Immutable X logoImmutable X


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Description

Immutable X claims to be the first Layer 2 for NFTs on Ethereum. It promises zero gas fees, instant trades and scalability for games, applications, marketplaces, without compromise.

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Risk summary

Technology

Validity proofs ensure state correctness

Each update to the system state must be accompanied by a ZK Proof that ensures that the new state was derived by correctly applying a series of valid user transactions to the previous state. Once the proof is processed on the Ethereum blockchain the L2 block is instantly finalized. The system state is represented using Merkle roots.

  1. Enforcing Consistency on the On-Chain State - StarkEx documentation

Zero knowledge STARK cryptography is used

Despite their production use ZK-STARKs proof systems are still relatively new, complex and they rely on the proper implementation of the polynomial constraints used to check validity of the Execution Trace.

  • Funds can be lost if the proof system is implemented incorrectly.

  1. STARK Core Engine Deep Dive

Data is not stored on chain

The balances of the users are not published on-chain, but rather sent to several well known and trusted parties, also known as committee members. A state update is valid and accepted on-chain only if at least a quorum of the committee members sign a state update.

  • 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. Validium - StarkEx documentation
  2. Availability Verifiers - StarkEx documentation

Operator

The system has a centralized operator

The operator is the only entity that can propose blocks. A live and trustworthy operator is vital to the health of the system. Typically, the Operator is the hot wallet of the StarkEx service submitting state updates for which proofs have been already submitted and verified.

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

  1. Operator - StarkEx documentation

Users can force exit the system

Force exit allows the users to escape censorship by withdrawing their funds. The system allows users to force the withdrawal of funds by submitting a request directly to the contract on-chain. The request must be served within a defined time period. If this does not happen, the system will halt regular operation and permit trustless withdrawal of funds.

  • Users can be censored if the operator refuses to include their transactions. They can still exit the system.

  1. Censorship Prevention - StarkEx documentation

Withdrawals

Regular exit

The user initiates the withdrawal by submitting a transaction on L2. When the block containing that transaction is proven the funds become available for withdrawal on L1. Finally the user submits an L1 transaction to claim the funds. This transaction does not require a merkle proof. When withdrawing NFTs they are minted on L1.

  1. Withdrawal - StarkEx documentation

Forced exit

If the user experiences censorship from the operator with regular exit they can submit their withdrawal requests directly on L1. The system is then obliged to service this request. Once the force operation is submitted if the request is serviced the operation follows the flow of a regular exit.

  1. Forced Operations - StarkEx documentation
  2. Forced Withdrawal - StarkEx documentation
  3. Full Withdrawal - StarkEx documentation

Emergency exit

If enough time passes and the forced exit is still ignored the user can put the system into a frozen state, disallowing further state updates. In that case everybody can withdraw by submitting a merkle proof of their funds with their L1 transaction.

  1. Forced Operations - StarkEx documentation
  2. Forced Withdrawal - StarkEx documentation
  3. Full Withdrawal - StarkEx documentation

Permissioned Addresses

The system uses the following set of permissioned addresses:

Governor 0x799E…4B5a

Can upgrade implementation of the system, potentially gaining access to all funds stored in the bridge. Currently there is no delay before the upgrade, so the users will not have time to migrate.

Validity proof must be signed by at least 5 of these addresses to approve state update.

SHARP Verifier Governor 0x3DE5…F5C6

Can upgrade implementation of SHARP Verifier, potentially with code approving fraudulent state. Currently there is no delay before the upgrade, so the users will not have time to migrate.

Operator 0x9B7f…927e

Allowed to update the state. When the Operator is down the state cannot be updated.

Smart Contracts

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

The system consists of the following smart contracts:

This contract stores the following tokens: ETH, IMX, USDC, OMI.

Committee 0x16BA…A295

Data Availability Committee (DAC) contract verifing data availability claim from DAC Members (via multisig check).

Starkware SHARP verifier used collectively by StarkNet, Sorare, Immutable X and rhino.fi. It receives STARK proofs from the Prover attesting to the integrity of the Execution Trace of these four Programs including correctly computed L2 state root which is part of the Program Output.

The current deployment carries some associated risks:

  • Funds can be stolen if a contract receives a malicious code upgrade. There is a 14 days delay on code upgrades.