How to Architect a Privacy-First Authentication Layer Using ZK Rollups

Traditional on-chain authentication, like signing a message with a private key, creates a permanent, public link between a user's wallet address and their actions. A privacy-first authentication layer decouples proof of identity from the identity itself. Using ZK rollups like StarkNet or zkSync, you can architect a system where users generate a zero-knowledge proof that they possess valid credentials (e.g., they are over 18, own a specific NFT, or are on an allowlist) without revealing the underlying data. The rollup verifies this proof and only posts the verification result to the main Ethereum chain, ensuring user privacy is preserved at the protocol level.

The core architecture involves three key components working off-chain: a Prover, a Verifier, and an Identity Manager. The Prover (client-side) uses a user's private credentials to generate a ZK-SNARK or STARK proof. The Verifier (a smart contract on the ZK rollup) contains the logic to check this proof against a set of public rules. The Identity Manager is a trusted service or decentralized protocol that issues and revokes the private credentials, such as semaphore-style identity commitments or zk-proofs of personhood. This separation ensures the rollup never sees the raw identity data.

To implement this, you first define your authentication logic in a circuit using a framework like Cairo (for StarkNet) or Circom (for zkSync). For example, a circuit could prove a user holds a credential signed by a specific issuer's public key. You then compile this circuit to generate a verifier contract. A user's client-side application would fetch their credential from the Identity Manager, use it to generate a proof with the circuit, and submit the proof to the verifier contract on the rollup. Upon successful verification, the contract emits an event or updates a state, granting the user access.

Consider a practical example: gated access to a DAO's private forum. Instead of checking a wallet's NFT holdings on-chain, a user proves membership with a ZK proof. The circuit logic checks that the user's private credential corresponds to a valid DAO membership. The verifier contract on the rollup validates this proof and mints a session token (as an ERC-20 or ERC-721) to the user's address. The forum's frontend then only needs to check for the presence of this session token, which is a privacy-preserving attestation of membership, with no link to the user's specific NFT or identity.

Key challenges in this architecture include managing the trust assumptions of the Identity Manager and ensuring circuit security. If the issuer of the private credentials is compromised, the entire system's integrity fails. Solutions involve using decentralized attestation networks like Worldcoin's Proof of Personhood or BrightID. Furthermore, circuit bugs are critical; formal verification tools for Cairo or audit-tested Circom templates are essential. The gas cost for verification is also concentrated on the rollup's prover, but this is significantly cheaper than performing complex checks directly on Ethereum L1.

To get started, explore frameworks like StarkNet's native support for privacy primitives or zkSync's ZK Stack. Tools such as snarkjs for Circom or the StarkNet Foundry for Cairo provide development environments. The end goal is an authentication flow where users can interact with dApps by proving "I am authorized" rather than "Here is who I am," fundamentally shifting the privacy paradigm in Web3. This architecture is foundational for compliant DeFi, private voting, and anonymous credential systems.

To follow this architecture guide, you need a foundational understanding of Zero-Knowledge Proofs (ZKPs) and rollup technology. Familiarity with Ethereum and its account abstraction concepts (ERC-4337) is also crucial. For development, ensure you have Node.js (v18 or later) and npm or yarn installed. You'll interact with tools like Hardhat or Foundry for smart contract development and testing. A basic grasp of TypeScript will be helpful for writing frontend integration code and interacting with ZK libraries.

The core of this system relies on specific ZK proving systems. We will use Circom for writing arithmetic circuits and snarkjs for generating and verifying proofs. These tools allow you to define the logic for a privacy-preserving authentication claim (e.g., "I own a credential without revealing it") as a circuit. You must install these globally or within your project: npm install -g circom snarkjs. Additionally, understanding the role of a verifier contract on the rollup is essential; this is a Solidity smart contract that validates the ZK proofs submitted by users.

Set up a local development chain to test your architecture. You can use Anvil (from Foundry) or run a local zkSync Era or Starknet devnet, depending on your chosen rollup stack. For example, to test on a zkSync Era local environment, you would use the zksync-cli. Configure your wallet (like MetaMask) to connect to this local network. This sandbox environment is where you will deploy your verifier contract and a mock Identity Registry—a smart contract that holds public commitments of user credentials without exposing the underlying data.

