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The blockchain proposed by Satoshi Nakamoto(Nakamoto, 2008) in 2008 is a distributed chained data structure, which has many advantages such as decentralization, non-tampering and non-forgeability, and is considered to be the future of financial service infrastructure(Huang, Li, Lai, & Chen, 2017). According to the number of central nodes or privileged nodes, the blockchain can be divided into three categories, namely public blockchain, consortium blockchain and private blockchain(Peters, & Panayi, 2016). The public blockchain is completely decentralized, which allows any node to obtain data and process transactions on the blockchain. However, consortium blockchain and private blockchain need to be authorized and verified by at least one organization before nodes can join. The consortium blockchain has the advantage of fast transaction processing, so it is widely used in cultural relics traceability(Liang, Zhang, Gu, Chen, Zhang, & Liu, 2020), medical data sharing(Shahna, Qamar, & Khalid, 2019), educational data sharing(Liang, Zhao, Zhang, Liu, & Zhang, 2020), etc.
However, the blockchain systems and operating mechanisms are various from different application scenarios. This phenomenon leads to the isolation of block information in different blockchains, resulting in the islanding effect of the blockchain(“Mauve Paper Vitalik”, n.d.). Therefore, how to exchange information and value across different blockchains has become the focus of research. In 2012, the InterLedger protocol was proposed to solve the coordination problem between different blockchain systems(Hope-Bailie, & Thomas, 2016). Since then, cross-chain technology has developed rapidly. Cross-chain technology aims to link independent blockchains and carries the value exchange function of different value system blockchains. Herlihy proposed hash-locking mode(Herlihy, 2018) in 2013. BlockStream proposed sidechain (Asgaonkar, & Krishnamachari, 2019) in 2014. In 2016, BTC-Relay proposed a relay-chain solution (Chow, 2016), which has become the mainstream cross-chain technology. In addition, technologies such as the off-chain payment channel of the Lightning Network at the layer-2 level(Poon, & Dryja, 2016) and the decentralized autonomous incentives in Plasma(Poon, & Buterin, 2017) are also worthy of attention. The architecture proposed in this paper uses the relay-chain scheme.
Blockchain ledger is open and transparent, so privacy protection has become a challenge. Unlike within-chain transactions only in one system, cross-chain will inevitably cause two systems to interact and affect each other. According to atomic transfer(Hope-Bailie, & Thomas, 2016), a problem with the cross-chain information of a chain will affect the entire cross-chain network. Recently, there have been many attacks on cross-chain transactions. In July 2021, due to the theft of the administrator's private key, the cross-chain project AnySwap was hacked and lost more than 8 million dollars. In August 2021, Poly Network, a cross-chain interoperability protocol, was attacked by hackers and lost more than 600 million dollars. It can be seen that the cross-chain security situation is very urgent. Therefore, how to ensure system security and protect privacy in the process of cross-chain transactions is a question worth considering.
In the blockchain, privacy issues are mainly divided into two categories: identity privacy and data privacy(Zhu, Gao, Shen, Li, Zheng, Mao, & Wu, 2017). This paper discusses the issue of identity privacy in the cross-chain process, that is, users hope that the public data content stored on the blockchain cannot obtain any useful information related to their identity. The identity privacy of cross-chain transactions is fundamentally different from that of within-chain transactions. Identity privacy refers to the association between user identity information and blockchain addresses. However, different blockchains have their own addresses to represent identity information. Therefore, cross-chain privacy protection must first solve the intercommunication of different blockchains’ identity information.