PoA Ethereum High Level Documentation

NOTE: This is from the old README

Ethereum Bridge Runtime Module

The main job of this runtime module is to keep track of useful information an Ethereum PoA chain which has been submitted by a bridge relayer. This includes:

Ethereum headers and their status (e.g are they the best header, are they finalized, etc.)
Current validator set, and upcoming validator sets

This runtime module has more responsibilties than simply storing headers and validator sets. It is able to perform checks on the incoming headers to verify their general integrity, as well as whether or not they've been finalized by the authorities on the PoA chain.

This module is laid out as so:

├── ethereum
│  └── src
│     ├── error.rs        // Runtime error handling
│     ├── finality.rs     // Manage finality operations
│     ├── import.rs       // Import new Ethereum headers
│     ├── lib.rs          // Store headers and validator set info
│     ├── validators.rs   // Track current and future PoA validator sets
│     └── verification.rs // Verify validity of incoming Ethereum headers

Currency Exchange Runtime Module

The currency exchange module is used to faciliate cross-chain funds transfers. It works by accepting a transaction which proves that funds were locked on one chain, and releases a corresponding amount of funds on the recieving chain.

For example: Alice would like to send funds from chain A to chain B. What she would do is send a transaction to chain A indicating that she would like to send funds to an address on chain B. This transaction would contain the amount of funds she would like to send, as well as the address of the recipient on chain B. These funds would now be locked on chain A. Once the block containing this "locked-funds" transaction is finalized it can be relayed to chain B. Chain B will verify that this transaction was included in a finalized block on chain A, and if successful deposit funds into the recipient account on chain B.

Chain B would need a way to convert from a foreign currency to its local currency. How this is done is left to the runtime developer for chain B.

This module is one example of how an on-chain light client can be used to prove a particular action was taken on a foreign chain. In particular it enables transfers of the foreign chain's native currency, but more sophisticated modules such as ERC20 token transfers or arbitrary message transfers are being worked on as well.

Ethereum Node

On the Ethereum side of things, we require two things. First, a Solidity smart contract to track the Substrate headers which have been submitted to the bridge (by the relay), and a built-in contract to be able to verify that headers have been finalized by the GRANDPA finality gadget. Together this allows the Ethereum PoA chain to verify the integrity and finality of incoming Substrate headers.

The Solidity smart contract is not part of this repo, but can be found here if you're curious. We have the contract ABI in the ethereum/relays/res directory.

Rialto Runtime

The node runtime consists of several runtime modules, however not all of them are used at the same time. When running an Ethereum PoA to Substrate bridge the modules required are the Ethereum module and the currency exchange module. When running a Substrate to Substrate bridge the Substrate and currency exchange modules are required.

Below is a brief description of each of the runtime modules.

Bridge Relay

The bridge relay is responsible for syncing the chains which are being bridged, and passing messages between them. The current implementation of the relay supportings syncing and interacting with Ethereum PoA and Substrate chains.

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