> For the complete documentation index, see [llms.txt](https://nervenetwork.gitbook.io/nerve/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://nervenetwork.gitbook.io/nerve/core-module-description/cross-chain-module.md).
# Cross-Chain Module
NerveChian's cross-chain interaction is divided into two parts:
1. NULS cross-chain ecological docking: realize the interaction of all blockchains in the NULS ecosystem based on the general NULS cross-chain protocol.
2. For independent public chains such as Bitcoin, Ethereum and Binance chain, NerveNetwork defines a set of interface protocols, which can facilitate the interaction of different blockchains. The cross-chain interface protocol includes the following aspects:
* address mapping
* creating multi-signature addresses/creating smart contracts
* transaction verification
* transaction assembly
* transaction broadcast
* signature verification
* additional signature
The architecture of cross-chain interaction protocol is designed as follows:
Every time a blockchain is docked, it needs to implement a set of interface protocol components for data interaction between the two chains.
A certain number of virtual bank nodes are selected from the consensus nodes to create and manage multi-signature addresses (smart contracts). Virtual banks are responsible for the verification of assets transfer in and execution of asset transfer out.\
Taking BTC as an example, the cross-chain interaction process is as follows:
Transfer in cross-chain assets (deposit - Cross In):
The user transfers BTC to the multi-signature account of the Bitcoin chain managed by the virtual bank, and fill in the remarks with his/her NerveNetwork address of Nerve-ADDR. The virtual bank of NerveNetwork monitors the transaction of the Bitcoin network, verifies the confirmation number, prevents the fork rollback attack, assembles a coin-creating transaction to the user's mapping address of Nerve-ADDR, and signs the transaction. Then it will broadcast the transaction, collect 66% signatures from the virtual bank, pack the transaction into the block, and then update the ledger. After that, the user has BTC assets in the NerveNetwork ecosystem. The actual BTC assets on Bitcoin network are guaranteed not to be used by the virtual bank.
Transfer out cross-chain assets (withdrawal - Cross Out):
The user assembles the transfer-out transaction, with the target address of Bitcoin address of BTC-ADDR, signs and broadcasts the transaction. Once the consensus node receives the transaction, it will validate the transaction signature and pack the transaction into the block after approving. After the block is confirmed, each node assembles the multi-signature transaction, and broadcasts it to the NerveNetwork. When the number of signatures is sufficient, the transaction will be broadcast to the Bitcoin main network, and BTC will be transferred to the BTC-ADDR from the Bitcoin multi-signature account. After that, the transaction is completed.
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