How to Design Upgradeable ZK-SNARK Systems

Upgradeable ZK-SNARK systems allow developers to patch security vulnerabilities, improve performance, and add new features to deployed zero-knowledge applications without breaking user trust or requiring a full migration. This is critical for long-lived protocols where the underlying cryptography or application logic may need to evolve. The core challenge is separating the proving/verification logic from the system's persistent state and user-facing interface, enabling one to change while preserving the other.

The most common architectural pattern is the proxy contract model, inspired by Ethereum's upgradeable smart contracts. A permanent, lightweight proxy holds the system's state and delegates all logic calls to a separate implementation contract. When an upgrade is needed, only the proxy's pointer to the implementation address is changed. For a ZK system, this means the contract storing verified proofs and managing permissions remains constant, while the underlying verifier logic—including the elliptic curve parameters or circuit constraints—can be replaced.

Managing the verifier key is a unique challenge. In a standard SNARK, a trusted setup generates a proving key and a verification key. To upgrade the circuit, you typically need a new setup and thus a new verification key. The upgradeable system must manage a registry of valid verification keys and allow authorized parties to deprecate old ones and add new ones. This can be done via a multi-sig governance contract or a timelock controller to ensure upgrades are transparent and non-malicious.

Consider a private voting application where the circuit checks a user's eligibility without revealing their identity. Version 1 uses the Groth16 proving system on the BN254 curve. An upgrade to Version 2 might switch to the PLONK proving system for universal trusted setups, improving flexibility. The upgrade process involves: 1) Deploying a new verifier contract with the PLONK verification key, 2) Proposing the new verifier address to the governance module, 3) After a time-locked vote, updating the proxy's implementation to a new adapter that routes proofs to the new verifier.

State migration between circuit versions must be handled carefully. If the new circuit requires different public inputs or interprets stored state differently, a migration contract may be needed to transform the old state format into the new one. For example, moving from a Merkle tree commitment to a Verkle tree would require re-computing and storing new root hashes for all users. This process should be permissioned and verifiable, often accompanied by a proof of correct state transition.

Security considerations are paramount. Use transparent proxies from libraries like OpenZeppelin to avoid storage collisions. Always verify the new circuit's correctness off-chain before an on-chain upgrade. Implement emergency pause functions in the proxy to halt operations if a bug is discovered. Ultimately, upgradeability introduces a trade-off: it reduces rigidity but adds complexity and centralization risk. The design must balance these factors to maintain the system's integrity and trustlessness.

A solid grasp of zero-knowledge proof fundamentals is essential. You should understand the roles of the prover and verifier, the concept of a circuit (often written in a domain-specific language like Circom or Cairo), and the trusted setup ceremony. Familiarity with the Groth16 or PLONK proving schemes is recommended, as they are commonly used in production systems like Tornado Cash and zkSync. Knowing the difference between a SNARK and a STARK will help you choose the right proof system for your application's trade-offs in proof size, verification cost, and trust assumptions.

Proficiency in smart contract development and upgrade patterns is non-negotiable. You will be deploying verifier contracts and managing upgrade logic on-chain. You must be comfortable with Solidity or Vyper and understand proxy patterns like the Transparent Proxy (OpenZeppelin) or the Universal Upgradeable Proxy Standard (UUPS). Each pattern has distinct security implications for managing admin rights and initialization. Experience with tools like Hardhat or Foundry for testing and deployment is crucial for simulating upgrades and verifying state integrity post-migration.

You need experience with circuit design and constraint systems. Writing a ZK circuit that correctly represents your program logic is the first step. This involves using frameworks such as Circom, where you define constraints that the prover must satisfy. A critical skill is minimizing the circuit size and constraint count, as this directly impacts proving time and gas costs for on-chain verification. Common pitfalls include unintentionally creating non-deterministic circuits or constraints that are too restrictive, which can break the system during an upgrade.

Understanding cryptographic primitives and library integration is key. Your system will rely on elliptic curve pairings (e.g., BN254, BLS12-381) and hash functions (e.g., Poseidon, MiMC) that are efficient in ZK circuits. You must know how to integrate audited libraries like snarkjs for proof generation or the circomlib for standard circuit templates. Security audits for both your circuit logic and the underlying cryptographic dependencies are mandatory before any mainnet deployment, as bugs can be catastrophic and irreversible.

Finally, you must plan for system architecture and state management. An upgradeable ZK system isn't just a new verifier contract; it may involve migrating user state or nullifier sets to prevent double-spends across versions. Design decisions include whether to allow parallel running of old and new verifiers, how to handle pending proofs during an upgrade, and how to securely deprecate old circuits. Documenting the upgrade process and having a rollback plan are critical operational prerequisites for any production system.

Designing an upgradeable ZK-SNARK system requires separating the proving logic from the verification contract. The core pattern involves a main verification contract that stores a commitment to the current trusted setup or verification key, while the proving logic and circuit definitions are managed off-chain. This allows developers to deploy new circuits—to fix bugs, improve performance, or add features—by simply updating the verification key reference in the main contract. Critical systems like zkSync Era and Polygon zkEVM employ variations of this pattern to maintain upgrade paths for their proving systems.

A common implementation uses a proxy pattern or a verification key registry. Instead of hardcoding the verification key, the verifier contract calls a registry to fetch the current key. This registry can be governed by a multisig or DAO. For example, you might structure it so Verifier.sol has a function verifyProof(bytes calldata proof, uint256[] calldata inputs) that internally calls KeyRegistry.getCurrentVk() to retrieve the correct verification key. This decoupling is essential because the cryptographic trust assumptions (the toxic waste of the trusted setup) are tied to the verification key; changing it is a major event that must be controlled and transparent.

When upgrading the circuit itself, you must consider state continuity. If your application manages on-chain state (like balances in a zk-rollup), the new circuit must be compatible with the existing state tree or commitment. This often requires a two-step migration: first, a new verifier is deployed and ratified; second, a state transition proof is generated to bridge the old state root to the new system. Failing to handle this can invalidate user assets. Always maintain the old verifier for a period to allow users to exit using old state proofs.

For developer tooling, frameworks like Circom and snarkjs don't natively manage upgrades. You must design your own pipeline. A robust setup includes: - Versioned circuit repositories, - Scripts to re-perform trusted setup phases for new circuits, - Automated verification key generation and registry updates. Storing circuit metadata (like the Circom compiler version and component versions) on-chain alongside the verification key is a best practice for reproducibility and audit trails.

Security considerations are paramount. The upgrade mechanism itself is a centralization vector. Mitigations include using timelocks for upgrades, requiring multiple signatures from geographically distributed parties, and implementing proof-of-innocence periods where users can challenge fraudulent upgrades. Furthermore, any new circuit must undergo rigorous re-auditing. The goal is to achieve upgradability without sacrificing the trust-minimized promise of zero-knowledge proofs. A well-architected system makes upgrades rare, transparent, and community-governed events.

Designing an upgradeable ZK-SNARK system begins with a clear separation of concerns. The core logic is divided into at least two components: a verification key registry and the proving logic. The registry, often implemented as a simple mapping on-chain, stores the current trusted verification key (VK). The proving logic, contained within a separate, upgradeable smart contract, uses this VK to verify proofs. This architecture, similar to the proxy pattern, allows the proving algorithm to be upgraded without requiring users to trust new keys, as long as the registry remains secure and immutable. A canonical example is the early design of zkSync's system, where the upgradeable contract held the logic and a separate contract managed the trusted state.

The most critical decision is the upgrade mechanism for the verification key. A purely centralized approach, where a single admin key can update the VK, offers maximum agility but minimal trustlessness. For decentralized systems, governance is essential. This typically involves a timelock-controlled multisig or a DAO vote to authorize key changes. The upgrade transaction must be highly visible, with proposals and execution delays (e.g., 1-2 weeks) to allow users to exit the system if they distrust the new parameters. Vitalik Buterin's writings on trusted setup ceremonies and upgradeability emphasize that transparency and user agency are non-negotiable for maintaining credibility in decentralized contexts.

Implementing the upgrade requires careful state management. The upgradeable prover contract must use delegatecall via a proxy (like OpenZeppelin's TransparentUpgradeableProxy) or an immutable diamond pattern (EIP-2535) to preserve its storage layout. When a new VK is approved, the governance contract calls a function like updateVerificationKey(bytes32 newVkHash) on the registry. The prover contract then reads from this registry for every verification. It is crucial to include a versioning system and event emissions for every key change, enabling indexers and frontends to track the system's evolution. A failure to preserve storage slots during an upgrade can permanently corrupt the contract state.

Security considerations are paramount. Beyond the governance model, you must protect against front-running attacks on key updates and ensure backward compatibility for pending proofs. A best practice is to implement a dual-key system during transitions: for a period, both the old and new VKs are accepted, allowing proofs generated with the old circuit to still be verified. Furthermore, the cryptographic parameters themselves must be secure; using a perpetual powers-of-tau trusted setup (like the one maintained by the Semaphore team) or a transparent setup (using STARKs or Halo2) can reduce the trust requirements for the initial circuit. Always audit both the circuit logic and the upgrade mechanics.

Finally, developer tooling and user communication define the system's success. Provide a clear SDK for generating proofs against the latest circuit and a block explorer plugin that displays the active VK and its change history. Document the fraud detection or validity proof model explicitly. For users, the interface should warn them if a major upgrade is pending. The end goal is a system where upgrades are possible for bug fixes and efficiency gains—like moving from Groth16 to Plonk—without undermining the foundational promise of trust-minimized verification. Successful implementations, such as incremental upgrades in Polygon zkEVM, demonstrate this balance in production.

Designing an upgradeable ZK-SNARK system requires balancing cryptographic security with practical development needs. The key is to isolate the proving logic (the circuit) from the verification logic (the verifier contract). This separation allows you to deploy new circuits without altering the on-chain verifier, using a system like a proxy pattern or a verifier registry. Always use a timelock and multisig for administrative upgrades to prevent unilateral changes and ensure community trust. The goal is to create a system that is as immutable as possible for users while allowing developers to fix bugs and improve performance.

For your next steps, begin by implementing a basic upgradeable verifier. A common approach is to use a VerifierRegistry contract that maps a circuitId (like a keccak256 hash of the circuit's verification key) to a verificationKey. Your main application contract would query this registry. When you need to upgrade, you deploy a new circuit, generate its verification key, and propose a governance transaction to update the registry mapping for that circuitId. Tools like SnarkJS for Groth16 or Circom's circom and snarkjs libraries are essential for circuit development and key generation.

Testing is critical. You must verify two main properties: functional correctness (the new circuit proves what you intend) and upgrade safety (the old proofs are invalidated if required). Use a forked testnet (e.g., via Foundry or Hardhat) to simulate the upgrade process end-to-end. Consider implementing a proof nullifier in your verifier contract to explicitly revoke proofs from a deprecated circuit version if your use case requires it, such as in privacy-preserving voting systems.

Look to existing implementations for reference. The Semaphore protocol uses upgradeable verifiers for its anonymity sets. zkSync Era and Scroll have sophisticated proving systems with upgrade paths managed by decentralized governance. Studying their documentation and source code provides practical insights into handling verification key management, versioning, and security assumptions.

Finally, remember that upgradeability adds complexity. Document the upgrade process clearly for your users and stakeholders. Make the verification keys and circuit source code available for audit. The most secure system is one whose upgrade mechanism is transparent, slow, and controlled by the community, ensuring the trustless properties of ZK-SNARKs are maintained throughout the system's evolution.