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Celer’s in ZK Proofs Mode Built with StarkEx from StarkWare.

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Risk summary


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


    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


    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.

      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:

          • 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.
          • Data Availability Committee
            There exists a Data Availability Committee with unknown members and an unverified smart contract.
          • SHARP Verifier Governor 0x3DE5…F5C6 (EOA)
            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.
          • Most Broker functionality is restricted only for the owner, it includes managing rides, setting prices or slippages, burning shares.
          • Allowed to update state of the system. When 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, USDC, USDT.
          • Committee 0xF000…28A9
            The source code of this contract is not verified on Etherscan.
          • Starkware SHARP verifier used collectively by StarkNet, Sorare, Immutable X and 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.
          • Broker manages investment strategies on L1 for tokens deposited to the system. Strategies invest in specific protocols, e.g. Compound and they escrow LP tokens as custom Wrapped tokens.

          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).