The cross-chain token bridge refers to the asset transfer channel between two heterogeneous chains. Through the token bridge, assets can be safely and reliably transferred on different heterogeneous chains. The token bridge solution in this article will be backed by the concept of cryptocurrency backed asset (CBA) model and decentralized backing technology.

CBA stands for Cryptocurrency Backed Asset. To build the CBA model, we need to build a Backing module on the source chain and an Issuing module on the target chain.

Backing Module: The module that locks and unlocks the native assets on the source chain. Issuing Module: The module that issues and burns the mapped assets on the target chain.

In the cross-chain CBA model, the two heterogeneous chains are called Source chain (backing blockchain) and Target chain (issuing blockchain) respectively. Cross-chain assets do not reside natively on either chain but as a conceptual medium to describe the process of cross-chain circulation, which involves locking backing assets on the source chain while issuing assets on the target chain. The assets issued by backing native assets on the source blockchain are called backed assets, or CBA for short. The security and redeemability of backed assets are determined by the security and reliability of decentralized backing technology. Darwinia cross-chain token bridge is a cross-chain bridge solution between the source chain and the target chain as well as a decentralized asset backing technology.

A Bridge Creator from the Source Chain (Backing BlockChain) deploys a contract called “Backing” on this chain; meanwhile, a Bridge Creator from Target Chain (Issuing BlockChain) deploys a contract named “Issuing” on their chain.

Users call the “register” interface in the Backing module by a transaction to register their original asset. And then relayers will synchronize this register message to the target chain. After this message is accepted by the target chain(must be proved), the Issuing module will create a corresponding mapping-contract to issue and store the mapped token.

A Requestor from the source chain locks a specific amount of native assets and assigns a receiving account on the target chain by calling the lock_and_remote_issue() interface of the Backing module. They can get the proof of locking from the source chain when succeeding. Then the relayer forwards the proof to the Issuing module on the target chain. In the end, the Issuing module writes the calculated amount of mapped tokens to the receiving account after verifying the proof.

The mapped tokens satisfy the standard format as the original tokens. Holders have the full authorities of the mapped token. And the mapped token can also be transferred from one account to another. Also, by using any swap pool, users can swap their mapped tokens to other tokens on the target chain.

A Requestor as an asset holder from the target chain calls the remote_unlock() interface in the Issuing module to burn an amount of mapped tokens on the target chain and assign a receiving account on the Source Chain. Then the relayer delivers the proof of burning to the Backing module on the source chain. In the end, the Backing module releases the locked assets to the assigned receiving account after verifying the proof.

Common decentralized backing technologies include:

• There are trust nodes or custodian mechanisms, where the trusted node or custodian uses multi-signature / threshold signature and other multi-centralized technologies to avoid single points of failure, such as Parity Bridge, ChainX, etc.

• Some solutions use collateralized bridges to ensure the redeemability of cross-chain assets, partially combined with Chain Relay. XClaim has the disadvantage that it is only suitable for homogeneous assets with good liquidity, and its economic feasibility is relatively weak.

• Fully use the Chain Relay for chain verification, such as BTCRelay, WaterLoo, Darwinia ChainRelay, etc. The prerequisite of using this solution is that the issuing chain needs to support the implementation of smart contracts, on-chain runtimes, hard forks, and other methods. Such blockchains like Chain Relay and Bitcoin network cannot be used as an issuing chain. The advantage is that it can support a variety of native assets, including illiquid assets and NFT assets. Economic feasibility depends on the type of Chain Relay. Sub-linear Chain Relay can achieve better economic feasibility.

Darwinia’s cross-chain token bridge solution is a two-way cross-chain token bridge based on chain relay. The chain relay we are developing adopts the design of the Darwinia Sublinear Relay, which has better performance and economic feasibility.

Darwinia Network is a decentralized cross-chain bridge network building on Substrate, which is the “Web 3.0 cross-chain bridge hub” of the cross-chain ecology. It provides a safe and general bridging solution, connects to Polkadot, Ethereum, TRON and other heterogeneous chains by cross-chain transfer of assets and remote chain calls. The application areas of Darwinia Network include DeFi, cross-chain NFT trading, games, etc.

Darwinia Network has gained a high reputation and recognition along the way to build the decentralized cross-chain bridge protocol. In 2020, Darwinia was written in Polkadot light-paper as one of the friends of Polkadot and Substrate. And Darwinia was selected to join Substrate Builder Program and Web3.0 Bootcamp, and for the outstanding work in Substrate Builder Program, Darwinia Network was officially awarded the Level 2 badge by Parity. The products and tools developed by Darwinia have been rewarded three W3F Grants.