Your authentication logic must be defined in a circuit file (auth.circom). A simple example circuit might prove knowledge of a secret that hashes to a public commitment stored in the registry. After writing the circuit, you compile it to generate the proving key, verification key, and Solidity verifier code using circom and snarkjs. The output verifier contract is what you deploy to your chosen rollup. The process involves several terminal commands for compilation, setup, and key generation, which we will detail in the implementation section.

Finally, plan your application's flow. The user's client-side application will use libraries like zkkit or snarkjs in the browser to generate a ZK proof locally using their secret. This proof, along with necessary public inputs, is sent to your backend or directly to a relayer (which could be an ERC-4337 bundler). The relayer submits the proof and a user operation to the verifier contract on the rollup. If the proof is valid, the user operation (e.g., a login transaction) is executed. You'll need to structure your project to handle this off-chain proof generation and on-chain verification seamlessly.

A privacy-first authentication layer built with ZK rollups allows users to prove their identity or credentials without revealing the underlying sensitive data. The core idea is to move the authentication logic and state management off-chain into a zkVM (like RISC Zero or SP1) or a zkEVM (like Polygon zkEVM or zkSync), while only submitting a cryptographic proof of a valid authentication event to the base layer (e.g., Ethereum). This architecture provides - user privacy by keeping personal data off-chain, - cost efficiency by batching proofs, and - security inherited from the underlying L1. The system's state is represented by a Merkle tree where each leaf corresponds to a user's hashed identity commitment.

The architecture consists of several key off-chain and on-chain components. The Off-Chain Prover is a server or client-side service that generates zero-knowledge proofs (ZKPs) for authentication logic. It uses a State Tree Manager to maintain the latest Merkle root of user identities. The On-Chain Verifier is a smart contract deployed on the base chain that receives and validates the ZK proofs. A Rollup Contract (or Bridge Contract) manages the canonical state root and handles proof submissions. For user interaction, a Client SDK provides methods for generating identity commitments and requesting proofs. Data availability for the off-chain state transitions can be managed via a Data Availability Committee or a validium setup using external chains like Celestia or EigenDA.

The user flow begins with registration, where a user generates a secret identity (like a private key) and computes a public identity commitment, typically a hash like hash(privateKey, salt). This commitment is sent to the off-chain prover, which adds it as a new leaf to the state Merkle tree and updates the root. To authenticate, the user requests a proof from the prover. The prover runs the authentication circuit, which verifies the user's secret against the commitment in the tree and outputs a proof of valid membership and correct state transition. This proof, along with the new state root, is sent to the on-chain verifier contract.

The on-chain verifier contract performs two critical checks. First, it validates the cryptographic proof using a pre-compiled verification key. Second, it ensures the old state root in the proof matches its current stored root. If both checks pass, the contract updates its stored root to the new one provided in the proof. This update is the only transaction that needs to be published on the base chain, and it can represent a batch of hundreds of individual authentications. The contract can then emit an event containing a nullifier—a unique hash per authentication—which frontends can listen for to grant access, preventing double-spending of the same proof.

For developers, implementing this requires choosing a ZK proving system and writing the authentication circuit. Using the Circom language with the SnarkJS library is a common approach for custom circuits. A simple circuit would take the user's private input (secret and salt), the current Merkle root, and a path of siblings as private inputs. The public inputs would be the nullifier and the new Merkle root. The circuit logic would verify that the hash of the secret and salt matches the leaf in the tree, and that the new root is correctly computed. An alternative is using a zkVM, where you write the prover logic in Rust (for RISC Zero) and the guest program handles the Merkle tree verification, offering easier development but potentially less optimization.

