A validity proof is a cryptographic proof, generated by a prover (often a specialized node), that attests to the correctness of a computation or state transition, such as the execution of a batch of transactions in a rollup. The proof is succinct, meaning it is small and fast to verify, allowing a verifier (like the main blockchain's consensus layer) to confirm the result's validity without re-executing the entire computation. This mechanism is the core innovation behind ZK-Rollups (Zero-Knowledge Rollups), enabling high-throughput, secure scaling by posting only the proof and minimal data to the base layer.
LABS
Glossary
Validity Proof
A validity proof is a cryptographic proof, such as a zk-SNARK or zk-STARK, that cryptographically guarantees the correctness of a state transition, used in ZK-rollups.
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definition
CRYPTOGRAPHIC VERIFICATION
What is a Validity Proof?
A formal proof that a blockchain transaction or state transition was executed correctly according to the network's rules.
The creation of a validity proof relies on advanced cryptographic systems, most commonly zk-SNARKs (Zero-Knowledge Succinct Non-Interactive Argument of Knowledge) or zk-STARKs (Zero-Knowledge Scalable Transparent Argument of Knowledge). These systems allow the prover to demonstrate that they possess certain knowledge (the correct execution trace) without revealing the underlying data, providing both scalability through data compression and strong privacy guarantees. The verification step is computationally cheap and constant-time, regardless of the complexity of the original computation, which is key to its efficiency.
Validity proofs provide the highest security model for Layer 2 scaling, known as cryptographic security or validity security. This is stronger than the fraud proof model used in Optimistic Rollups, as the underlying chain (Layer 1) only accepts state updates accompanied by a valid proof. There is no challenge period where transactions can be disputed; invalidity is mathematically impossible once the proof is verified. This allows for near-instant finality for the Layer 2 state on the base chain.
The primary trade-off for this security and finality is prover complexity. Generating validity proofs is computationally intensive, requiring specialized hardware and significant time, which can increase centralization pressures and latency for proof generation. Furthermore, supporting general-purpose smart contract execution with validity proofs (as in a ZK-EVM) is a major engineering challenge, though rapid advancements are being made. Systems like zkSync Era, Starknet, and Polygon zkEVM are leading implementations of this technology.
Beyond scaling, validity proofs enable powerful applications like private transactions on public chains, as the proof can verify payment validity without revealing sender, receiver, or amount. They are also foundational for trustless bridges and interoperability protocols, where one chain can verify the state of another using a lightweight proof. As the technology matures, validity proofs are poised to become a fundamental cryptographic primitive for building verifiable and efficient decentralized systems.
key-features
VALIDITY PROOF
Key Features
Validity proofs are cryptographic certificates that verify the correctness of off-chain state transitions, enabling secure and efficient Layer 2 scaling.
01
Cryptographic Guarantee
A validity proof is a succinct cryptographic argument, often a zk-SNARK or zk-STARK, that mathematically proves a batch of transactions was executed correctly according to the rules of the underlying blockchain. This allows a verifier (e.g., an L1 smart contract) to confirm the integrity of off-chain computations with minimal on-chain verification cost.
02
Trustless Security
Unlike optimistic rollups that rely on a fraud-proof challenge period, validity proofs provide cryptographic finality. Once a proof is verified on the L1, the state transition is immediately considered valid and irreversible, eliminating trust assumptions and withdrawal delays for users.
03
Data Availability
For a validity proof to be meaningful, the input data (transaction data) must be available. Systems like zkRollups typically post this data as calldata on the L1, ensuring anyone can reconstruct the state and verify the proof's claims independently, a property known as data availability.
04
Scalability Engine
By moving computation off-chain and submitting only a small proof for verification, validity proofs drastically reduce the on-chain computational and storage burden. This enables massive scalability, allowing networks like zkSync and Starknet to process thousands of transactions per second while settling on Ethereum.
