validity proof

A validity proof is a cryptographic proof, posted alongside a rollup batch, that mathematically guarantees the batch's state transition was computed correctly. Where an optimistic rollup says 'assume this is right unless someone proves otherwise,' a validity-proof rollup says 'here is a proof that it is right — verify it and you need trust nothing else.' This is the engine of ZK rollups such as zkSync, StarkNet, Polygon zkEVM, Scroll, and Linea, and it is built from succinct proof systems (zk-SNARKs or zk-STARKs).

The mechanics rest on succinctness: the proof is tiny and cheap to verify even though the computation it attests to may involve thousands of transactions. The rollup runs the batch, generates a proof that the new state root follows correctly from the old one and the transactions, and submits that proof to a verifier contract on Ethereum. The contract checks it in milliseconds of computation, and if it passes, the new state root is accepted as final immediately — there is no window in which it might later be reverted. Note that 'zero-knowledge' is partly a misnomer here: the headline benefit is succinct verifiable computation; privacy (hiding inputs) is an optional extra most rollups do not use.

Compared with fraud proofs, validity proofs give two decisive advantages: there is no multi-day challenge period, so withdrawals can finalize as soon as the proof verifies (often within an hour or two, limited by proving time), and security does not depend on a watchful honest challenger being online and uncensored — math alone enforces correctness. The costs are heavier: generating proofs is computationally expensive, and building a prover for the full EVM (a zkEVM) is hard, which is why these systems matured later than optimistic rollups. Data availability is still required — a validity proof shows the state is correct but you still need the data to know what that state is and to use the chain.