Validity Proof

A validity proof is a succinct cryptographic argument that verifies the correctness of a batch of transactions. This proof is small and fast to verify on-chain, regardless of the size of the computation it represents. This enables massive scalability by moving heavy computation off-chain.

• Example: A SNARK proof verifying 10,000 transactions might be only ~200 bytes.
• Contrasts with fraud proofs, which require re-executing the disputed transaction on-chain.

Security relies on cryptographic assumptions (e.g., the hardness of discrete logarithms) rather than economic incentives or honest majority assumptions. Once a valid proof is posted and verified on the base layer (L1), the state transition is final. There is no challenge period or window for disputing the result, providing instant withdrawal guarantees.

• Key Property: Offers strong cryptographic security akin to the base layer itself.

Users do not need to trust the operator (prover) of the scaling network. They only need to trust the cryptographic soundness of the proof system and the security of the underlying L1. This creates a trustless bridge where security is inherited from Ethereum or another settlement layer.

• Core Benefit: Eliminates the need for honest majority assumptions required by optimistic rollups.

When validity proofs are implemented using Zero-Knowledge Proofs (ZKPs), they can provide transaction privacy. A ZK-SNARK or ZK-STARK proof can validate that a state transition is correct without revealing the underlying transaction data.

• Use Case: zkRollups like zkSync and StarkNet use this for scalable payments and smart contracts.
• Note: Privacy is an optional feature enabled by specific ZKP constructions, not all validity proof systems.

The system involves two distinct roles:

• Prover: A powerful node that executes transactions off-chain and generates a cryptographic proof of correctness. This is computationally intensive (proof generation).
• Verifier: A smart contract on the L1 that checks the proof. This is computationally cheap (proof verification). This separation is what allows the L1 to securely scale by outsourcing work.

Different cryptographic engines generate validity proofs, primarily:

• SNARKs (Succinct Non-Interactive Argument of Knowledge): Small proofs, fast verification. Require a trusted setup ceremony. Used by zkSync, Polygon zkEVM.
• STARKs (Scalable Transparent ARguments of Knowledge): Larger proofs, but no trusted setup. Considered post-quantum resistant. Used by StarkNet. Both provide the core feature of succinct verifiability.

Recursive proof systems like Plonky2 (used by Polygon zkEVM) allow a proof to verify other proofs, enabling efficient proof aggregation and parallelization.

• Mechanism: A single final proof can attest to the validity of a massive blockchain state by recursively combining proofs of smaller blocks.
• Benefit: Dramatically reduces the time and cost to generate proofs for large batches, making ZK-Rollups more practical for high-throughput environments.
• Innovation: Represents a significant advancement in proof recursion and proof generation speed.

A Volition is a hybrid system that gives users a choice per transaction: to post data on-chain (like a ZK-Rollup) for maximum security or off-chain (like a Validium) for lower cost.

• Flexibility: This architecture, pioneered by StarkNet, allows applications to optimize for security or cost based on the transaction's value.
• Related: Solutions like EigenDA and Celestia provide modular data availability layers that can be used by validity-proof systems, separating execution from data consensus.

While ZK-Rollups focus on scaling, the underlying zero-knowledge proof technology is inherently private. Standalone applications use validity proofs to enable confidential transactions and computations.

• Private L2s: Networks like Aztec use ZKPs to hide transaction amounts and participants.
• Identity & Credentials: Protocols (e.g., Worldcoin, zkPass) use ZK proofs to verify claims (like humanity or age) without revealing underlying data.

Trust-minimized bridges are adopting validity proofs to securely verify state transitions or message passing between blockchains.

• Light Client Bridges: Use ZK proofs to verify the consensus of one chain on another (e.g., proving an Ethereum block header is valid on a different chain).
• Proof Verification: A bridge can verify a validity proof from a source chain to unlock assets on a destination chain, reducing trust assumptions compared to multi-sig models.

A critical supporting infrastructure. Generating ZK proofs (especially STARKs) is computationally intensive. Decentralized networks of provers are emerging as a service layer.

• Specialized Hardware: Provers often use GPUs or dedicated ASICs.
• Market Dynamics: These networks allow rollups to outsource proof generation, creating a competitive market for cost and speed. Projects like Georli, Ingonyama, and Succinct are building in this space.

A validity proof (e.g., a zk-SNARK or zk-STARK) cryptographically guarantees computational integrity, requiring users to trust only the mathematical soundness of the cryptography and the correct implementation of the prover and verifier. This is a significant reduction from optimistic rollups, which require trust in a set of honest watchers to challenge invalid state during a fraud proof window.

The security of many validity proof systems (particularly zk-SNARKs) depends on a trusted setup ceremony to generate proving and verification keys. If this process is compromised, false proofs could be generated. Transparent setups (zk-STARKs) or ongoing ceremony audits mitigate this risk. The prover software itself is also a critical trust component and potential attack vector.

A validity proof ensures state transition correctness, but not that the underlying transaction data is published. Users must separately trust that data availability is ensured, typically by posting data to a base layer like Ethereum. Without available data, users cannot reconstruct the state or generate future proofs, leading to a system halt. This is a core consideration for validium and volition architectures.

Most validity-proof systems (zk-Rollups) have upgradeable smart contracts controlled by a multi-sig or DAO. This introduces an admin key risk, where a malicious upgrade could steal funds or censor transactions. The security model therefore includes trust in the governance process. Timelocks and escape hatches are common risk-mitigation mechanisms.

Validity proofs rely on specific cryptographic assumptions:

• zk-SNARKs typically depend on the hardness of elliptic curve pairings (not quantum-resistant).
• zk-STARKs rely on collision-resistant hashes (considered post-quantum secure). A future break in these underlying primitives could invalidate the security guarantees, making cryptographic agility important for long-term security.

Validity-proof systems prioritize safety (funds cannot be stolen) over liveness (ability to transact). If the sequencer or prover fails, the system may halt, but funds remain safe due to cryptographic guarantees. Users can force transactions via censorship resistance mechanisms, but these often require interacting directly with the base layer contract, which can be costly and slow.

A cryptographic method allowing one party (the prover) to prove to another (the verifier) that a statement is true without revealing any information beyond the validity of the statement itself. It is the foundational cryptographic primitive for validity proofs.

• Types: zk-SNARKs, zk-STARKs, Bulletproofs.
• Core Property: Provides succinctness (proofs are small and fast to verify) and zero-knowledge (no data leakage).

A Layer 2 scaling solution that batches hundreds of transactions off-chain, generates a validity proof (typically a zk-SNARK or zk-STARK) for the correctness of the batch, and posts this single proof to the underlying Layer 1 (e.g., Ethereum).

• Security: Inherits the security of the L1 via cryptographic proofs.
• Example: zkSync, StarkNet, Polygon zkEVM.
• Key Benefit: Enables high throughput and low fees while maintaining strong security guarantees.

The security mechanism used in Optimistic Rollups. It operates on a challenge-response model where transactions are assumed valid (optimistic) but can be challenged during a dispute window. A fraud proof is generated to demonstrate that an invalid state transition occurred.

• Contrast with Validity Proof: Reactive vs. proactive; requires watchers and has a delay for fund withdrawal.
• Example: Optimism, Arbitrum (though some use hybrid models).

A virtual machine compatible with the Ethereum EVM that can produce validity proofs for the correctness of its execution. It allows developers to deploy existing Ethereum smart contracts with minimal changes while leveraging ZK-proof scaling.

• Types: Varies in level of EVM equivalence (e.g., bytecode-level, language-level).
• Goal: Maintain developer familiarity and tooling compatibility while enabling ZK-rollup benefits.

A critical property of a validity proof system where the size of the proof is very small (e.g., a few hundred bytes) and the time required to verify it is extremely fast (e.g., milliseconds), regardless of the complexity of the original computation.

• Why it matters: Makes posting and verifying proofs on-chain economically feasible.
• zk-SNARKs: Provide very small, constant-sized proofs.
• zk-STARKs: Provide slightly larger proofs but with faster prover times and no trusted setup.

A one-time ceremonial procedure required to generate the public parameters (Common Reference String) for certain zk-SNARK circuits. If compromised, it could allow the creation of false validity proofs.

• Security Consideration: A potential point of centralization or vulnerability.
• Trustless Alternatives: zk-STARKs and some newer SNARK constructions (e.g., PLONK with universal setup) eliminate or minimize this requirement.