ZIP-0800: Zoo-Lux Bridge Protocol

Abstract

This proposal defines the cross-chain bridge protocol between Zoo L2 (Chain ID: 200200) and Lux Network (C-Chain, X-Chain, B-Chain). The bridge is built on the Lux Teleport architecture (LP-6332) and secured by MPC (Multi-Party Computation) threshold signatures with a 2/3 supermajority requirement. It supports bridging of ZRC-20 fungible tokens, ZRC-721 wildlife NFTs (with metadata preservation), ZRC-1155 multi-tokens, and native ZOO/LUX gas tokens. The protocol ensures that conservation metadata, provenance chains, and token-bound account states are faithfully preserved across chains.

Motivation

The Zoo ecosystem requires seamless value transfer between Zoo L2 and Lux Network for:

Liquidity Access: ZOO tokens must trade on Lux DEXs and access Lux DeFi liquidity pools.
Settlement: Conservation fund disbursements settled on Lux C-Chain benefit from its finality guarantees.
Cross-Chain NFTs: Wildlife NFTs minted on Zoo must be exhibitable and tradeable on Lux marketplaces.
Compute Payment: AI inference on Hanzo (via Lux) must be payable with ZOO tokens (see ZIP-0801).
Ecosystem Unity: ZIP-0042 mandates cross-ecosystem interoperability. The bridge is the physical infrastructure for that mandate.

Existing generic bridges (e.g., LayerZero, Wormhole) lack support for conservation-specific metadata, provenance chains, and token-bound account synchronization. A purpose-built bridge ensures these Zoo primitives survive cross-chain transfers intact.

Specification

Architecture Overview

Zoo L2 (200200)                                              Lux Network
┌─────────────────────┐                                ┌─────────────────────┐
│  ZooBridgeGateway   │◄──────── MPC Relayers ────────►│  LuxBridgeGateway   │
│                     │         (threshold 2/3)         │                     │
│  ┌───────────────┐  │                                │  ┌───────────────┐  │
│  │ Token Locker  │  │    ┌──────────────────────┐    │  │ Token Minter  │  │
│  │ (lock/unlock) │  │    │   MPC Signer Set     │    │  │ (mint/burn)   │  │
│  └───────────────┘  │    │                      │    │  └───────────────┘  │
│  ┌───────────────┐  │    │  S1  S2  S3 ... Sn   │    │  ┌───────────────┐  │
│  │ Metadata      │  │    │  (n >= 7, t = 2n/3)  │    │  │ Metadata      │  │
│  │ Serializer    │  │    └──────────────────────┘    │  │ Deserializer  │  │
│  └───────────────┘  │                                │  └───────────────┘  │
│  ┌───────────────┐  │                                │  ┌───────────────┐  │
│  │ TBA Freezer   │  │                                │  │ TBA Mirror    │  │
│  └───────────────┘  │                                │  └───────────────┘  │
└─────────────────────┘                                └─────────────────────┘

Bridge Gateway Interface

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.20;

/**
 * @title IZooBridgeGateway
 * @notice Gateway contract for Zoo-Lux cross-chain bridge.
 * @dev Deployed on both Zoo L2 and Lux C-Chain.
 */
interface IZooBridgeGateway {

    enum AssetType {
        NATIVE,     // ZOO or LUX native gas token
        ZRC20,      // Fungible token
        ZRC721,     // Non-fungible token
        ZRC1155     // Multi-token
    }

    struct BridgeMessage {
        bytes32 messageId;          // Unique message identifier
        uint256 sourceChainId;      // Origin chain
        uint256 destinationChainId; // Target chain
        address sender;             // Initiator on source chain
        address recipient;          // Recipient on destination chain
        AssetType assetType;        // Type of asset being bridged
        address tokenContract;      // Token contract on source chain
        uint256 tokenId;            // Token ID (0 for fungibles)
        uint256 amount;             // Amount (1 for NFTs)
        bytes metadata;             // Serialized conservation metadata
        uint256 nonce;              // Replay protection
        uint256 deadline;           // Message expiry timestamp
    }

