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Challenger Specification

Table of Contents

Description

The Challenger is an off-chain agent that listens for faulty claims made about the state of the L2 on the data availability layer. It is responsible for challenging these incorrect claims and ensuring the correctness of all finalized claims on the settlement layer.

The Challenger agent is intended to be ran as a permissionless service by participants of the network alongside a rollup-node. Challenger agents will be rewarded in the form of the bond attached to the claims they disprove.

Terminology

  • data availability layer - In the context of this document, the data availability layer is the generic term for the location where claims about the state of the layer two are made. In the context of Optimism, this is Ethereum Mainnet.
  • settlement layer - In the context of this document, the settlement layer is the location of the bridge as well as where funds deposited to the rollup reside. In the context of Optimism, this is Ethereum Mainnet.
  • L2 - In the context of this document, the layer two of the Optimistic Rollup. In the context of Optimism, this is the Optimism Mainnet.
  • rollup-node - In the context of this document, the rollup node describes the rollup-node specification. In the context of Optimism, this is the implementation of the rollup-node specification, the op-node.

Event and Response Lifecycle

The Challenger agent is expected to be able to listen for and respond to several different events on the data availability layer. These events and responses are parameterized depending on the type of dispute game being played, and the Challenger listens to different events and responds uniquely to each of the different game types. For specification of dispute game types, see the Dispute Game Interfaces specification and Dispute Game specification.

GameType.FAULT

Warning
The FAULT game type is not yet implemented. In the first iteration of Optimism's decentralization effort, challengers will respond to ATTESTATION games only.

Events and Responses TODO

GameType.ATTESTATION

Events and Responses

  • L2OutputOracle.OutputProposed The L2OutputOracle contract emits this event when a new output is proposed on the data availability layer. Each time an output is proposed, the Challenger should check to see if the output is equal the output given by the optimism_outputAtBlock endpoint of their rollup-node.
    • If it is, the Challenger should do nothing to challenge this output proposal.
    • If it is not, the Challenger should respond by creating a new DisputeGame with the DisputeGameType.ATTESTATION gameType, the correct output root as the rootClaim, and the abi-encoded l2BlockNumber of the correct output root as the extraData. Attestation OutputProposed Diagram
  • DisputeGameFactory.DisputeGameCreated A new dispute game has been created and is ready to be reviewed. The Challenger agent should listen for this event and check if the rootClaim of the AttestationDisputeGame created by the DisputeGameFactory is equal to the output root of their rollup-node at the game's l2BlockNumber.
    • If it is, the Challenger should sign the EIP-712 typeHash of the struct containing the AttestationDisputeGame's rootClaim and l2BlockNumber. The Challenger should then submit the abi-encoded signature to the AttetationDisputeGame's challenge function.
    • If it is not, the Challenger should do nothing in support of this dispute game. Attestation DisputeGameCreated Diagram

A full diagram and lifecycle of the Challenger's role in the ATTESTATION game type can be found below:

Attestation Diagram

GameType.VALIDITY

TODO

Warning
The VALIDITY game type is not yet implemented. In the first iteration of Optimism's decentralization effort, challengers will respond to ATTESTATION games only. A validity proof based dispute game is a possibility, but fault proof based dispute games will be the primary focus of the team in the near future.

Events and Responses TODO