Zero-Knowledge Rollup
A zero-knowledge (ZK) rollup is a Layer 2 that bundles transactions and posts a cryptographic proof to the base chain showing that the whole batch was executed correctly. The base chain checks the proof instead of re-running every transaction.
How a ZK rollup works
Transactions are executed off the base chain and grouped into batches. A component called a prover then generates a validity proof, a compact piece of math that demonstrates the new state follows correctly from the old one. A smart contract on Ethereum verifies this proof, which is far cheaper than repeating all the computation.
Despite the name, most ZK rollups are not about privacy. They use zero-knowledge proof techniques mainly for their ability to prove correctness succinctly, while the transaction data is often still published openly.
How it differs from an optimistic rollup
An optimistic rollup assumes batches are valid and waits for a challenge period in which fraud can be proven. A ZK rollup proves validity up front. Once a proof is verified on the base chain, the state is final there, so withdrawals through the official bridge do not need a week-long wait. In practice they can still take some hours, because generating proofs takes time and computing power.
Examples and trade-offs
zkSync Era, Starknet, Scroll and Linea are examples of ZK rollups. Making them compatible with Ethereum's virtual machine, known as a zkEVM, is technically hard, so compatibility varies. Proof generation is computationally expensive, and the cryptography and circuits are complex, which makes bugs harder to spot.
Risks to understand
A validity proof only guarantees what the circuit actually checks. A bug in the prover or verifier could allow an invalid state. Like other rollups, many ZK rollups still have upgradeable contracts, a single sequencer and centralized provers, so users trust the operators to some degree.
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Frequently asked questions
Does a ZK rollup make my transactions private?
Usually not. Most ZK rollups publish transaction data; the zero-knowledge part proves correctness rather than hiding details.
Are ZK rollups more secure than optimistic rollups?
They rely on mathematics rather than on someone watching and challenging fraud, but they add cryptographic complexity. Each design has different risks.
Why are ZK rollups cheap if proofs are expensive?
The proving cost is shared across thousands of transactions in a batch, and verifying the proof on the base chain is relatively cheap.
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