Single Round Fraud Proof

A single round fraud proof is a cryptographic challenge protocol that allows a verifier to prove a state transition is invalid in one on-chain transaction, without requiring interactive, multi-step games. This is a key innovation for Optimistic Rollups, which operate on the principle that state updates are presumed valid unless proven otherwise (fraudulent). Unlike earlier multi-round interactive fraud proofs, which required a lengthy back-and-forth challenge game on the parent chain (like Ethereum), the single-round model compresses the entire dispute into a single, verifiable data submission. This dramatically reduces the dispute resolution time and gas costs associated with proving fraud.

The mechanism relies on a fault proof program—a self-contained, deterministic computation—that is executed on-chain to verify a specific step in a rollup's state transition. When a challenger submits a fraud proof, they provide the pre-state, the contested transaction(s), and the alleged invalid post-state. The parent chain's execution environment (e.g., an Ethereum Virtual Machine or a dedicated verifier contract) then re-executes the step. If the on-chain execution result differs from the state root posted by the rollup's sequencer, the fraudulent state update is reverted. This design shifts the computational burden of verification to the chain itself for one critical step, rather than replaying an entire transaction batch.

Implementing single round fraud proofs requires careful engineering of the underlying data availability and state commitment schemes. The system must guarantee that all data necessary for the fault proof program—such as specific transaction inputs and intermediate Merkle proofs—is available on-chain, typically via data availability layers or calldata. Projects like Arbitrum Nitro pioneered this approach with its Arbitrum Virtual Machine (AVM) and one-step proof system. The primary advantage is a short, predictable challenge period (often as low as 24 hours), enhancing capital efficiency for users waiting for withdrawals, while maintaining strong security guarantees derived from the parent chain's validators.

A single-round fraud proof is a cryptographic challenge protocol that enables a verifier to prove a state transition is invalid by submitting a single, self-contained transaction to the L1 (Layer 1) blockchain. Unlike interactive, multi-round proofs, this mechanism does not require a lengthy back-and-forth challenge game. Instead, the challenger submits a succinct proof—often a Merkle proof of specific transaction data and the disputed state—directly to a smart contract on the base chain. This contract then verifies the proof's logic in one execution, immediately slashing the fraudulent proposer's bond and reverting the invalid state if the challenge is successful.

The process relies on data availability and the publication of all transaction data to the L1. To construct a proof, a watcher node must first detect a discrepancy between a rollup block's proposed post-state root and the correct outcome computed from the published transaction batch. The proof itself typically involves providing the pre-state, the transaction in question, and the relevant Merkle proofs to demonstrate the correct execution step. The on-chain verifier, a simplified virtual machine or custom logic, re-executes this single step. If the challenger's computation yields a different output than the proposer's, the fraud is conclusively proven.

Key advantages of the single-round model are its finality speed and reduced on-chain footprint. Since the dispute concludes after one on-chain transaction, invalid state can be corrected within one L1 block time, enhancing security and user experience. This contrasts with multi-round systems where challenges can take days. However, it imposes greater computational burden on the L1 during verification and requires all necessary data for the proof to be available on-chain. Implementations like Arbitrum Nitro use this model, compiling fraud proofs to WebAssembly for efficient on-chain verification.

The security model hinges on the presence of at least one honest and vigilant participant, often called a watcher or validator, who is monitoring the rollup's state. This actor must be willing to spend gas to submit the proof, incentivized by a share of the slashed bond. The system's safety is therefore conditional on the honest minority assumption—that a single honest party with sufficient resources exists. The design elegantly balances trust minimization with scalability, moving complex computation off-chain while retaining the cryptographic guarantee that the L1 can always correct errors.

A Single Round Fraud Proof is a cryptographic challenge protocol used in optimistic rollups where a state transition's validity is disputed and resolved in a single interactive round between a challenger and the prover (typically the sequencer). This contrasts with multi-round fraud proofs, dramatically reducing the time and on-chain data required to settle disputes. The mechanism relies on a bisection protocol where the two parties iteratively narrow down their disagreement to a single instruction or state transition step, which is then verified on-chain.

The core technical implementation involves a commitment scheme, such as a Merkle tree of the transaction batch and resulting state roots. When a challenger submits a fraud proof, they provide the minimal data needed to pinpoint the disagreement—often just two adjacent state roots and the transaction data between them. The on-chain verifier contract then executes this single-step computation to determine which party is correct. This design minimizes gas costs and dispute resolution latency, making it practical for Ethereum's block gas limits.

Key advantages include fast finality for honest users, as the safety net against invalid state transitions does not require long waiting periods typical of simpler designs. However, it imposes higher data availability requirements, as all transaction data must be posted to the underlying layer-1 chain for the challenge to be possible. Protocols like Arbitrum Nitro and Optimism's Cannon fault proof system employ sophisticated single-round designs, using interactive fraud proofs to ensure the security of the rollup's state is cryptographically enforced by the Ethereum network.