05
Privacy Potential
The cryptographic nature of validity proofs allows for transaction privacy. By default, zkRollups like zkSync do not reveal sender/receiver details in proofs, but full privacy (hiding amounts, etc.) requires specific application design, as seen in protocols like Aztec.
06
Proof Generation & Verification
- Prover: An off-chain node that generates the proof, a computationally intensive process requiring specialized hardware.
- Verifier: A lightweight on-chain smart contract that checks the proof's validity, a process that is fast and cheap. This asymmetry is the core efficiency gain, shifting the heavy work off-chain.
how-it-works
ZK-ROLLUP MECHANICS
How a Validity Proof Works
A technical breakdown of the cryptographic mechanism that allows a blockchain to verify the correctness of off-chain transactions without re-executing them.
A validity proof is a cryptographic attestation, generated by a prover, that mathematically guarantees the correctness of a batch of transactions executed off-chain. This proof, often a zk-SNARK or zk-STARK, is submitted to a smart contract on the parent chain (Layer 1), which verifies it using a fixed verification algorithm. The core innovation is that the verifier only needs to check the proof, not the entire computation, enabling secure scaling. If the proof is valid, the resulting state root is accepted; if invalid, it is rejected, ensuring only correct state transitions are finalized.
The process begins with a prover (e.g., a sequencer node) executing a batch of transactions within a zk-rollup and generating a new state root. It then creates a validity proof that cryptographically binds this new state to the previous one, proving that every transaction was executed according to the rollup's rules—checking signatures, balances, and smart contract logic—without revealing the transaction details. This proof is succinct, meaning its size and verification time are tiny compared to re-executing the batch. The system's security is rooted in computational integrity: forging a valid proof for incorrect computations is considered computationally infeasible.
Upon receiving the proof and the new state root, the verifier contract on Layer 1 performs a fixed computation to check the proof's cryptographic signatures. This verification is gas-efficient and constant in cost, regardless of the batch size. This creates a powerful trust model: the security of the rollup inherits the security of the underlying L1, as the L1 acts as the ultimate arbiter of truth. Validity proofs enable sovereign validity, where the canonical state is defined by provable correctness, not by a committee's signatures or a fraud challenge window.
Contrast this with optimistic rollups, which use fraud proofs and assume transactions are valid unless challenged during a dispute window. Validity-proof systems provide instant finality for the L1, as the state is finalized as soon as the proof is verified. Key implementations include zkSync Era, Starknet, and Polygon zkEVM. The primary trade-off is prover complexity: generating these proofs requires significant computational resources, though verification remains simple and cheap for the L1.
types-of-proofs
PROOF SYSTEMS
Types of Validity Proofs
A validity proof is a cryptographic proof that a state transition (e.g., a batch of transactions) was executed correctly according to the rules of the system. Different proof systems offer varying trade-offs in performance, trust assumptions, and complexity.
01
ZK-SNARKs
ZK-SNARKs (Zero-Knowledge Succinct Non-Interactive Argument of Knowledge) are a class of validity proofs characterized by their small size and fast verification. They require a trusted setup ceremony to generate public parameters but produce proofs that are extremely efficient to verify on-chain. Key properties include:
- Succinctness: Proofs are only a few hundred bytes.
- Non-interactivity: The proof is submitted without needing a verifier's challenge.
- Zero-Knowledge: The proof can hide transaction details (optional). Used by: Zcash (zk-SNARKs), early zkRollups.
02
ZK-STARKs
ZK-STARKs (Zero-Knowledge Scalable Transparent Argument of Knowledge) are a validity proof system designed to be post-quantum secure and transparent, meaning they do not require a trusted setup. They trade off slightly larger proof sizes (tens of kilobytes) for faster prover times and enhanced scalability. Key properties include:
- Transparency: Relies on public randomness, eliminating trusted setup.
- Scalability: Prover and verifier times scale quasi-linearly.
- Post-Quantum Security: Based on collision-resistant hashes. Used by: Starknet, Polygon Miden.