    /**
     * @notice Initiates a bridge transfer from this chain to the destination.
     */
    function bridgeOut(
        uint256 destinationChainId,
        address recipient,
        AssetType assetType,
        address tokenContract,
        uint256 tokenId,
        uint256 amount
    ) external payable returns (bytes32 messageId);

    /**
     * @notice Completes a bridge transfer on the destination chain.
     *         Only callable by the MPC relayer with threshold signatures.
     */
    function bridgeIn(
        BridgeMessage calldata message,
        bytes calldata mpcSignature
    ) external;

    /**
     * @notice Returns the MPC signer set address (threshold verification contract).
     */
    function mpcSignerSet() external view returns (address);

    /**
     * @notice Returns whether a message ID has been processed (replay protection).
     */
    function processedMessages(bytes32 messageId) external view returns (bool);

    event BridgeOutInitiated(
        bytes32 indexed messageId,
        address indexed sender,
        uint256 destinationChainId,
        AssetType assetType,
        address tokenContract,
        uint256 tokenId,
        uint256 amount
    );

    event BridgeInCompleted(
        bytes32 indexed messageId,
        address indexed recipient,
        uint256 sourceChainId,
        AssetType assetType,
        address tokenContract,
        uint256 tokenId,
        uint256 amount
    );

    event BridgeMessageFailed(
        bytes32 indexed messageId,
        string reason
    );
}

MPC Threshold Signature Scheme

The bridge is secured by an MPC signer set using GG20 (Gennaro-Goldfeder 2020) threshold ECDSA:

Parameter	Value
Minimum signers (n)	7
Threshold (t)	ceil(2n/3)
Key generation	Distributed key generation (DKG)
Signature scheme	Threshold ECDSA (secp256k1)
Rotation period	30 days
Slashing	Signers stake 10,000 LUX; slashed for equivocation

/**
 * @title IMPCSignerSet
 * @notice Verifies MPC threshold signatures for bridge messages.
 */
interface IMPCSignerSet {
    /**
     * @notice Verifies that a message was signed by the threshold of MPC signers.
     * @param messageHash The keccak256 hash of the bridge message.
     * @param signature The aggregated MPC signature.
     * @return valid True if the signature meets the threshold requirement.
     */
    function verifyThresholdSignature(
        bytes32 messageHash,
        bytes calldata signature
    ) external view returns (bool valid);

    /**
     * @notice Returns the current signer set configuration.
     */
    function signerSetInfo() external view returns (
        uint256 totalSigners,
        uint256 threshold,
        uint256 rotationEpoch,
        bytes32 publicKeyHash
    );

    /**
     * @notice Proposes a signer set rotation. Requires governance approval.
     */
    function proposeRotation(
        address[] calldata newSigners,
        uint256 newThreshold
    ) external;

    event SignerSetRotated(
        uint256 epoch,
        uint256 totalSigners,
        uint256 threshold,
        bytes32 publicKeyHash
    );
}

Asset Bridging Mechanics

Native Token (ZOO/LUX)

Zoo to Lux: ZOO locked in ZooBridgeGateway. Wrapped ZOO (wZOO) minted on Lux as LRC-20 (LP-3800).
Lux to Zoo: wZOO burned on Lux. ZOO unlocked on Zoo L2.
LUX follows the reverse path (locked on Lux, wrapped on Zoo).

ZRC-20 Tokens

Zoo to Lux: Tokens locked. Bridged LRC-20 representation minted on Lux with matching metadata per LP-3800.
Lux to Zoo: Bridged tokens burned. Original ZRC-20 unlocked. Conservation extensions (donation rate, impact credits) are restored from on-chain state.

ZRC-721 NFTs

Zoo to Lux: NFT locked. Species metadata serialized and included in BridgeMessage.metadata. Provenance chain snapshot uploaded to IPFS and CID included. TBA frozen (ZIP-0703). Mirror LRC-721 minted on Lux with full metadata.
Lux to Zoo: Mirror burned. Original NFT unlocked. TBA unfrozen. New provenance record added: "bridged_back".

