LP-0025Finalfraud-proofsinteractivedisputeverificationoptimistic

LP-025: Fraud Proofs

Abstract

Lux fraud proofs enable optimistic execution verification through an interactive bisection game. An asserter posts a state commitment for a batch of transactions. If a challenger disagrees, they engage in a binary search over execution steps to isolate the single disputed instruction. An on-chain referee contract verifies that one instruction in a MIPS/RISC-V emulator, resolving the dispute with O(log n) on-chain interactions for n execution steps.

Specification

Assertion

An asserter posts a state commitment:


Assertion {
    batchIndex      uint64     // which transaction batch
    preStateRoot    [32]byte   // state root before batch execution
    postStateRoot   [32]byte   // claimed state root after execution
    numSteps        uint64     // total VM execution steps
    bond            uint256    // stake posted (forfeited if wrong)
}

The assertion enters a CHALLENGE_WINDOW (default 7 days) during which any party can challenge.

Bisection Game

If challenged, the asserter and challenger engage in interactive bisection:

1. Challenger posts ChallengeAssertion referencing the disputed assertion

2. Both parties iteratively bisect the execution trace:

Asserter provides midpoint state hash at step n/2
Challenger indicates which half they dispute
Repeat until a single step is isolated

3. Total rounds: ceil(log2(numSteps)) -- typically 40-50 rounds for 10^12 steps

One-Step Proof

The isolated step is verified on-chain by executing a single instruction in a minimal VM emulator:


OneStepProof {
    preState    MemoryProof   // Merkle proof of memory/registers before step
    instruction []byte        // the single instruction to execute
    postState   [32]byte      // claimed state after instruction
}

The referee contract executes the instruction against preState and compares the result to postState. If they differ, the asserter is slashed. If they match, the challenger is slashed.

Bonds and Incentives

Assertion bond: 1000 LUX (returned if unchallenged or if asserter wins)
Challenge bond: 1000 LUX (returned if challenger wins)
Timeout: each bisection round has a MOVE_TIMEOUT (1 hour). If a party fails to respond, they forfeit.
Reward: the winner receives the loser's bond minus gas costs

Application

Fraud proofs apply to:

1. Optimistic rollups: L2 chains posting state roots to Lux L1

2. DA fraud proofs: proving incorrect erasure coding (LP-023)

3. Cross-chain disputes: verifying Warp message correctness (LP-021)

Security Considerations

1. Liveness assumption: at least one honest party must monitor assertions and challenge fraud within the window.

2. Delay attack: a malicious asserter can delay finality by up to CHALLENGE_WINDOW + (log2(steps) * MOVE_TIMEOUT). Bond requirements make this expensive.

3. Referee correctness: the on-chain instruction emulator must be equivalent to the off-chain VM. Formal verification of the emulator is recommended.

Reference

| Resource | Location |

|---|---|

| Fraud proof contracts | github.com/luxfi/standard/contracts/fraud/ |

| VM emulator | github.com/luxfi/node/fraud/emulator/ |

| DA fraud proofs | LP-023 |

Copyright

Licensed under the MIT License.