What is the Difference between ZK-Rollup and Optimistic Rollup?

ZK-Rollup and Optimistic Rollup are both Layer-2 scaling technologies that take transactions off-chain, compute the resulting state there, and submit a commitment to this new state back to the blockchain. The key difference between them is the type of proof each protocol relies on. Optimistic Rollup relies on fraud proofs: a new state is assumed to be valid, unless proof to the contrary (a proof of fraud) is submitted to the blockchain within some timeframe. ZK-Rollup relies on validity proofs: a new state will always be presented to the blockchain with proof that it is in fact valid. For additional background, check out StarkWare’s comparison of validity proofs vs. fraud proofs.

ZK-Rollup has a few important advantages over Optimistic Rollup:

Security: In ZK-Rollup, the system is always in a valid state. In Optimistic Rollup, an invalid state may exist, certainly until a fraud proof is submitted, and possibly indefinitely. Specifically, a 51% attack in OR can result in theft of funds, whereas in ZK-Rollup it could only allow for the reverting of transactions.

Capital Efficiency: ZK-Rollup is 1000X more capital efficient than Optimistic Rollup.

Scale: Optimistic Rollup consumes blockchain resources that grow linearly with the volume of transactions. ZK-Rollup can be designed (and so we did with StarkEX!) to consume exponentially less blockchain resources. StarkEx consumes an almost fixed amount of blockchain resources: they grow logarithmically with the volume of transactions, thus allowing for massive transaction throughput.
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