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Hyperbridges

Introduction

In the Shared bridge document we described how the L1 smart contracts work to support multiple chains, and we emphasized that the core feature is hyperbridging, but we did not outline the hyperbridges themselves. This is because hyperbridges are mostly L2 contracts. In this document we describe what hyperbridges are, and specify the necessary infrastructure.

Hyperbridge description

Hyperbridges are trustless and cheap general native bridges between hyperchains, allowing cross-chain function calls. Trustlessness is achieved by relying on the main hyperchain bridge to send a compressed message to L1, which is then sent to and expanded on the destination hyperchain.

Technically they are a system of smart contracts that build on top of the enshrined L1<>L2 validating bridges, and can interpret messages sent from L2 to L2 by verifying Merkle proofs. They are built alongside the protocol, they can transfer the native asset of the ecosystem, and they can be used for asynchronous function calls between hyperchains.

Hyperbridges

The trustless nature of hyperbridges allows the ecosystem to resemble a single VM. To illustrate imagine a new hyperchain joining the ecosystem. We will want ether/Dai/etc. to be accessible on this hyperchain. This can be done automatically. There will be a central erc20 deployer contract in the ecosystem, which will deploy the new ERC20 contract via the hyperbridge. After the contract is deployed it will be able to interact other Dai contracts in the ecosystem.

High Level design

Hyperbridging

L1

For the larger context see the Shared Bridge document, here we will focus on

  • HyperMailbox (part of Bridgehub). Contains the Hyperroot, root of Merkle tree of Hyperlogs. Hyperlogs are the L2->L1 SysLogs that record the sent hyperbridge messages from the L2s.

L2 Contracts

  • Outbox system contract. It collects the hyperbridge txs into the hyperlog of the hyperchain.
  • Inbox system contract. This is where the hyperroot is imported and sent to L1 for settlement. Merkle proofs are verified here, tx calls are started from here, nullifiers are stored here (add epochs later)