ZRC-1155 Multi-Tokens

Batch bridging: Multiple token IDs and amounts can be bridged in a single message. The metadata field contains serialized TokenTypeInfo for each bridged type.
Soulbound tokens MUST NOT be bridged. The gateway rejects any bridgeOut call for non-transferable token IDs.

Metadata Serialization

Conservation metadata is serialized using ABI encoding for deterministic cross-chain reconstruction:

// ZRC-721 metadata serialization
bytes memory metadata = abi.encode(
    speciesMetadata,           // SpeciesMetadata struct
    provenanceSnapshotCID,     // IPFS CID of full provenance chain
    conservationFundAddress,   // Conservation royalty fund
    royaltySplit,              // (conservationBps, creatorBps)
    tbaAddress,                // Token-bound account address on source chain
    tbaBalanceSnapshot         // ZOO balance in TBA at bridge time
);

Fee Structure

Operation	Fee	Recipient
Native token bridge	0.1% of amount	MPC signer set (operational costs)
ZRC-20 bridge	0.05% of amount	50% signers, 50% conservation fund
ZRC-721 bridge	Flat 10 ZOO	Conservation fund
ZRC-1155 batch	5 ZOO per token type	Conservation fund

Liveness and Safety

Liveness: If the MPC signer set is unavailable for > 72 hours, a governance-controlled emergency withdrawal path activates, allowing users to reclaim locked assets with a 7-day timelock.
Safety: Messages expire after deadline (default 24 hours). Expired messages are rejected. Each messageId can only be processed once (replay protection via processedMessages mapping).
Finality: Bridge messages are only relayed after source chain finality. Zoo L2 inherits sub-second finality from Lux primary network (ZIP-0015). Lux C-Chain requires 2 seconds for finality.

Rationale

Why MPC over multisig? MPC threshold ECDSA produces a single on-chain signature regardless of signer set size, reducing gas costs. Multisig requires per-signer signature verification on-chain, which scales linearly with signer count.

Why not use the Lux native bridge (Teleport) directly? Teleport (LP-6332) provides the transport layer. ZIP-0800 adds the application layer for conservation metadata serialization, TBA synchronization, and conservation-specific fee routing. We build on Teleport, not replace it.

Why freeze TBAs on bridge? A TBA's assets could be double-spent if the NFT and its account were active on both chains simultaneously. Freezing ensures exactly-once asset access.

Why conservation fee routing? The bridge is ecosystem infrastructure. Directing a portion of bridge fees to conservation aligns infrastructure usage with the Zoo Foundation's mission (ZIP-0500).

Backwards Compatibility

The bridge is compatible with LP-3800 (Bridged Asset Standard) and LP-6000 (B-Chain Bridge Specification). Bridged tokens on Lux appear as standard LRC-20/LRC-721/LRC-1155 tokens and are usable in any Lux DeFi protocol. The conservation metadata is stored in auxiliary mappings on the Lux gateway and does not interfere with standard token interfaces.

Security Considerations

MPC Key Compromise: If t signers are compromised, the bridge is vulnerable. Mitigation: 30-day key rotation, geographic distribution of signers, hardware security modules (HSMs), and slashing for equivocation.
Replay Attacks: Each messageId is tracked in processedMessages. The nonce and deadline fields provide additional replay protection.
Metadata Tampering: The messageHash covers all fields including metadata. MPC signature verification ensures metadata integrity.
Reentrancy: bridgeIn mints or unlocks tokens before external calls. Implementations MUST follow checks-effects-interactions pattern.
Censorship: If MPC relayers censor specific messages, the 72-hour liveness fallback allows governance to force-process messages.
Oracle Price Manipulation: Bridge fees denominated in ZOO are not subject to price oracle manipulation since they are percentage-based or flat-rate.
TBA Freeze Race Condition: The TBA must be frozen before the bridge message is relayed. The bridgeOut function MUST atomically lock the NFT and freeze the TBA in a single transaction.
Signer Set Rotation: During rotation, both old and new signer sets are valid for a 24-hour overlap period. Messages signed by either set are accepted during this window.

Zoo-Lux Bridge Protocol