03
Bulletproofs
Bulletproofs are short, non-interactive zero-knowledge proofs that do not require a trusted setup. They are particularly efficient for proving statements about pedersen commitments, such as range proofs (e.g., proving a value is non-negative without revealing it). While not as succinct as SNARKs for general computation, they are a core component in confidential transaction protocols.
- No trusted setup.
- Short proofs for specific arithmetic circuit types.
- Widely used for confidential assets in Monero and Mimblewimble-based chains.
04
PLONK & Universal SNARKs
PLONK (Permutations over Lagrange-bases for Oecumenical Noninteractive arguments of Knowledge) is a universal and updatable SNARK system. Its key innovation is a single, reusable trusted setup ceremony that can support any program up to a certain size, which can be updated by a decentralized group. This makes it highly practical for evolving blockchain applications.
- Universal Setup: One ceremony for many circuits.
- Updatable: The toxic waste can be refreshed.
- Efficient: Balances prover speed and proof size. Used by: Aztec, many modern zkRollup implementations.
05
Proof Aggregation & Recursion
Proof aggregation and recursion are techniques to combine multiple validity proofs into a single proof, dramatically reducing on-chain verification costs for systems like rollups. Recursive proofs are proofs that verify other proofs.
- Aggregation: Merges proofs from multiple batches.
- Recursion: A proof verifies the correctness of another proof's verification.
- Finality: A single, small proof can attest to the validity of thousands of transactions or even an entire day's blocks. This is a critical scaling component for zkEVM rollups.
06
Validity vs. Fraud Proofs
This is the fundamental architectural choice for Layer 2 scaling. Validity proofs (zk-Rollups) cryptographically guarantee correctness, allowing for instant withdrawal finality. Fraud proofs (Optimistic Rollups) assume correctness but allow a challenge period where anyone can submit a proof of fraud to revert invalid state. Key Differences:
- Finality: Validity proofs offer immediate finality; fraud proofs have a 7-day challenge delay.
- Security Model: Validity proofs rely on cryptography; fraud proofs rely on economic incentives and at least one honest verifier.
- Cost: Validity proofs have higher prover costs but lower on-chain data costs.
ecosystem-usage
VALIDITY PROOF
Ecosystem Usage
Validity proofs, also known as zero-knowledge proofs (ZKPs) or succinct non-interactive arguments of knowledge (SNARKs/STARKs), are cryptographic primitives that enable one party (the prover) to convince another (the verifier) that a statement is true without revealing the underlying data. In blockchain, they are the core mechanism powering ZK-Rollups and other scaling solutions.
04
On-Chain Gaming & Autonomous Worlds
Validity proofs enable complex, verifiable game logic to be executed off-chain with on-chain settlement. The game state progresses off-chain, and a proof of each turn's correctness is periodically submitted to the blockchain.
- Fully on-chain games (e.g., Dark Forest) use ZKPs to hide player positions while proving move validity.
- Reduces gas costs by moving computation off-chain.
- Ensures provably fair and deterministic game mechanics without revealing all data.
05
Decentralized Identity & Credentials
Validity proofs allow users to prove claims about their identity or credentials without revealing the underlying documents. This is foundational for self-sovereign identity (SSI) and decentralized attestations.
- Prove you are over 18 without revealing your birth date.
- Prove membership in a DAO or possession of a specific NFT for gated access.
- Verifiable Credentials (VCs) use ZKPs to provide selective disclosure, enhancing privacy in DeFi and social applications.
