Xai
| Website | xai.games |
|---|---|
| Docs | xai-foundation.gitbook.io/xai-network |
| Explorer | explorer.xai-chain.net |
| Repository | github.com/OffchainLabs/nitro |
| Social | @xai_gamesXaiSentryNodesDiscord |
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Funds can be stolen if
Funds can be lost if
Users can be censored if
MEV can be extracted if
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.
Data availability
External (DAC)Proof construction relies fully on data that is NOT published on chain. There exists a Data Availability Committee (DAC) with a threshold of 5/6 that is tasked with protecting and supplying the data.
Exit window
NoneThere is no window for users to exit in case of an unwanted upgrade since contracts are instantly upgradable.
Sequencer failure
Self sequenceIn 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.
Proposer failure
Self proposeAnyone can become a Proposer after 6d 8h of inactivity from the currently whitelisted Proposers.
Fraud proofs ensure state correctness
After some period of time, the published state root is assumed to be correct. For a certain time period, one of the whitelisted actors can submit a fraud proof that shows that the state was incorrect. The challenge protocol can be subject to delay attacks. After the state root is published, there is also a trusted entity, called Challenger, that signs it and submits the signature to a Referee smart contract. The signatures submitted to the referee are used then verified by sentry nodes. The role of sentry nodes is to verify (assert) the submitted state root after it has been submitted. There is no integrated way to flag an invalid state root, sentry nodes will have to raise the alarm by external means, making them just observation nodes.
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
- Arbitrum Glossary: Challenge Period
- RollupUser.sol - Etherscan source code, onlyValidator modifier
- Referee.sol - Etherscan source code, submitChallenge function
- Referee.sol - Etherscan source code, submitAssertionToChallenge function
- Solutions to Delay Attacks on Rollups
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 5 out of 6.
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.
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.
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. Anyone can become a Proposer after approximately 6d 8h (45818 blocks) of inactivity from the currently whitelisted Proposers.
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.
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.
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.
The system uses the following set of permissioned addresses:
Admin of UpgradeExecutor, ChallengeManager, Outbox, L1ERC20Gateway, Bridge, L1GatewayRouter, Inbox.
They can submit new state roots and challenge state roots. Some of the operators perform their duties through special purpose smart contracts.
Central actors allowed to submit transaction batches to L1.
Multisig that can execute upgrades via the UpgradeExecutor.
The system consists of the following smart contracts:
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.
Contract managing Inboxes and Outboxes. It escrows the native token used for gas on the chain. This contract stores the following tokens: XAI.
Main entry point for the Sequencer submitting transaction batches.
Entry point for users depositing ETH and sending L1 -> L2 messages.
Contract that allows L2->L1 calls, i.e. messages initiated on L2 which eventually resolve in execution on L1.
Contract allowed to upgrade the system.
Contract that allows challenging invalid state roots. Can be called through the RollupProxy.
Contract used to perform the last step of a fraud proof.
Contract used to perform the last step of a fraud proof.
Contract used to perform the last step of a fraud proof.
Contract used to perform the last step of a fraud proof.
Contract used to perform the last step of a fraud proof.
Router managing token <–> gateway mapping.
This is the contract where Xai Sentry Keys to run a node are minted.
The referree contract allows to create new challenges (state root reports) from the permissioned challenger, collects assertions from sentry nodes, and distributes esXAI rewards for operating a sentry node. The referee contract is also a whitelisted address in the esXAI token contract, which allows it to initiate arbitrary esXAI token transfers.
Value Locked is calculated based on these smart contracts and tokens:
Contract managing Inboxes and Outboxes. It escrows XAI sent to L2.
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).
