A zk-rollup is a scaling system that executes transactions outside a base blockchain, groups the results and submits cryptographic validity proofs for verification on that base layer. The verifier checks that the proposed state transition follows the rollup's encoded rules. Sharing publication and verification costs across a batch can reduce the cost per transaction.
Proofs and data perform different jobs
A prover generates evidence linking an earlier state, the processed operations and a proposed new state. A settlement contract verifies that evidence before accepting the update. Users can receive an earlier operator confirmation, but that is different from final settlement on the base chain.
A rollup also publishes sufficient data on its base layer to reconstruct its state, using mechanisms such as Ethereum calldata or blobs. A validium keeps the corresponding data outside that layer, introducing different availability assumptions. A validity proof alone does not supply missing state data.
What the name does not guarantee
“ZK” does not mean every transaction detail is private. Many systems use proof technology primarily to verify computation.
There is no optimistic fraud-challenge period inherent to this model, but proving, publication and settlement can still delay withdrawals. Faulty proof circuits, vulnerable contracts, upgrade controls and operator outages remain relevant risks. Application compatibility and transaction costs also vary by implementation.