Key design considerations include deciding on data availability—a validium for higher throughput or a zkRollup for higher security—and managing trust assumptions for the off-chain prover. Using a decentralized network of provers or allowing users to generate proofs locally with a client SDK can reduce centralization risks. Furthermore, the system must handle key management and recovery schemes, potentially using social recovery or multi-factor setups. This architecture is foundational for applications like private DAO voting, gated content access, and anonymous credential systems, moving beyond traditional Web2 OAuth to a user-owned, privacy-preserving standard.

The identity verification circuit is the cryptographic engine of a privacy-first authentication layer. Built using a ZK-SNARK framework like Circom or Halo2, its primary function is to generate a proof that a user's private input data satisfies a specific public verification condition. For example, a circuit can prove a user is over 18 by verifying a birthdate is before a certain timestamp, without disclosing the actual date. The circuit's public inputs (like the current date) and outputs (a boolean isVerified) are published on-chain, while all private witness data (the user's birthdate) remains off-chain and confidential.

Design begins by defining the precise claim to be proven. Common patterns include membership proofs (e.g., proving inclusion in a credential whitelist), range proofs (e.g., age or balance checks), and signature verification (proving ownership of a private key). The circuit code, written in a domain-specific language, encodes the logical constraints between these inputs. For instance, a simple age-check in a Circom template might look like:

codeCopy
template AgeCheck(n) {
    signal private input birthYear;
    signal input currentYear;
    signal output isAdult;

    // Constraint: currentYear - birthYear >= 18
    component gt = GreaterThan(n);
    gt.in[0] <== currentYear - birthYear;
    gt.in[1] <== 18;
    isAdult <== gt.out;
}

This creates a constraint system where a valid proof is only possible if the private birthYear is at least 18 years before the currentYear.

Optimizing the circuit is critical for performance and cost. The number of constraints directly impacts proof generation time and the gas cost of on-chain verification. Techniques include using lookup tables for static data, optimizing elliptic curve operations, and minimizing non-deterministic computations. The circuit must also be designed for determinism—the same inputs must always produce the same outputs—and should include safeguards against common vulnerabilities like underflow/overflow in arithmetic operations. Auditing the circuit logic is as important as auditing a smart contract.

Once designed, the circuit is compiled into an R1CS (Rank-1 Constraint System) or a similar intermediate representation, and a trusted setup ceremony generates the proving and verification keys. The proving key is used client-side to generate proofs, while the verification key is deployed to a smart contract on the ZK rollup. This separation ensures the verification logic is immutable and publicly accessible, enabling anyone to verify a proof's validity by checking it against the on-chain verification key and public inputs, completing the trustless authentication loop.

The off-chain proof sequencer is the private server-side component that handles the heavy computational lifting for your ZK-based authentication layer. Its primary responsibilities are to: receive authentication requests from your application's frontend, generate a zero-knowledge proof (ZKP) that validates the user's credentials against the on-chain policy without revealing them, and batch multiple proofs into a single transaction for efficient L1 settlement. This architecture ensures sensitive user data, like biometric templates or password hashes, never leaves your secure environment.

For development, you can build this sequencer using a framework like SnarkJS for Groth16 proofs or Circom for circuit design. A typical Node.js implementation involves setting up an Express.js server with endpoints to: POST /auth-request to receive a user's public commitment and witness data, POST /generate-proof to run the proving key against the witness using your ZK circuit, and a cron job that periodically calls POST /submit-batch to your L1 smart contract. The sequencer must securely manage the proving key and maintain a queue of pending proofs for batching.

A critical design decision is choosing your proof system and trusted setup. For authentication, Groth16 or PLONK are common choices due to their small proof sizes. You must run a Powers of Tau ceremony or use a pre-existing trusted setup for your circuit. The sequencer signs each batch submission with a private key, which your L1 Verifier contract will check. For high throughput, consider implementing proof generation in a separate worker pool using WebAssembly versions of your proving library to avoid blocking the main server thread.

