Thread by @bkiepuszewski

1/ The easiest way to understand the difference between L2 Rollup and a sidechain such as @0xPolygon is to inspect closer the exit procedure. Below is tx withdrawing 450,000 USDC from @0xPolygon child chain: ethtx.info/0x5c5f80a7dab5… 👇

2/ First thing to notice is that to perform the exit user needs to submit the chunk of data (input data) containing, among other info, merkle proof for the exit. This data can only be obtained from Matic nodes, it is impossible to construct it just by observing L1

3/ On a Rollup data would be available on L1, so even if all Matic nodes were down, users would still be able to exit their tokens. That's not the case here - you need to get that data from Matic nodes

4/ Next, contract verifies MerkleProof against previously submitted checkpoint. Again - if Matic nodes chose not to commit a checkpoint, exit would be impossible. But it's worse than that - they can submit fraudulent checkpoint state assigning all users' tokens to themselves

5/ And then simply exit them by submitting the merkle proof and verifying it against this fraudulent state (note that any state can be submitted and there is no validity check)

6/ TL/DR: users need to trust validators for the safety of their funds and the ability to exit them. Validators can easily block funds on a sidechain (by withholding data) or seize them (by assigning all tokens to themselves)