Author: Haotian, Cryptocurrency Researcher Source: X (Twitter) @tmel0211
As you may have noticed, OP-Rollup has a 7-day challenge period, but so far there hasn’t been any effective battle-tested cases. Originally, it was just assumed to be optimistic, but it turned out to be really “optimistic”? Why are there so few challenge cases in OP-Rollup? What would happen if Sequencer acts maliciously without any challenges? Is OP Stack another remedy? How effective can Arbitrum’s BOLD be? After reading this article, you may have a deeper understanding of OP-Rollup.
In simple terms, layer2 chains will batch a large number of transactions to the mainnet, and the mainnet will change the state only after the L2 transactions have gone through the validation stage. Currently, OP-Rollup uses a fraud-proof validation method, which means that any validators on the L2 chain can initiate challenges and raise questions. Within 7 days, if the challenger succeeds, the mainnet will reject the previous state change, and the L2 chain needs to repackage and submit a new state declaration. If the challenger fails, their staked assets will be forfeited.
This submit-then-challenge model makes sense, after all, the purpose of layer2 as an extension layer is to prioritize efficiency. However, you still need to have “challenges” happening, right? Even if there are very few challenges, it can still be justified. It is a bit unreasonable to make everyone believe that L2 chains will never act maliciously without any challenges.
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At the current stage, Optimism does not have a challenge mechanism. If you ask, they will tell you “In Development”. Whether it has been actually developed is unknown, but if their Sequencer is centralized, there is not much need for challenges at this stage. Do they have to slap themselves in the face to show loyalty to the mainnet? Arbitrum has its own internal testing challenge mechanism called BOLD, which attempts to transform the challenge target from permissioned to permissionless, paving the way for the decentralization of the Sequencer.
Because the challenge mechanism is actually a self-correcting mechanism, challenges, if not necessary, will only cause delays in network operation or even trigger crashes. If the incentive mechanism for challenges is designed improperly, and some malicious nodes are willing to spend a lot of money to challenge, it is possible to cause network crashes by acting maliciously. Therefore, when it comes to the design of the challenge mechanism, layer2 participants in the OP-Rollup track will be very cautious.
So, what would happen if there are no challenges? Here’s a simple example: layer2 can create a fake transfer transaction in the Sequencer, and after the validation by Validators and confirmation by the mainnet, this fictitious transfer can be legally withdrawn on the mainnet. The impact on the layer2 network can be imagined.
Currently, centralized Sequencers don’t have to worry about this problem, it’s simple. It doesn’t make sense to shuffle their own assets between hands. But what if the Sequencer becomes decentralized? If there is a thief node among them, this problem becomes inevitable.
To some extent, the script of Optimism is, because they are afraid of malicious challenges, the Sequencer needs to be centralized, and because the Sequencer is centralized, there is no need for challenges. Well, it makes sense.
But this story is obviously not attractive enough for pitching. So Optimism chose the soft distributed approach of OP Stack to solve the centralization problem in its Sequencer. Because if one day, the SuperChain family is formed, the OP Stack family will each have a seat in the Sequencer governance decision-making, governing and making decisions together. Isn’t this also a constraint on the risk of centralization? Currently, Optimism is pushing forward with OP Stack quite quickly, at least faster than the challenger mechanism in development…
On the other hand, Arbitrum’s script is relatively straightforward. Since everyone wants to see the challenge mode happen, Arbitrum constructs a set of patterns for it. Bold, in its internal testing, is doing this. I have studied the complex operation mechanism of Bold and will simplify and share it:
1) Layer2 publishes a state assertion to the mainnet Rollup contract; 2) Challenge Manage queries this assertion and initiates a challenge; 3) Each challenge requires a pledge of assets, and the challenger can submit evidence to the Edge Tracker, including the details of each step of state changes, function calls, parameters, etc.; 3) If the challenger does not agree with this evidence, they need to continue to pledge assets and continue the challenge, but each time it will consume costs; 4) A third-party Chain watcher monitors the evidence submitted throughout the challenge process, giving some reference to judge which party is honest, and ultimately the edgeChallengeManage contract determines who wins and who loses.
If this Bold challenge mode is truly applied, it would be a good solution. But let’s think about it from another perspective. Why would challengers burn money to challenge? Has the official mentioned any incentives for successful challenges? If the incentives are set high, there will definitely be people willing to challenge at any cost. If the incentives are set too low, the cost of the challenge may exceed the profits, so what’s the point of challenging? Therefore, in my opinion, even if the BOLD challenge mechanism goes online in the future, it is like military exercises, with limited actual impact. It’s just something for everyone to observe.
Think about it, even if the challenge is successful, what then? During the 7-day challenge period, if the challenger succeeds, L2 quietly executes a hard fork, who would notice? Or even, what’s wrong with adjusting the network state by upgrading the mainnet Rollup contract?
Therefore, whether the OP-Rollup challenge exists or not doesn’t have such a big impact on layer2 players. The important thing is the trust and consensus of OP-Rollup in everyone’s mind.
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