Arbitrum Nova logoArbitrum Nova


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Tokens:

Milestones
Mainnet for everyone
2022 Aug 9th
Whitelist got removed, there are no restrictions on who can transact with the network.

Description

Arbitrum Nova is an AnyTrust chain that aims for ultra low transaction fees. Nova differs from Arbitrum One by not posting transaction data on chain, but to Data Availability Committee.

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Risk summary

Technology

Fraud proofs ensure state correctness

After some period of time, the published state root is assumed to be correct. For a certain time period, usually one week one of the whitelisted actors can submit a fraud proof that shows that the state was incorrect.

  • 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).

  1. Executing and Securing the Chain - Arbitrum documentation
  2. RollupUserFacet.sol#L281 - Etherscan source code, onlyValidator modifier

Data is not stored on chain

Users transactions are not published on-chain, but rather sent to several well known and trusted parties, also known as committee members (DAC). New transaction batch needs a BLS signature with the required DAC quorum, for example 9/10. This signature is not verified by L1, however external Validators will skip the batch if BLS signature is not valid resulting. This will reslt in a fraud proof challenge if this batch is included in a consequitive 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.

  • 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.

  1. AnyTrust Chains - Arbitrum documentation

Operator

The system has a centralized sequencer

While proposing blocks 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.

  1. Validators - Arbitrum documentation
  2. If the sequencer is malicious - Arbitrum documentation

Users can force any transaction

Because the state of the system is based on transactions submitted on-chain and anyone can submit their transactions there it allows the users to circumvent censorship by interacting with the smart contract directly.

  1. Submitting Transactions - Arbitrum documentation

Withdrawals

Regular exit

The user initiates the withdrawal by submitting a transaction on L2. 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.

  1. L2 to L1 Messages Lifecycle - Arbitrum documentation
  2. Rules for Confirming or Rejecting Rollup Blocks - Arbitrum documentation
  3. Mainnet for everyone - Arbitrum Blog

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.

  1. Tradeable Bridge Exits - Arbitrum documentation

Other considerations

EVM compatible smart contracts are supported

Arbitrum Nova 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.

  1. Arbitrum Nitro Sneak Preview

Smart Contracts

A diagram of the smart contract architecture
A diagram of the smart contract architecture

The system consists of the following smart contracts:

ProxyAdmin 0x71D7…7148

This contract is an admin of most other contracts allowed to upgrade their implementations. It is owned by a 4-of-6 multisig.

Main contract implementing Arbitrum Nova 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.

Main entry point for the Sequencer submitting transaction batches to a Rollup.

Entry point for users depositing ETH and sending L1 --> L2 messages. Deposited ETH is escowed in a Bridge contract.

Contract managing Inboxes and Outboxes. It escrows ETH sent to L2. This contract stores the following tokens: ETH.

Contract managing an interactive fraud challenge process.

OneStepProofEntry 0x7Adc…0Fc5

Contract managing adjudication logic for EVM implementation in WASM used by the fraud proofs.

ProxyAdmin (2) 0xa8f7…e560

This is a different proxy admin for the three gateway contracts below. It is also owned by a 4-of-6 multisig.

Router managing token <--> gateway mapping.

Main entry point for users depositing ERC20 tokens. Upon depositing, on L2 a generic, "wrapped" token will be minted. This contract can store any token

Main entry point for users depositing ERC20 tokens that require minting custom token on L2.

L1DaiGateway 0x97f6…A786

Custom DAI Gateway, main entry point for users depositing DAI to L2 where "canonical" L2 DAI token managed by MakerDAO will be minted. Managed by MakerDAO.

L1Escrow 0xA2e9…aAd3

DAI Vault for custom DAI Gateway managed by MakerDAO. This contract stores the following tokens: DAI.

The current deployment carries some associated risks:

  • Funds can be stolen if a contract receives a malicious code upgrade. There is no delay on code upgrades (CRITICAL).