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In optimistic roll-ups, blocks (aggregated transactions) are published on the Ethereum main net. The main net functions as a security layer for the roll-up in the sense that it leverages the security and decentralization features of the Ethereum main net to ensure the integrity and finality of the roll-up transactions.

The roll-up operators generate cryptographic proofs, known as fraud proofs, that demonstrate the correctness of the roll-up state or expose any invalid or malicious behavior. What if there was no dispute over an invalid block and it was published on-chain, and after the challenge period is over, the invalid block is detected? Is it too late then? If not, how can it be reverted? If it is too late, will the operators continue building on top of the invalid block?

I know that operators do not build on top of invalid blocks since they will get slashed, but is this block invalid if it is finalized?

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The best answer is probably "undefined". Before speculating on an answer, it is important to state the 2 main assumptions of an optimistic rollup:

  1. a trust model of 1-of-N: there are many actors, and the system works as long as at least one of them does what you expect them to. Just one honest node is needed to submit fraud proofs, and few systems can do better than 1-of-N.

  2. censorship resistance of Ethereum mainnet (the L1 chain) so that fraud proofs cannot be censored from getting published in time.

The question is out of bounds, so a speculative answer would be yes: nodes would likely accept a block built on an invalid, unchallenged, finalized block.

Further speculation is unproductive as it depends on details like what the invalid block/s did and when they were discovered. Perhaps the more severe the invalid block is (like Bitcoin's value overflow), the sooner it would be discovered, and the higher likelihood that the stakeholders of the rollup will fork to fix the issue.

EDIT: There is a concept of attention challenges that can strengthen the trust model and is discussed in this detailed article: https://medium.com/onther-tech/optimistic-rollup-is-not-secure-enough-than-you-think-cb23e6e6f11c

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  • Nice. Yes, there will probably be hard forks and stakeholders will try their best to invalidate the malicious block. But, if the challenge period is short enough, the network congestion could prevent the challenge transaction from being included (Or at least the challenge interaction is completed). After this, the proposers will continue building on the invalid block without forking because of miners' rationality. Anyway, I assume your answer supports my assumption and I'll mark it as a valid answer. Nov 5, 2023 at 12:08
  • You pointed out another important assumption that the challenge period is long enough. To your point of miner's rationality, I think it is more like most of them will be running the default code that allows them to build on a possibly invalid block. It seems this default code could include code that validates the block and submits a fraud proof? Someone familiar with actual implementations could write an answer?
    – eth
    Nov 5, 2023 at 18:41
  • Yes, you might be right. If the builders validate the blocks themselves then probably not many are going to build on top of the invalid block. I will leave this question open for now to see if someone more familiar with the code would answer. Nov 6, 2023 at 1:07

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