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Overview

The Chromia token bridge enables seamless transfer of Ethereum Virtual Machine (EVM)-compatible tokens and NFTs between EVM-compatible chains and Chromia. This compatibility ensures that Chromia dapps can communicate with the broader Ethereum ecosystem, enhancing interoperability and simplifying project development.

The entire bridging mechanism is based on the Ethereum Interoperability Framework (EIF), designed to bridge any data to and from EVM chains. EIF allows developers to use its Rell module and EVM smart contracts to set up their bridges for specific data needs in their dapps. Chromia uses the EIF Rell module to facilitate communication between Chromia and EVM-compatible chains, enabling developers to build Chromia dapps that can interact with Ethereum and other platforms.

There are two main ways to bridge:

  • Pegged bridge (H-bridge): The pegged bridge makes a copy of your token on Chromia, managed by the Chromia network for safe transfers. This implementation within the EIF framework is designed for token bridging. It is ideal for developers who want to bridge standard tokens without creating their own bridges. The pegged bridge replicates or 'pegs' the original token on the Chromia blockchain, with these pegged tokens managed by the Chromia network to ensure secure transfers. Data incoming from EVM chains is evaluated according to the same consensus rules as the rest of the Chromia network. Outgoing data from Chromia to EVM chains is validated by smart contracts on the EVM chain, which require a supermajority of providers to have signed the block, adhering to the greater than ⅔ consensus requirement as the rest of the network.
  • Originals NFT protocol: This protocol allows projects from various blockchains to migrate their NFTs (non-fungible tokens) to Chromia. This migration includes metadata, images, and other relevant information associated with the NFT, transforming them into Originals on Chromia. When fungible tokens from EVM-compatible chains (like Ethereum) are brought to Chromia, they are converted into a format that follows the FT4 protocol.

How does the bridge work?​

Chromia uses the following bridge mechanisms:

  • Lock and mint – Lock assets on the source chain and mint assets on the destination chain.

  • Burn and release – Burn assets on the Chromia chain and release assets on the destination chain.

Rell structureRell structure

Lock and mint​

  1. Deposit: Users deposit tokens (ERC20, ERC721) on an EVM-compatible chain, which are locked in a smart contract on that chain.
  2. Monitor: Chromia validator nodes monitor the deposit event on the EVM-compatible chain.
  3. Detect: Upon detecting the deposit event, Chromia validator nodes create, validate, and execute a special transaction using a specific Rell operation called __evm_block.
  4. Mint: This transaction mints equivalent pegged tokens for the user on the Chromia chain.

When the user wishes to withdraw the pegged tokens from Chromia back to the original token on the EVM-compatible chain, the bridge follows the Burn and release mechanism.

Burn and release​

  1. Burn: The pegged token is burned on the Chromia chain.
  2. Withdrawal event: A withdrawal event is emitted on the Chromia chain.
  3. Query proof: Users can query the token withdraw event Merkle proof on the Chromia node.
  4. Release: They can then use the proof data to call the smart contract for releasing the original token on the EVM-compatible chain.