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 1d 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 8h challenge period.

Data availability

External (DAC)

Proof construction relies fully on data that is NOT published onchain. There exists a Data Availability Committee (DAC) with a threshold of 1/1 that is tasked with protecting and supplying the data.

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 17h of inactivity from the currently whitelisted Proposers.

Technology

Data is not stored on chain

Users transactions are not published on-chain, but rather sent to external trusted parties, also known as committee members (DAC). Members of the DAC collectively produce a Data Availability Certificate (comprising BLS signatures from a quorum) guaranteeing that the data behind the new transaction batch will be available until the expiry period elapses (currently a minimum of two weeks). This signature is not verified by L1, however external Validators will skip the batch if BLS signature is not valid resulting. This will result in a fraud proof challenge if this batch is included in a consecutive state update. It is assumed that at least one honest DAC member that signed the batch will reveal tx data to the Validators if Sequencer decides to act maliciously and withhold the data. If the Sequencer cannot gather enough signatures from the DAC, it will “fall back to rollup” mode and by posting the full data directly to the L1 chain. The current DAC threshold is 1 out of 1.

Funds can be lost if the external data becomes unavailable (CRITICAL).
Users can be censored if the committee restricts their access to the external data.

Inside AnyTrust - Arbitrum documentation

Learn more about the DA layer here:

AlienX DAC

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 8h, 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 1d 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 0xff30…4f25

Central actors allowed to submit transaction batches to L1.

Validators/Proposers 0x32AD…E4A3

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

AlienXMultisig 0x32f6…bb27

This is a Gnosis Safe with 5 / 7 threshold. Can upgrade the smart contract system (via UpgradeExecutor) at any time and gain access to all funds.

AlienXMultisig participants (7) 0x12ee…C76A 0x3560…0391 0xa179…b052 0x4c02…D66C 0xbD81…8186 0x740E…666e 0x26F1…622c

Those are the participants of the AlienXMultisig.

Smart contracts

A diagram of the smart contract architecture

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

RollupProxy 0x6fa8…7f39 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: