Crypto glossary
Validity proof
A validity proof is a succinct cryptographic proof that a batch of transactions was executed correctly, checked by a contract before the new state is accepted.
A validity proof lets a rollup convince Ethereum that its new state follows correctly from the previous one, without Ethereum re-executing every transaction. A prover runs the batch and produces a SNARK or STARK; a verifier contract checks it in a fraction of the cost, and only then accepts the new state root. Invalid state transitions simply cannot be finalised.
This is the foundation of ZK rollups and validiums. Because correctness is shown upfront, withdrawals do not need a week-long challenge window and can complete once the proof is verified, typically within hours or faster as proving speeds up. The same technique is used to build trust-minimised bridges and light clients.
The name ZK is partly a misnomer: most rollup proofs are used for succinctness, not to hide data. Proofs are only as sound as the circuits and verifier code, which are complex and have contained bugs, so many systems keep security councils able to intervene. Generating proofs also requires significant computing power.
Example
A ZK rollup bundles thousands of transfers, a prover generates a single proof in minutes, and the Ethereum verifier contract checks it in one transaction before updating the rollup's state.