Non-fungible Token Bridging

Pegged NFT Bridge

This is intended for existing NFTs that have an origin chain. The architecture is:

• The NFTBridge contract is deployed on the origin chain, and the PegNFT contract is deployed on the destination chain.

• NFTBridge: to lock or release the original NFT

• PegNFT: to mint or burn the pegged NFT

• Relayers will be deployed on both the source chain and the destination chain in order to fetch the SGN’s multisig attestations and call the NFTBridge and PegNFT contracts.

When transferring an NFT from the origin chain to the destination chain, the bridge mints a "copy" of the NFT on the destination chain that is pegged to the original NFT on the source chain. In the same style as in Scenario 1 and Scenario 2 of the fungible token transfer, the Pegged NFT bridge process is supported by the Lock-and-Mint and Burn-and-Release models.

Multi-Chain Native (MCN) NFT Bridge

Different from the Pegged NFT, the MCN NFT does not have the notion of an “origin chain” or "original NFT". The MCN NFT Bridge only applies to newly deployed NFTs that follow this template. The architecture is:

• The MCNNFT contract is deployed on all chains you would like to bridge the NFT to, this is to burn or mint the NFT.
• Celer’s State Guardian Network (SGN) serves as a message router for cross-chain transfers. It is a Proof-of-Stake (PoS) blockchain built on Tendermint. Validator nodes must stake $CELR to join the consensus process of the SGN.

• Relayers will be deployed on all chains to fetch the SGN’s multisig attestations and call the MCNNFT contract.

When transferring an MCN NFT from one chain to another, the bridge process is supported by a Burn-and-Mint model. In this way, there is always only one unique and native NFT existing across all chains.

Bridging Fee Structure

This is the same as the Fungible Token Bridging model, the Non-Fungible Token Bridging Fee = Base Fee + Protocol Fee, whereas Base Fee is to cover the destination chain gas cost for minting the NFT and the Protocol Fee is paid to the State Guardian Network (SGN) validators and stakers in return for their services of relaying the bridging message that is passed across the chains. The Protocol Fee is proportional to the message size, determined by the NFT token’s address, the token ID, and the token URL length. In most cases, the Protocol Fee is very low.