Optimistic Challenge

A critical time-bound period during which a published state root can be challenged. This is the primary security parameter for optimistic rollups.

• Duration: Typically 7 days (e.g., Arbitrum, Optimism).
• Function: Allows any verifier to compute and submit a fraud proof demonstrating an invalid transaction.
• Consequence: If unchallenged, the state is considered final after the window closes.

The multi-round protocol used to resolve a challenge without re-executing the entire disputed transaction batch. It employs bisection to pinpoint the exact step of disagreement.

• Process: The challenger and the sequencer recursively narrow down the dispute to a single instruction.
• Efficiency: Minimizes on-chain computation by only verifying the contentious step on Layer 1.
• Implementation: Used by systems like Arbitrum Nitro.

A cryptoeconomic system that disincentivizes malicious challenges and faulty assertions by requiring stakes (bonds) from participants.

• Assertion Bond: Posted by the sequencer when proposing a new state. Slashed if proven fraudulent.
• Challenge Bond: Posted by the challenger. Slashed if the challenge is invalid (e.g., a false alarm).
• Purpose: Ensures challenges are economically rational and not spam.

Defines the two-stage finality of transactions in an optimistic system: provisional and absolute.

• Provisional Finality: Achieved when a state root is published. Funds can be used within the rollup, but withdrawal to Layer 1 is delayed.
• Absolute Finality: Achieved after the fraud proof window expires without a successful challenge. Enables trustless withdrawals.
• User Impact: Creates a trade-off between speed and guaranteed security for cross-chain assets.

The architectural choice for fraud proof complexity, balancing cost, speed, and trust assumptions.

• Single-Round Proof: A monolithic proof that re-executes the entire disputed batch on-chain (conceptually simpler, but gas-intensive).
• Multi-Round (Interactive) Proof: Uses the interactive dispute game to drastically reduce on-chain work (more complex, but far more efficient).
• Evolution: Modern systems like Optimism's Cannon are moving towards single-round, non-interactive fault proofs.

Decentralized services or actors that monitor the rollup chain and automatically submit challenges when fraud is detected.

• Role: Provide liveness for the challenge mechanism, ensuring someone is always watching.
• Incentive: Often earn a portion of the slashed bond as a reward.
• Importance: Critical for user security, as most users cannot monitor chains 24/7. Examples include Upshot and Kleros.

The critical time-bound parameter in all optimistic systems. It defines the period during which a fraud proof can be submitted to challenge an asserted state. Characteristics:

• Duration: Ranges from minutes (bridges) to days (rollups).
• Security Assumption: At least one honest, vigilant actor exists.
• User Impact: Creates a withdrawal delay for assets moving from the optimistic system to its base layer.

A cryptographic proof submitted during the challenge period to demonstrate that a proposed state transition is invalid. It is the technical mechanism that makes an optimistic challenge actionable.

• Constructed by: A verifier who detects incorrect state data.
• Required for: Successfully disputing and rolling back a fraudulent transaction batch.
• Contrasts with: Validity proofs (like ZKPs), which require proof of correctness upfront.

A mandatory time delay (e.g., 7 days) during which newly published state commitments can be challenged before being finalized. This is the "window of opportunity" for the optimistic challenge process.

• Purpose: Allows sufficient time for verifiers to detect and submit fraud proofs.
• Trade-off: Introduces a withdrawal delay for users but enables higher throughput and lower computation costs compared to immediate verification.
• Finality: Transactions achieve full finality only after this period expires without a successful challenge.

Any network participant who monitors state commitments and has the economic incentive to submit a fraud proof. They are the active agents in the optimistic challenge model.

• Role: Independently re-executes transactions or checks state roots to validate correctness.
• Incentive: Often receives a reward (a portion of the sequencer's bond) for successfully challenging fraud.
• Key Property: The system's security only requires one honest and active verifier to be secure.

The node responsible for ordering transactions, producing batches, and posting state commitments to the parent chain (e.g., Ethereum). It is the entity whose work is subject to optimistic challenges.

• Primary Function: Provides fast transaction confirmation and execution.
• Bond: Typically must post a substantial financial stake (bond) that can be slashed if a fraud proof against its work is successful.
• Centralization Concern: Often a single, permissioned entity in early Optimistic Rollup implementations.

A Layer 2 scaling solution that uses an optimistic challenge mechanism as its core security guarantee. It assumes transactions are valid by default and only uses fraud proofs for dispute resolution.

• Data Availability: Transaction data is posted to Layer 1, enabling anyone to reconstruct state and verify challenges.
• Efficiency: Moves computation and state storage off-chain, dramatically increasing throughput.
• Examples: Arbitrum and Optimism are the two major implementations of this architecture.

The guarantee that the data necessary to reconstruct the state and verify fraud proofs is published and accessible to all network participants. It is a prerequisite for any functional optimistic challenge system.

• Location: Typically achieved by posting transaction batch data to the Layer 1 blockchain (e.g., Ethereum calldata).
• Critical Role: Without it, verifiers cannot independently compute the correct state, rendering fraud proofs impossible and breaking the security model.