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Arbitrum Orbit

Arbitrum orbit finalizes L2 state commitments when a trusted proposer submits the L2's state_hash to the RollupCore contract on the L1. These submissions occur at designated intervals. Anyone acting as a challenger can dispute an invalid submissions. If no challenge arises within a specific timeframe, the submitted state_hash is considered final. This approach leverages a permissioned proposer and challenger.

type H256 = [u8; 32];
type Bloom = [u8; 256];
type U256 = [u64; 4];
type H64 = [u8; 8];
 
struct GlobalState {
	pub block_hash: H256,
	pub send_root: H256,
	pub inbox_position: u64,
	pub position_in_message: u64,
}
 
enum MachineStatus {
	Running = 0,
	Finished = 1,
	Errored = 2,
	TooFar = 3,
}
 
struct Header {
	parent_hash: H256,
	uncle_hash: H256,
	coinbase: H160,
	state_root: H256,
	transactions_root: H256,
	receipts_root: H256,
	logs_bloom: Bloom,
	difficulty: U256,
	number: U256,
	gas_limit: u64,
	gas_used: u64,
	timestamp: u64,
	extra_data: Vec<u8>,
	mix_hash: H256,
	nonce: H64,
	base_fee_per_gas: Option<U256>,
	withdrawals_hash: Option<H256>,
	blob_gas_used: Option<u64>,
	excess_blob_gas_used: Option<u64>
}

State Hash Construction

The state_hash is a hash of the concatenation of the GlobalState hash, MachineStatus and the sequencer inbox count.

state_hash = keccak256(global_state_hash || inbox_max_count || machine_status)
  • The global_state_hash is hash of the GlobalState.
  • The inbox_max_count is the sequencer inbox message count.
  • The machine_status is the MachineStatus.

Verifiying the state hash existence

To verify the existence of a state hash, we need to verify it's existence in the Rollup contract in a finalized L1 chain. This requires the fields present in the structure described below:

struct ArbitrumPayloadProof {
	/// Arbitrum header that corresponds to the node being created
	arbitrum_header: CodecHeader,
	/// Global State as recorded in the NodeCreated event that was emitted for this node
	global_state: GlobalState,
	/// Machine status as recorded in the NodeCreated event that was emitted for this node
	machine_status: MachineStatus,
	/// Inbox max count as recorded in the NodeCreated event that was emitted for this node
	inbox_max_count: U256,
	/// Key used to store the node  in the _nodes mapping in the RollupCore as recorded in the
	/// latestNodeCreated field of the NodeCreated event
	node_number: u64,
	/// Proof for the state_hash field in the Node struct inside the _nodes mapping in the
	/// RollupCore
	storage_proof: Vec<Vec<u8>>,
	/// RollupCore contract proof in the ethereum world trie
	contract_proof: Vec<Vec<u8>>,
}

Inputs:

payload: Arbitrum payload containing proofs and state information root: MPT root of the L1 rollup_core_address: Address of the Rollup Core contract

Outputs:

state_root: State root of the L2

Steps:

  • Verify Contract Storage Root:

Use the provided proof (payload.contract_proof) and Rollup Core address to retrieve the storage root of the rollup core contract.

  • Verify Header Extra Data:

Compare the send_root field in the global state (payload.global_state.send_root) with the extra data field in the Arbitrum header (payload.arbitrum_header.extra_data). If they don't match, return an error indicating a mismatch.

  • Verify Header Hash:

Compute the header hash and compare the computed hash with the block_hash field in the global state (payload.global_state.block_hash). If they don't match, return an error indicating a mismatch.

  • Compute State Hash:

Use the provided global state (payload.global_state), machine status (payload.machine_status), and inbox max count (payload.inbox_max_count) to calculate the expected state_hash.

  • Verify State Hash Proof:

Derive the slot hash for the state_hash within the rollup core contract's storage using the node number (payload.node_number). Use the proof (payload.storage_proof) and the storage root of the rollup contract extracted in a previous step to retrieve the value associated with the slot hash. Check if a value was found. If not, return an error indicating the state hash wasn't found in the proof. Compare the retrieved value to the computed state hash. If they don't match, return an error indicating a mismatch.

If all steps complete successfully then we can accept the state_root, timestamp and block_number in the provided block header.

Implementation