LP-0034Finalbridgeb-chaincross-chainmpclock

LP-034: Bridge VM

Abstract

The Bridge VM runs the B-chain, a purpose-built chain for cross-chain asset custody and settlement. It manages the lock/mint/burn/release lifecycle for bridged assets across 270+ chain IDs. The B-chain holds custodied assets in MPC-controlled vaults (LP-019), tracks per-chain collateral invariants, and processes bridge operations as first-class VM transactions rather than smart contract calls.

Specification

Vault Model

Each supported external chain has a vault on the B-chain:


Vault {
    chainID         uint256
    custodiedAssets  map[assetID]uint256  // locked balances per asset
    mintedSupply    map[assetID]uint256  // minted bridged tokens
    signerGroupKey  [32]byte             // MPC aggregate key for this vault
    dailyLimit      uint256
    dailyVolume     uint256
    active          bool
}

The collateral invariant custodiedAssets[asset] >= mintedSupply[asset] is enforced at the VM level.

Transaction Types

| Tx Type | Description |

|---|---|

| LockTx | Lock assets in a vault (initiates bridge-out) |

| MintTx | Mint bridged tokens (requires MPC signature, completes bridge-in) |

| BurnTx | Burn bridged tokens (initiates bridge-back, always succeeds) |

| ReleaseTx | Release custodied assets (requires MPC signature, completes bridge-back) |

| RotateSignerTx | Propose MPC key rotation (7-day timelock, per LP-016) |

| RegisterVaultTx | Register a new chain vault (governance) |

Bridge Flow

Inbound (External -> Lux):

1. User locks tokens on external chain (observed by MPC watchers)

2. MPC group signs a MintTx on the B-chain

3. B-chain VM verifies MPC aggregate signature, increments mintedSupply, mints bridged tokens

4. Bridged tokens are Warp-transferred to the user's target Lux chain (C/Zoo/Hanzo/etc.)

Outbound (Lux -> External):

1. User submits BurnTx on B-chain (always succeeds, no pause gate)

2. MPC group observes the burn, signs a release on the external chain

3. B-chain VM decrements mintedSupply and custodiedAssets

Warp Integration

The B-chain is a Warp message hub. Bridged token minting on target chains (C-chain, application L1/L2 chain-VMs) is triggered by Warp messages from the B-chain. This means bridge operations have Lux-native finality without relying on external relayers.

Rate Limiting

Per-vault daily limits bound blast radius:

dailyLimit: maximum value bridged per chain per day
Auto-pause: if dailyVolume exceeds dailyLimit, the vault pauses until the next UTC day

Security Considerations

1. Exit guarantee: BurnTx is never gated by pause state. Users can always initiate exit.

2. MPC custody: no single key controls vault assets. Threshold signature required for all mints and releases.

3. Per-chain isolation: a compromised vault on chain A does not affect chain B's vault.

Reference

| Resource | Location |

|---|---|

| Bridge VM | github.com/luxfi/node/vms/bridgevm/ |

| MPC signing | LP-019 |

| OmnichainRouter | LP-016 |

| Warp messaging | LP-021 |

Copyright

Licensed under the MIT License.