ring\" signature that represents the members of the ring [18; 19], but it cannot be attributed to any specific signer. The ring set is a collection without any central authority, providing features such as non-repudiation, privacy, and traceability, significantly enhancing the privacy of blockchain transactions. CryptoNote [20] is one of the most prominent protocols based on ring signature technology. It utilizes linkable ring signatures to conceal the sender's address in transactions. Monero [21], a cryptocurrency, is built upon this key technology. Monero implements ring confidential transactions (RingCT), which leverage homomorphic commitments and ring signatures to achieve range proofs, thereby hiding transaction amounts while ensuring public verifiability. Due to its anonymity and anti-counterfeiting properties, ring signature technology has been widely applied in fields such as intellectual property protection and anonymous voting. However, ring signature privacy protection schemes also face security threats. Research studies [4; 22; 23] have demonstrated privacy vulnerabilities in ring signature technology. In such schemes, the length of signatures increases as more members join, and there are also issues related to malicious repudiation of signatures by signers and vulnerability to key selection attacks. The stealth address technology primarily aims to hide transaction addresses and provide privacy protection for the recipients [24]. The sender computes a temporary address before the transaction, and the payment tokens are sent to that address.