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Introduction to Chromia token bridge

The Chromia token bridge enables seamless transfer of Ethereum Virtual Machine (EVM)-compatible fungible and non-fungible tokens 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.

How does the bridge work?​

Chromia uses the following bridge mechanisms:

  • Lock and mint – Locks assets on the source chain and mints equivalent tokens on the destination chain.

  • Burn and release – Burns tokens on the Chromia chain and releases the original tokens on the source chain.

Rell structureRell structure

Lock and mint​

  1. Deposit: A user deposits tokens (e.g., ERC20 or ERC721) on an EVM-compatible chain, locking them into a smart contract.
  2. Monitor: Chromia's validator nodes monitor this deposit event on the EVM chain.
  3. Detect: Upon detecting the event, Chromia's validator nodes create, validate, and execute a transaction using a special Rell operation called __evm_block.
  4. Mint: Chromia mints pegged tokens equivalent to the locked tokens on the Chromia chain.

If the user wants to transfer the pegged tokens back to the original chain, the Burn and release mechanism is used.

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 retrieve the withdrawal event's Merkle proof from Chromia nodes.
  4. Release: Using the proof, users call the smart contract on the EVM chain to release the original tokens.