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

ZK-Rollups and Optimistic Rollups 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 they rely on.

Optimistic Rollups rely 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 a certain timeframe. ZK-Rollups, in contrast, rely 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-Rollups have a few important advantages over Optimistic Rollups:

Security: With ZK-Rollups, the system is always in a valid state. With Optimistic Rollups, 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-Rollups it could only result in transactions being reverted.

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

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

ZK-Rollups and Optimistic Rollups 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 they rely on.

Optimistic Rollups rely 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 a certain timeframe. ZK-Rollups, in contrast, rely 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-Rollups have a few important advantages over Optimistic Rollups:

Security: With ZK-Rollups, the system is always in a valid state. With Optimistic Rollups, 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-Rollups it could only result in transactions being reverted.

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

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

Updated on: 14 / 07 / 2022