About the First Successful Challenge on Kroma Mainnet

Written by Harry (@StackDigest)

TL;DR

• Kroma Network has been secured by its unique permissionless validator mechanism since the mainnet launch in September last year, employing a ZK fault proof system.
• On April 1st, an incorrect output was submitted, and on April 5th, a challenge against this output was successfully executed — marking the first successful challenge in rollup history.
• This article shares our experiences and takeaways from this event, outlining the future direction of the Kroma Network.

Challenge System of Kroma

Kroma Network has been secured from day-one by a permissionless validator system utilizing ZK fault proofs.

Designed so that just one honest validator can detect an incorrect output, Kroma’s challenge system ensures that no one can manipulate withdrawals or censor them at the L1 Bridge. This design allows users to perform withdrawals to L1 in a permissionless manner.

For detailed information about Kroma’s challenge system, please check this Medium article.

Challenge on Kroma Mainnet, Successfully Proving Faults

4/1–4/4 Faulty Output submitted, Challenge created

Due to an issue with the sequencer’s L1 origin on April 1st, an incorrect block was generated and subsequently, an incorrect L2 output was submitted. Following the submission, 7 challengers created 12 challenges against this output.

However, at that time, Kroma Network’s Colosseum contract was unable to properly verify proofs. This was because the withdrawalsRoot included in the block headers following Kroma v1.2.1 release was not yet reflected in the Colosseum’s proof verification process. Consequently, all attempts by the challengers to proveFault failed, and the generated challenges were all canceled due to Challenger Timeout.

4/5 Faulty Output Deleted Successfully

The Kroma team was aware of issues with the Colosseum, and included a patch in Kroma v1.3.4, which was released and applied to the mainnet on April 5th. Subsequently, Kroma’s validator successfully created a challenge and executed the proveFault function.

First-ever Success of Challenge in the History of Ethereum

Approximately three years after the launch of the first Optimistic Rollup, Arbitrum, in May 2021, this event represents the first successful fault proof verification and challenge execution in a mainnet environment.

This success reaffirms that the Kroma Network is securely protected by a permissionless validator system and the challenge mechanism.

Takeaway: What the Optimistic Rollup Community Should Keep in Mind

So, what have we learned from this? These experiences let us know that challenges often arise not just from hostile actions but from mistakes. The incorrect output, in this case, was not created with malicious intent but was mistakenly submitted due to a fault.

Thus, challenges can occur without any malicious intentions. Issues like abnormalities in the L1 client viewed by the validator or failures to update the L2 client can lead to challenges stemming simply from validator errors or faulty patches, not necessarily indicating a critical flaw or manipulation attempt in the chain.

As various optimistic rollups with permissionless validator systems are poised for release, the community should be aware that challenges may occur more frequently than anticipated. Users of these systems (Kroma, Arbitrum, Optimism and so on) need not panic when a challenge occurs but should wait for announcements explaining the outcome and context of the challenge.

Road to Security and Decentralization

Fault Proof System V2, Step to Stage 2 Rollup

Vitalik Buterin recently stated at Ethereum has blob. Where do we go from here?:

By the end of the year, I think our standards should increase and we should only treat a project as a rollup if it has actually reached at least stage 1.

This statement underscores the importance of achieving a more robust and fault-resistant framework in rollups to ensure their long-term viability and security.

Kroma Network is currently a Stage 0 rollup, mainly because the fault proof system is not yet perfect, particularly in scenarios where:

1. Last submitted output is deleted, so that the bisection between asserter and challenger cannot be progressed, leading to the victory of the challenger.
2. There are errors in the prover or verifier, or a ZK Soundness error occurs.

To ensure the correct processing of outputs and withdrawals, even in these cases, Kroma has a Security Council composed of 10 trustworthy entities responsible for deleting incorrect outputs and safeguarding the network in edge cases.

Kroma team is working on Fault Proof System V2, which promises numerous improvements over the current system. The primary goals for this upgrade includes:

• The honest one will always win the dispute challenge and invalid output cannot be finalized.
• Valid output can be finalized in max upper bound time.
• ZK proof guarantees that it is the result of executing L2 txs batches.

Transitioning To Stage 2 Rollup

The implementation of Fault Proof System V2 is expected to facilitate Kroma’s transition from a Stage 0 to a Stage 2 rollup. This advancement will eliminate the need for a Security Council, thereby decentralizing the network further and placing the responsibility of maintaining network security directly in the hands of the community’s validators. This shift is anticipated to bolster decentralization and security within the network, making it more resilient against faults and capable of self-regulation.

Stay tuned for further updates!

About Kroma

As Asia’s leading game-centric Layer 2 solution, Kroma is an Optimistic Rollup with a ZK fault-proof system based on the OP Stack and utilizing Scroll’s zkEVM. Kroma will transition to a universal ZK Rollup once the generation of ZK proofs becomes more cost-efficient and faster — using our original project, Tachyon.

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About the First Successful Challenge on Kroma Mainnet was originally published in Kroma on Medium, where people are continuing the conversation by highlighting and responding to this story.