Kroma
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Description
Kroma aims to develop an universal ZK Rollup based on the Optimism Bedrock architecture. Currently, Kroma operates as an Optimistic Rollup with ZK fault proofs, utilizing a zkEVM based on Scroll. Kroma’s goal is to eventually transition to a ZK Rollup once the generation of ZK proofs becomes more cost-efficient and faster.
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Risk analysis
State validation
Fraud proofs (INT, ZK)Fraud proofs allow actors watching the chain to prove that the state is incorrect. Interactive proofs (INT) require multiple transactions over time to resolve. ZK proofs are used to adjudicate the correctness of the last step. The challenge protocol can be subject to delay attacks and can fail under certain conditions. The current system doesn’t use posted L2 txs batches on L1 as inputs to prove a fault, meaning that DA is not enforced.
Data availability
On chainAll of the data needed for proof construction is published on chain.
Upgradeability
7d delayThe code that secures the system can be changed arbitrarily and users don’t have enough time to react because of the 7d challenge period delay.
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 12h delay on this operation.
Proposer failure
Self proposeAnyone can be a Proposer and propose new roots to the L1 bridge.
Rollup stage
Kroma is a Technology
Fraud Proofs ensure state correctness
Kroma uses an interactive fraud proof system to find a single block of disagreement, which is then ZK proven. The zkEVM used is based on Scroll. Once the single block of disagreement is found, the challenger is required to present ZK proof of the fraud. When the proof is validated, the incorrect state output is deleted. The Security Council can always override the result of the challenge, it can also delete any L2 state root at any time. If the malicious attester and challenger collude and are willing to spend bonds, they can perform a delay attack by engaging in continuous challenge resulting in lack of finalization of the L2 state root on L1. The protocol can also fail under certain conditions.
Withdrawals can be delayed if the fraud proof system is under a delay attack.
Funds can be lost if the cryptography is broken or implemented incorrectly.
All transaction data is recorded on chain
All executed transactions are submitted to an on chain smart contract. The execution of the rollup is based entirely on the submitted transactions, so anyone monitoring the contract can know the correct state of the rollup chain.
State derivation
Kroma nodes source code, including full node, proposer and validator, can be found here. Also, the geth server, source maintained here, is a fork of go-ethereum. For more details on how they are different from the Optimism implementation, see here. The instructions to run the proposer (called validator) and the ZK prover, are documented here.
Data batches are compressed using the zlib algorithm with best compression level.
The genesis file can be found here.
L2 blocks derivation from L1 data plus the format and architecture of batch submission is documented here.
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.
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.
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.
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
OP stack chains are pursuing the EVM Equivalence model. No changes to smart contracts are required regardless of the language they are written in, i.e. anything deployed on L1 can be deployed on L2.
Permissions
The system uses the following set of permissioned addresses:
MultiSig (currently 4 / 9) that is a guardian of KromaPortal, privileged Validator that does not need a bond and privileged actor in Colosseum contract that can remove any L2Output state root regardless of the outcome of the challenge.
Members of the SecurityCouncil.
Central actor allowed to commit L2 transactions on L1.
Actor allowed to pause withdrawals. Currently set to the Security Council.
Smart contracts
The system consists of the following smart contracts:
The L2OutputOracle contract contains a list of proposed state roots which Proposers assert to be a result of block execution. Anyone can participate as a Proposer by depositing in the ValidatorPool. A root can be proposed every 1h.
Can be upgraded by: SecurityCouncil
Upgrade delay: 7d delay
The OptimismPortal contract is the main entry point to deposit funds from L1 to L2. It also allows to prove and finalize withdrawals. This contract stores the following tokens: ETH.
Can be upgraded by: SecurityCouncil
Upgrade delay: 7d delay
It contains configuration parameters such as the Sequencer address, the L2 gas limit and the unsafe block signer address.
Can be upgraded by: SecurityCouncil
Upgrade delay: 7d delay
The L1ERC721Bridge contract is the main entry point to deposit ERC721 tokens from L1 to L2.
Can be upgraded by: SecurityCouncil
Upgrade delay: 7d delay
The L1 Cross Domain Messenger contract sends messages from L1 to L2, and relays messages from L2 onto L1. In the event that a message sent from L1 to L2 is rejected for exceeding the L2 epoch gas limit, it can be resubmitted via this contract’s replay function.
Can be upgraded by: SecurityCouncil
Upgrade delay: 7d delay
Timelock contract behind which the ProxyAdmin is. There is a 7d delay.
Contract allowed to start upgrades, dismiss challenges and delete roots. It is also designated as a guardian, meaning it can pause withdrawals.
Can be upgraded by: SecurityCouncil
Upgrade delay: 7d delay
Controls the Timelock. It is governed using a Soulbound NFT.
Admin of the L2OutputOracle, Timelock, KromaPortal, SystemConfig, SecurityCouncil, L1CrossDomainMessenger, L1ERC721Bridge, ZKVerifier, Colosseum, L1StandardBridge, UpgradeGovernor, SecurityCouncilToken, ValidatorPool proxies. It’s effectively controlled by the Security Council. The proxy is behind a Timelock.
Contract used to challenge state roots and prove fraud. The SecurityCouncil can interfere by deleting challenges and roots.
Can be upgraded by: SecurityCouncil
Upgrade delay: 7d delay
Contract used to manage the Proposers. Anyone can submit a deposit and bond to a state root, or create a challenge. It also manages the Proposer rotation for each submittable block using a random selection. If the selected proposer fails to publish a root within 30m, then the submission becomes open to everyone.
Can be upgraded by: SecurityCouncil
Upgrade delay: 7d delay
Trie contract used to prove withdrawals.
ZK verifier used to verify the last step of a fraud proof, which corresponds to a block.
Can be upgraded by: SecurityCouncil
Upgrade delay: 7d delay
Contract used to compute hashes. It is used by the ZKMerkeTrie. The contract has been generated using the circomlibjs library.
Value Locked is calculated based on these smart contracts and tokens:
Main entry point for users depositing ERC20 token that do not require custom gateway.
Main entry point for users depositing ETH.
The current deployment carries some associated risks:
Funds can be stolen if a contract receives a malicious code upgrade. There is a 7d delay on code upgrades.
