In the event of a sequencer failure, users can force transactions to be included in the project’s chain by sending them to L1. There is a 2d delay on this operation.

State validation

Fraud proofs (INT)

No actor outside of the single Proposer can submit fraud proofs. Interactive proofs (INT) require multiple transactions over time to resolve. The challenge protocol can be subject to delay attacks. There is a 6d challenge period.

Data availability

Onchain

All of the data needed for proof construction is published on Ethereum L1.

Exit window

None

There is no window for users to exit in case of an unwanted regular upgrade since contracts are instantly upgradable.

Proposer failure

Self propose

Anyone can become a Proposer after 12d 8h of inactivity from the currently whitelisted Proposers.

Rollup stage

Parallel is a

Stage 0

Optimistic Rollup.

The requirement for available node software is under review

Learn more about Rollup stages

Please keep in mind that these stages do not reflect rollup security, this is an opinionated assessment of rollup maturity based on subjective criteria, created with a goal of incentivizing projects to push toward better decentralization. Each team may have taken different paths to achieve this goal.

Technology

All transaction data is recorded on chain

All executed transactions are submitted to an on chain smart contract. The execution of the rollup is based entirely on the submitted transactions, so anyone monitoring the contract can know the correct state of the rollup chain.

State validation

Updates to the system state can be proposed and challenged by a set of whitelisted validators. If a state root passes the challenge period, it is optimistically considered correct and made actionable for withdrawals.

State root proposals

Whitelisted validators propose state roots as children of a previous state root. A state root can have multiple conflicting children. This structure forms a graph, and therefore, in the contracts, state roots are referred to as nodes. Each proposal requires a stake, currently set to 0.1 ETH, that can be slashed if the proposal is proven incorrect via a fraud proof. Stakes can be moved from one node to one of its children, either by calling stakeOnExistingNode or stakeOnNewNode. New nodes cannot be created faster than the minimum assertion period by the same validator, currently set to 15m. The oldest unconfirmed node can be confirmed if the challenge period has passed and there are no siblings, and rejected if the parent is not a confirmed node or if the challenge period has passed and no one is staked on it.

Funds can be stolen if none of the whitelisted verifiers checks the published state. Fraud proofs assume at least one honest and able validator (CRITICAL).

How is fraud proven - Arbitrum documentation FAQ

Challenges

A challenge can be started between two siblings, i.e. two different state roots that share the same parent, by calling the startChallenge function. Validators cannot be in more than one challenge at the same time, meaning that the protocol operates with partial concurrency. Since each challenge lasts 6d, this implies that the protocol can be subject to delay attacks, where a malicious actor can delay withdrawals as long as they are willing to pay the cost of losing their stakes. If the protocol is delayed attacked, the new stake requirement increases exponentially for each challenge period of delay. Challenges are played via a bisection game, where asserter and challenger play together to find the first instruction of disagreement. Such instruction is then executed onchain in the WASM OneStepProver contract to determine the winner, who then gets half of the stake of the loser. As said before, a state root is rejected only when no one left is staked on it. The protocol does not enforces valid bisections, meaning that actors can propose correct initial claim and then provide incorrect midpoints.

Fraud Proof Wars: Arbitrum Classic

Operator

The system has a centralized sequencer

While forcing transaction is open to anyone the system employs a privileged sequencer that has priority for submitting transaction batches and ordering transactions.

MEV can be extracted if the operator exploits their centralized position and frontruns user transactions.

Sequencer - Arbitrum documentation

Users can force any transaction

Because the state of the system is based on transactions submitted on the underlying host chain and anyone can submit their transactions there it allows the users to circumvent censorship by interacting with the smart contract on the host chain directly. After a delay of 2d in which a Sequencer has failed to include a transaction that was directly posted to the smart contract, it can be forcefully included by anyone on the host chain, which finalizes its ordering.

Withdrawals

Regular exit

The user initiates the withdrawal by submitting a regular transaction on this chain. When the block containing that transaction is finalized the funds become available for withdrawal on L1. The process of block finalization usually takes several days to complete. Finally the user submits an L1 transaction to claim the funds. This transaction requires a merkle proof.

Tradeable Bridge Exit

When a user initiates a regular withdrawal a third party verifying the chain can offer to buy this withdrawal by paying the user on L1. The user will get the funds immediately, however the third party has to wait for the block to be finalized. This is implemented as a first party functionality inside Arbitrum’s token bridge.

Tradeable Bridge Exits - Arbitrum documentation

Autonomous exit

Users can (eventually) exit the system by pushing the transaction on L1 and providing the corresponding state root. The only way to prevent such withdrawal is via an upgrade.

Other considerations

EVM compatible smart contracts are supported

Arbitrum One uses Nitro technology that allows running fraud proofs by executing EVM code on top of WASM.

Funds can be lost if there are mistakes in the highly complex Nitro and WASM one-step prover implementation.

Inside Arbitrum Nitro

Permissions

The system uses the following set of permissioned addresses:

Sequencers (2) 0x40ac…9Df4 0x7a6B…ca3A

Central actors allowed to submit transaction batches to L1.

Validators/Proposers 0xcCE4…6FeA

They can submit new state roots and challenge state roots. Some of the operators perform their duties through special purpose smart contracts.

RollupOwner (2) 0xefCf…dD5C ParallelMultisig

Can execute upgrades via the UpgradeExecutor, potentially stealing all funds.

ParallelMultisig RollupOwner

This is a Gnosis Safe with 3 / 5 threshold. Multisig that can execute upgrades via the UpgradeExecutor.

ParallelMultisig participants (5) 0xb655…9E49 0x9EBe…f515 0x1781…aCD4 0x0049…49Dd 0x909e…7695

Those are the participants of the ParallelMultisig.

Smart contracts

A diagram of the smart contract architecture

The system consists of the following smart contracts on the host chain (Ethereum):

L1GatewayRouter 0x5D65…FB96 Implementation (Upgradable)Admin

Router managing token <–> gateway mapping.

RollupProxy 0x6594…9877 Implementation #1 (Upgradable)Implementation #2 (Upgradable)Admin

Main contract implementing Arbitrum One Rollup. Manages other Rollup components, list of Stakers and Validators. Entry point for Validators creating new Rollup Nodes (state commits) and Challengers submitting fraud proofs.

Implementation used in: