# A Universal Privacy-Preserving Multi-Blockchain Aggregated Identity Scheme
## 1 Introduction  
### Background and Related Solutions  
While cryptocurrencies bring convenience, they are also used by criminals due to their anonymity [1]. Cryptocurrency-related crimes such as rug pull [2], extortion, money laundering, and terrorist financing, involve huge amounts of money and have a negative impact on the blockchain ecosystem. For example, the attack on the bnb chain on 7 October 2022 [3], resulted in a total loss of USD 570 million. The attackers used the cross-chain bridge Stargate Protocol to transfer about USD 100 million in assets, and these losses are irreversible. For the problems brought on by anonymity, deanonymization [4] is the straightforward solution. Deanonymization solutions combine on-chain and off-chain data to analyze on-chain transactions and infer their identity. However, due to the difficulty ofobtaining useful off-chain data and the existence of some anonymous cross-chain exchange services, it is not easy to find the real world identity associated with a wallet account with these solutions. In addition, deanonymization can only investigate crimes after they have occurred but cannot prevent crimes from happening. Compared to deanonymization schemes, identity management schemes can help regulators determine rather than speculate on the identities of accounts on the blockchain. In addition, identity management solutions can effectively reduce the occurrence of related crimes by creating a circle of trust between verified virtual asset service providers (VASPs) and verified users. So, identity management is a better way to deal with cryptocurrency-based crimes.  
[PERSON] et al. [5] pointed out that the absence of certification creates obstacles to its wider acceptance in e-commerce and official uses. They proposed a bitcoin authentication system, in which a central trusted authority controls the user's bitcoin address. In this way, the cryptocurrency asset balances and transaction records of these users are completely exposed to the regulator, which threatens privacy. In the scheme proposed by [PERSON], [PERSON], et al. [6], the generation of the user's certificate is independent of the public keys of the wallet accounts, which avoids the need for registration when the user wants to use a new account. However, the corresponding certificate should be attached to the transaction, which requires an additional cost. [PERSON] et al. [7] proposed KYCE on Ethereum. In this scheme, the identity provider adds the dependency data of the user who passed the Know your customer (KYC) check to a cryptographic accumulator, and provides the user with a witness. However, each transaction requires the witness described above to be put into the transaction data, which also requires an additional cost. Decentralized identity (DID) [8], Self-Sovereign Identity (SSI) [9], and Soulobound tokens [10] can also serve the purpose of identity tracking. DID schemes [11; 12; 13] identify identities by using each wallet account as a DID and provide verifiable credentials of user attributes. [PERSON] et al. proposed SBT to build the identity system of Web3. SBT schemes [14; 15; 16] bind identity credentials to specific wallet accounts by issuing a non-transferable Non-Fungible Token (NFT). However, these schemes can only be built on blockchains that support smart contracts. As mentioned above, there is currently no identity management solution that unconditionally supports all public blockchains. If identity management can be implemented on all blockchains, it will be more difficult for criminals to transfer assets. Besides, these schemes present a privacy threat because the identity provider knows each user's asset balance and transaction history, which discourages users from actively registering their identities with regulators.