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Manual Fault Proof Interactions

Creating a Game

The process of disputing an output root starts by creating a new dispute game. There are conceptually three key inputs required for a dispute game:

  • The output root being disputed
  • The agreed output root the derivation process will start from
  • The L1 head block that defines the canonical L1 chain containing all required batch data to perform the derivation

The creator of the game selects the output root to dispute. It is identified by its L2 block number which can be used to look up the full details in the L2 output oracle.

The agreed output root is defined as the output root immediately prior to the disputed output root in the L2 output oracle. Therefore, a dispute game should only be created for the first invalid output root. If it is successfully disputed, all output roots after it are considered invalid by inference.

The L1 head block can be any L1 block where the disputed output root is present in the L2 output oracle. Proposers should therefore ensure that all batch data has been submitted to L1 before submitting a proposal. The L1 head block is recorded in the BlockOracle and then referenced by its block number.

Creating a game requires two separate transactions. First the L1 head block is recorded in the BlockOracle by calling its checkpoint function. This records the parent of the block the transaction is included in. The BlockOracle emits a log Checkpoint(blockNumber, blockHash, childTimestamp).

Now, using the L1 head along with output root info available in the L2 output oracle, cannon can be executed to determine the root claim to use when creating the game. In simple cases, where the claim is expected to be incorrect, an arbitrary hash can be used for claim values. For more advanced cases cannon can be used to generate a trace, including the claim values to use at specific steps. Note that it is not valid to create a game that disputes an output root, using the final hash from a trace that confirms the output root is valid. To dispute an output root successfully, the trace must resolve that the disputed output root is invalid. This is indicated by the first byte of the claim value being set to the invalid VM status (0x01).

The game can then be created by calling the create method on the DisputeGameFactory contract. This requires three parameters:

  • gameType - a uint8 representing the type of game to create. For fault dispute games using cannon and op-program traces, the game type is 0.
  • rootClaim - a bytes32 hash of the final state from the trace.
  • extraData - arbitrary bytes which are used as the initial inputs for the game. For fault dispute games using cannon and op-program traces, this is the abi encoding of (uint256(l2_block_number), uint256(l1_checkpoint)).
    • l2_block_number is the L2 block number from the output root being disputed
    • l1_checkpoint is the L1 block number recorded by the BlockOracle checkpoint

This emits a log event DisputeGameCreated(gameAddress, gameType, rootClaim) where gameAddress is the address of the newly created dispute game.

The helper script, create_game.sh can be used to easily create a new dispute game and also acts as an example of using cast to manually create a game.

Performing Moves

The dispute game progresses by actors countering existing claims via either the attack or defend methods in the FaultDisputeGame contract. Note that only attack can be used to counter the root claim. In both cases, there are two inputs required:

  • parentIndex - the index in the claims array of the parent claim that is being countered.
  • claim - a bytes32 hash of the state at the trace index corresponding to the new claim’s position.

The helper script, move.sh, can be used to easily perform moves and also acts as an example of using cast to manually call attack and defend.

Performing Steps

Attacking or defending are the only available actions before the maximum depth of the game is reached. To counter claims at the maximum depth, a step must be performed instead. Calling the step method in the FaultDisputeGame contract counters a claim at the maximum depth by running a single step of the cannon VM on chain. The step method will revert unless the cannon execution confirms the claim being countered is invalid. Note, if an actor's clock runs out at any point, the game can be resolved.

The inputs for step are:

  • claimIndex - the index in the claims array of the claim that is being countered
  • isAttack - Similar to regular moves, steps can either be attacking or defending
  • stateData - the full cannon state witness to use as the starting state for execution
  • proof - the additional proof data for the state witness required by cannon to perform the step

When a step is attacking, the caller is asserting that the claim at claimIndex is incorrect, and the claim for the previous trace index (made at a previous level in the game) was correct. The stateData must be the pre-image for the agreed correct hash at the previous trace index. The call to step will revert if the post-state from cannon matches the claim at claimIndex since the on-chain execution has proven the claim correct and it should not be countered.

When a step is defending, the caller is asserting that the claim at claimIndex is correct, and the claim for the next trace index (made at a previous level in the game) is incorrect. The stateData must be the pre-image for the hash in the claim at claimIndex.

The step function will revert with ValidStep() if the cannon execution proves that the claim attempting to be countered is correct. As a result, claims at the maximum game depth can only be countered by a valid execution of the single instruction in cannon running on-chain.

Populating the Pre-image Oracle

When the instruction to be executed as part of a step call reads from some pre-image data, that data must be loaded into the pre-image oracle prior to calling step. For local pre-image keys, the pre-image must be populated via the FaultDisputeGame contract by calling the addLocalData function. For global keccak256 keys, the data should be added directly to the pre-image oracle contract.

Resolving a Game

The final action required for a game is to resolve it by calling the resolve method in the FaultDisputeGame contract. This can only be done once the clock of the left-most uncontested claim’s parent has expired. A game can only be resolved once.

There are no inputs required for the resolve method. When successful, a log event is emitted with the game’s final status.

The helper script, resolve.sh, can be used to easily resolve a game and also acts as an example of using cast to manually call resolve and understand the result.