SCALING MECHANISM COMPARISON
Validity Proof vs. Fraud Proof
A technical comparison of the two primary cryptographic proof systems used in blockchain scaling solutions like optimistic and zk-rollups.
| Feature | Validity Proof (e.g., zk-Rollup) | Fraud Proof (e.g., Optimistic Rollup) |
|---|---|---|
Core Security Guarantee | Cryptographic proof of state correctness | Economic challenge period for incorrect state |
Proof Generation | Computationally intensive (ZK-SNARK/STARK) | Only required if a challenge is issued |
Withdrawal Finality to L1 | Immediate (after proof verification) | Delayed by challenge period (e.g., 7 days) |
On-Chain Data Requirement | State diff + validity proof | Full transaction data for challenges |
Trust Assumption | Cryptographic (trustless) | Economic (1-of-N honest verifier assumption) |
EVM Compatibility | Complex (zkEVM required) | Native (full EVM equivalence) |
Typical Transaction Cost | Higher prover cost, lower L1 data cost | Lower compute cost, higher L1 data cost |
security-considerations
VALIDITY PROOF
Security Considerations
Validity proofs are cryptographic guarantees that a state transition is correct, but their security depends on the underlying assumptions and implementation details.
01
Trust Assumptions
A validity proof's security is defined by its trust model. ZK-Rollups rely on cryptographic assumptions (e.g., computational hardness of problems like discrete logarithms) and a trusted setup, making them trust-minimized. Optimistic Rollups rely on a cryptoeconomic security model, assuming at least one honest actor will submit a fraud proof during the challenge period.
02
Prover & Verifier Integrity
The system's security hinges on the correct implementation of the prover (which generates the proof) and verifier (which checks it). Bugs in this code can lead to invalid state transitions being accepted. This requires extensive formal verification and auditing of the circuit logic and smart contracts.
03
Data Availability
For a validity proof to be verified, the underlying data must be available. If transaction data is withheld (data withholding attack), users cannot reconstruct the state or create proofs for future transactions. This is mitigated by posting data to a data availability layer like Ethereum's calldata or a dedicated Data Availability Committee (DAC).
04
Upgradeability & Centralization Risks
Many proof systems have upgradeable contracts or rely on a centralized sequencer. A malicious upgrade or sequencer can potentially censor transactions or deploy faulty code. Security depends on decentralization of these roles and robust governance mechanisms with timelocks.
05
Cryptographic Assumptions & Breakage
Validity proofs depend on the long-term security of specific elliptic curves (e.g., BN254, BLS12-381) and hash functions. A future cryptographic breakthrough (e.g., quantum computing breaking elliptic curve cryptography) could invalidate all historical proofs. Systems may require post-quantum secure upgrades.
06
Economic & Liveness Guarantees
Even with a valid proof, the system must ensure liveness. This includes:
- Sequencer liveness: Ability to submit proofs and data.
- Withdrawal guarantees: Users must be able to exit to L1 even if the L2 halts via escape hatches or force inclusion mechanisms.
- Bonding & slashing for provers to ensure honest behavior.
VALIDITY PROOFS
Common Misconceptions
Clarifying the technical realities behind zero-knowledge proofs and their role in blockchain scaling.
A validity proof is a cryptographic proof, often a zero-knowledge proof (ZKP), that cryptographically attests to the correctness of a batch of transactions or state transitions without revealing their underlying data. It works by having a prover (e.g., a sequencer) generate a succinct proof that a computation was executed correctly, which a verifier (e.g., a smart contract on a base layer like Ethereum) can check with minimal computational effort. This allows for the secure off-chain execution of transactions, with only the tiny proof posted on-chain for verification, enabling scaling solutions like ZK-Rollups. The most common proof systems used are zk-SNARKs and zk-STARKs.
VALIDITY PROOFS
Frequently Asked Questions
A deep dive into the cryptographic mechanism that enables trust-minimized scaling for blockchains.
A validity proof is a cryptographic proof that attests to the correctness of a batch of transactions or a state transition, allowing a verifier to be convinced of a computation's result without re-executing it. It is the core mechanism behind zero-knowledge rollups (ZK-rollups). The proof is generated off-chain by a prover and submitted on-chain to a smart contract, the verifier. This verifier contract can check the proof's validity with minimal computational effort, ensuring the new state root posted on the main chain (Layer 1) is correct. This process enables massive scalability by moving execution off-chain while maintaining the security guarantees of the underlying blockchain.
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