What is a Validity Proof?
A validity proof is a cryptographic guarantee that every transaction in a batch is valid. Used by ZK-rollups, validity proofs are generated by the rollup operator and verified by the Layer 1 contract. Unlike fraud proofs (used by optimistic rollups), validity proofs don’t rely on a challenge window — every batch is proven correct before it’s accepted.
This means ZK-rollup withdrawals are fast (minutes to hours instead of 7 days) because there’s no need to wait for a challenge period.
Validity Proofs vs. Fraud Proofs
| Aspect | Validity Proof (ZK-Rollup) | Fraud Proof (Optimistic Rollup) |
|---|---|---|
| Security model | Prove correctness upfront | Assume correct, challenge if wrong |
| Who generates proofs? | Rollup operator (prover) | Third-party challengers |
| Verification cost | Every batch verified on L1 | Only on dispute |
| Withdrawal speed | Fast (minutes-hours) | Slow (7 days) |
| Trust assumption | Cryptographic soundness | At least one honest watcher |
How Validity Proofs Work
- The rollup processes a batch of transactions off-chain
- The prover generates a ZK proof (zk-SNARK or zk-STARK) that all transactions are valid
- The proof and compressed transaction data are posted to Layer 1
- The Layer 1 contract verifies the proof
- If the proof is valid, the state transition is finalized immediately
The proof guarantees that:
- All transactions had sufficient balances
- No double-spending occurred
- All smart contract logic executed correctly
- The state transition follows protocol rules
Benefits of Validity Proofs
- No challenge window: State is final as soon as the proof is verified
- Fast withdrawals: Withdraw to L1 in hours, not days
- Stronger security: Cryptographic guarantee, not economic assumption
- Data compression: Only the proof and state diff are posted to L1, reducing costs
Limitations
- High prover cost: Generating ZK proofs is computationally expensive
- Complex implementation: Building a ZK-rollup is significantly harder than an optimistic rollup
- Prover centralization risk: If only a few entities can generate proofs efficiently, they become centralization points
ZK-Rollups Using Validity Proofs
| Rollup | Proof System | Withdrawal Time |
|---|---|---|
| zkSync Era | zk-SNARK (Boojum) | Hours |
| Polygon zkEVM | zk-SNARK (Plonky2) | Hours |
| Starknet | zk-STARK (Stone) | Hours |
| Scroll | zk-SNARK (Halo2) | Hours |
| Linea | zk-SNARK (Plonk) | Hours |
Frequently Asked Questions
Q: Why don’t all rollups use validity proofs? A: Generating ZK proofs is computationally expensive and requires specialized knowledge. Optimistic rollups are simpler to build and have lower operating costs. As ZK technology matures, more rollups are expected to adopt validity proofs.
Q: Can a validity proof be faked? A: Under standard cryptographic assumptions, no. A fake validity proof would require breaking the underlying zero-knowledge proof system (zk-SNARK or zk-STARK), which is considered computationally infeasible — unless the trusted setup is compromised (for zk-SNARKs without universal setups).
Q: What are the prover costs? A: Generating a ZK proof for a batch of thousands of transactions can take minutes to hours of computation on specialized hardware (GPUs, FPGAs, or ASICs). This cost is amortized across all transactions in the batch.