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For Rollups like Optimism and Arbitrum, what happens if the sequencer colludes with a proposer so:

  1. The sequencer withholds data
  2. The proposer publishes an invalid state update (so he can drain the L1 bridge).

Some people have argued that the sequencer "should" make data available. But how can you guarantee that ? If the sequencer is malicious he can just not publish data right ? That entity can just disable that part of the code.

What am I getting wrong ?

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I'm not super familiar with optimistic systems, I'm more from the ZK-rollup side, but I believe the basic concepts are somewhat similar.

I think all Layer 2 networks are currently mostly centralized. It's very possible that a sequencer (or any other centralized service which is needed for the process) misbehaves.

Furthermore, I don't think any of the networks have proper escape hatch mechanisms in place yet, such as fraud proofs. If these were fully implemented, a user in L2 could always engage the escape hatch himself and forcefully transfer his assets back to L1. But, as long as these are not implemented, users have to trust the services.

However, the situation is not as grim as it sounds. There are huge economic incentives for all participants to act honestly. If someone starts cheating, they could gain big money in the short term, but in any longer term it means huge losses for the entire network. Possibly the death of the entire network. Most parties understand what is at stake: any dishonest behavior will result in the loss of trust and therefore, loss of future revenue.

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So to answer my own question after doing more research.

Fraud / fault proofs need to access the data published to L1 by the sequencer in order to construct a proof in case of fraud.

So:

  1. The sequencer cannot post invalid state proofs to the L1 bridge.
  2. If the sequencer withholds some data, users can just force include their tx through the L1 contract.

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