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Introduction

Tornado Nova implements a cross-chain bridge system that enables seamless interaction between Ethereum mainnet (L1) and Gnosis Chain (formerly xDai, L2). The bridge infrastructure consists of two main components:
  • L1Unwrapper: Handles ETH↔WETH bridging and account registration on mainnet
  • CrossChainGuard: Enables cross-chain governance via the Arbitrary Message Bridge (AMB)

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Architecture

The bridge system leverages two distinct bridge protocols:
1

OmniBridge for Asset Transfers

The OmniBridge protocol facilitates ERC20 token transfers between chains. L1Unwrapper extends WETHOmnibridgeRouter to handle ETH/WETH conversions automatically.
2

AMB for Governance Messages

The Arbitrary Message Bridge (AMB) enables cross-chain contract calls, allowing mainnet governance to control L2 contracts via CrossChainGuard.

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ETH↔WETH Bridging Flow

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L1 → L2 (Mainnet to Gnosis Chain)

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L2 → L1 (Gnosis Chain to Mainnet)

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Key Features

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Automatic ETH/WETH Conversion

Users don’t need to manually wrap ETH. The L1Unwrapper handles:
  • Deposits: Wraps ETH to WETH before bridging
  • Withdrawals: Unwraps WETH to ETH after bridging

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Account Registration

The bridge system includes TornadoPool account registration, allowing users to register their public keys during the bridging process. 
struct Account {
  address owner;
  bytes publicKey;
}

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L1 Fee Mechanism

Withdrawals to L1 include a configurable fee to cover gas costs:
  • Fee can be sent to a designated l1FeeReceiver contract
  • If no receiver is set, fee goes to tx.origin (the relayer)
  • Enables subsidized transactions and fee sharing logic

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Cross-Chain Governance

The CrossChainGuard pattern enables secure cross-chain administration:
The guard verifies that calls originate from the correct chain and authorized owner address through the AMB bridge, preventing unauthorized cross-chain access.

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Bridge Protocols

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OmniBridge

Purpose: ERC20 token transfers between chains Documentation: TokenBridge OmniBridge Docs Key Interface: 
interface IOmnibridge {
  function relayTokensAndCall(
    address token,
    address receiver,
    uint256 value,
    bytes calldata data
  ) external;
}

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Arbitrary Message Bridge (AMB)

Purpose: Cross-chain contract calls and governance Documentation: AMB Development Guide Key Interface: 
interface IAMB {
  function messageSender() external view returns (address);
  function messageSourceChainId() external view returns (bytes32);
}

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Security Considerations

Bridge Trust AssumptionsThe bridge system relies on the security of both OmniBridge and AMB protocols. Users should be aware that:
  • Bridge validators control asset transfers
  • Cross-chain messages are subject to bridge confirmation times
  • Bridge upgrades or governance changes could affect functionality

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L1Unwrapper Security

  • Only accepts WETH tokens to prevent malicious token bridging
  • Validates msg.sender is the bridge contract in onTokenBridged()
  • Enforces correct data length (64 bytes) for recipient and fee data
  • Uses SafeMath for all arithmetic operations

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CrossChainGuard Security

  • Immutable configuration (bridge, chainId, owner) set at deployment
  • Triple verification in isCalledByOwner():
    1. Caller must be the AMB bridge contract
    2. Message must originate from the correct chain
    3. Message sender must be the authorized owner

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External Resources

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