## 2 Related Work
For hash locking-based approaches, Hashed Time-Lock Contracts (HTLCs) originally emerged as an alternative to centralized switching because they support cross-chain atomic operations [15]. HTLC technology uses hash locks [16] and time locks [17] to enforce the atomicity of operations, usually between two sides. The trader makes a trade by promising to provide proof of encryption to the other party before the time runs out. An HTLC is used in Bitcoin for conditional or cross-chain payments, namely atomic exchange [18; 19; 20]. Atomic swapping can be thought of as a distributed commitment [21], able to send off a Byzantine adversary. Therefore, atomic cross-chain switching is a distributed atomic transaction [20], which is settled on the chain. Side chains are two existing blockchains that are interoperable [22; 23], extensible (for example, through blockchain sharing [24]), and upgradable [25]. Side chains allow for different interactions between participating blockchains, the most common being the transfer of assets between the main and side chains (bidirectional hooking) [26; 27]. A relay solution is an SPV [28] client of the source blockchain running on the target blockchain, capable of verifying transactions [29]. The relay/side chain does not rely on a trusted third party to help it verify the transaction; rather, it is verified by the receiving chain itself after receiving the transmit chain data. A can read B, but B cannot read A; if the A-C chain can read all chains, it should also be an \"on-chain\" intermediary, and the whole process is \"A-C-B\". The trunk is more like the dispatch center of multiple chains. When a \"side chain\" links many main chains, it becomes a trunk chain. Side chains need to anchor each other's data. The private keys of various assets are controlled by distributed nodes, and the original chain assets are mapped to the cross-chain to ensure the interconnection of various assets in the blockchain system. The core of distributed private key control is distributed control management; that is, the ownership and use rights of assets are separated, and the control rights of digital assets on the original chain are safely transferred to the decentralized system. A distributed private key blocks a chain inside the private key, which is divided into N and put it into N shares. To share it with N participants, everyone can grasp some of the private key participants and can only collect K of them after the distribution of the private key, to restore the integrity of a private key, to restore a full private key, to be able to access the private key assets, or to perform an unlock. In other words, distributed private key control transfers account control through the distributed storage of user account private keys on the network. A comparison of the four existing cross-chain methods is shown in Table 1.  
By summarizing the above four mainstream cross-chain methods, it can be found that they are all value exchanges for \"coin\" and designed to provide interoperability between cryptocurrency systems [13], aiming to improve the transaction processing capacity of Bitcoin-dominated tokens. Besides the hash locking method, other methods include connecting the token network with the public chain network for token transaction performance improvement and transaction verification. However, blockchain is no longer limited to the field of cryptocurrency and is expected to become a data storage and management technology that is as widely used as a database. Therefore, the blockchain cross chain should not be limited to the transfer of assets and should focus on transaction interoperation between blockchains. With the popularity of the alliance chain, cross-chain technology should also be applied to the scope of the alliance chain. In this paper, we will design and implement the cross-chain interoperation of a blockchain based on Hyperledger Fabric, an important project in the federation chain.