The cross-chain token bridge refers to the asset transfer channel between two heterogeneous chains. Through the token bridge, assets can be safely and reliably transferred on different heterogeneous chains. The token bridge solution in this article will be backed by the concept of cryptocurrency backed asset (CBA) model and decentralized backing technology.

CBA stands for Cryptocurrency Backed Asset. To build the CBA model, we need to build a Backing module on the source chain and an Issuing module on the target chain.

Backing Module: The module that locks and unlocks the native assets on the source chain. Issuing Module: The module that issues and burns the mapped assets on the target chain.

In the cross-chain CBA model, the two heterogeneous chains are called Source chain (backing blockchain) and Target chain (issuing blockchain) respectively. Cross-chain assets do not reside natively on either chain but as a conceptual medium to describe the process of cross-chain circulation, which involves locking backing assets on the source chain while issuing assets on the target chain. The assets issued by backing native assets on the source blockchain are called backed assets, or CBA for short. The security and redeemability of backed assets are determined by the security and reliability of decentralized backing technology. Darwinia cross-chain token bridge is a cross-chain bridge solution between the source chain and the target chain as well as a decentralized asset backing technology.

A Bridge Creator from the Source Chain (Backing BlockChain) deploys a contract called “Backing” on this chain; meanwhile, a Bridge Creator from Target Chain (Issuing BlockChain) deploys a contract named “Issuing” on their chain.

Users call the “register” interface in the Backing module by a transaction to register their original asset. And then relayers will synchronize this register message to the target chain. After this message is accepted by the target chain(must be proved), the Issuing module will create a corresponding mapping-contract to issue and store the mapped token.

A Requestor from the source chain locks a specific amount of native assets and assigns a receiving account on the target chain by calling the lock_and_remote_issue() interface of the Backing module. They can get the proof of locking from the source chain when succeeding. Then the relayer forwards the proof to the Issuing module on the target chain. In the end, the Issuing module writes the calculated amount of mapped tokens to the receiving account after verifying the proof.

The mapped tokens satisfy the standard format as the original tokens. Holders have the full authorities of the mapped token. And the mapped token can also be transferred from one account to another. Also, by using any swap pool, users can swap their mapped tokens to other tokens on the target chain.

A Requestor as an asset holder from the target chain calls the remote_unlock() interface in the Issuing module to burn an amount of mapped tokens on the target chain and assign a receiving account on the Source Chain. Then the relayer delivers the proof of burning to the Backing module on the source chain. In the end, the Backing module releases the locked assets to the assigned receiving account after verifying the proof.

Common decentralized backing technologies include:

• There are trust nodes or custodian mechanisms, where the trusted node or custodian uses multi-signature / threshold signature and other multi-centralized technologies to avoid single points of failure, such as Parity Bridge, ChainX, etc.

• Some solutions use collateralized bridges to ensure the redeemability of cross-chain assets, partially combined with Chain Relay. XClaim has the disadvantage that it is only suitable for homogeneous assets with good liquidity, and its economic feasibility is relatively weak.

• Fully use the Chain Relay for chain verification, such as BTCRelay, WaterLoo, Darwinia ChainRelay, etc. The prerequisite of using this solution is that the issuing chain needs to support the implementation of smart contracts, on-chain runtimes, hard forks, and other methods. Such blockchains like Chain Relay and Bitcoin network cannot be used as an issuing chain. The advantage is that it can support a variety of native assets, including illiquid assets and NFT assets. Economic feasibility depends on the type of Chain Relay. Sub-linear Chain Relay can achieve better economic feasibility.

Darwinia’s cross-chain token bridge solution is a two-way cross-chain token bridge based on chain relay. The chain relay we are developing adopts the design of the Darwinia Sublinear Relay, which has better performance and economic feasibility.

Darwinia Network is a decentralized cross-chain bridge network building on Substrate, which is the “Web 3.0 cross-chain bridge hub” of the cross-chain ecology. It provides a safe and general bridging solution, connects to Polkadot, Ethereum, TRON and other heterogeneous chains by cross-chain transfer of assets and remote chain calls. The application areas of Darwinia Network include DeFi, cross-chain NFT trading, games, etc.

Darwinia Network has gained a high reputation and recognition along the way to build the decentralized cross-chain bridge protocol. In 2020, Darwinia was written in Polkadot light-paper as one of the friends of Polkadot and Substrate. And Darwinia was selected to join Substrate Builder Program and Web3.0 Bootcamp, and for the outstanding work in Substrate Builder Program, Darwinia Network was officially awarded the Level 2 badge by Parity. The products and tools developed by Darwinia have been rewarded three W3F Grants.