Here is a simplified code snippet for the core proof generation endpoint using SnarkJS:

javascriptCopy
app.post('/generate-proof', async (req, res) => {
  const { witness } = req.body;
  try {
    const { proof, publicSignals } = await snarkjs.groth16.fullProve(
      witness,
      "./circuit.wasm",
      "./proving_key.zkey"
    );
    // Store proof in batch queue
    batchQueue.push({ proof, publicSignals });
    res.json({ status: 'proof_generated', proofId: uuidv4() });
  } catch (error) {
    res.status(500).json({ error: 'Proof generation failed' });
  }
});

Finally, the sequencer must be highly available and secure. Deploy it within a private VPC with strict firewall rules, only allowing connections from your frontend and your L1 RPC node. Use HTTPS with mutual TLS for all API calls. Monitor performance metrics like proof generation time and batch gas costs. The sequencer's operational integrity is essential, as downtime would block user logins. For production resilience, run multiple sequencer instances behind a load balancer with automated failover, ensuring private keys are managed via a hardware security module (HSM) or cloud KMS.

The on-chain verifier is the final, trust-minimized component of your architecture. Written in Solidity or a zkVM-compatible language like Cairo or Noir, this contract contains the verification logic for the specific zk-SNARK or zk-STARK circuit you designed. Its sole function is to accept a cryptographic proof and public inputs, then return a boolean result. For a login system, the public input is typically a hashed version of the user's identity (like an identityCommitment), while the proof cryptographically attests that the user knows the corresponding private credentials without revealing them.

Before deployment, you must compile the verification key from your circuit into a format the contract can use. Using a framework like snarkjs for Circom or the Starknet toolchain, you generate a verifier.sol contract. This auto-generated code includes a verifyProof function. For example, a basic function signature might be function verifyProof(uint[2] memory a, uint[2][2] memory b, uint[2] memory c, uint[1] memory input) public view returns (bool). The parameters a, b, and c represent the elliptic curve points of the zk-SNARK proof.

Deploy this contract to your target L1 or L2 network, such as Ethereum Mainnet, Arbitrum, or Polygon zkEVM. The choice involves a trade-off: L1 offers maximum security but higher cost per verification, while an L2 or specific zkRollup like zkSync Era or Starknet provides lower fees and can natively support your verifier. After deployment, record the contract address—it becomes the single source of truth for your application. All authentication requests will ultimately query this contract to validate proofs.

Integrate the deployed verifier with your off-chain prover service. Your backend authentication API, after receiving a proof from a client, must call the verifier contract's verifyProof function. Use a Web3 library like ethers.js or viem for this on-chain call. A successful verification confirms the user's identity claim is valid according to the circuit rules. This step completes the loop, creating a system where authentication is permissionless and relies on cryptographic truth rather than a trusted server's database.

The architecture you've implemented provides a robust foundation. User credentials are verified off-chain via a zkSNARK or zkSTARK proof, which is then posted to a rollup chain like zkSync Era or Starknet. The on-chain verifier contract only checks the proof's validity, never the underlying private data. This model offers data minimization, user sovereignty, and gas efficiency compared to storing credentials directly on-chain.

To extend this system, consider implementing advanced features. Account abstraction (ERC-4337) can create smart contract wallets that use your ZK proofs as a custom signature scheme. Social recovery mechanisms can be built using ZK proofs to verify guardians without exposing social graphs. For cross-chain applications, you can use a canonical verifier on Ethereum and relay proofs to other chains via LayerZero or Axelar for a unified identity layer.

Security must remain a primary focus. Regularly audit your circuit logic with tools like Circom's circomspect and the verifier contract. Implement a timelock or multi-sig for any upgrades to the verifier. Monitor for vulnerabilities in the underlying proving system, such as trusted setup ceremonies for SNARKs or soundness bugs in STARK arithmetization.

The next step is to integrate this authentication layer into a real application. Start by adding it to a decentralized application (dApp) requiring gated access, like a private DAO or a credential-based airdrop. Use The Graph to index successful verification events for user dashboards. Measure key metrics: proof generation time, verification gas cost, and user adoption rates to iterate on the design.

Finally, stay updated with evolving standards. The Ethereum Attestation Service (EAS) provides a schema registry that can be combined with ZK proofs for attestations. Polygon ID and Sismo are developing reusable protocol-level primitives for ZK credentials. Engaging with these communities and contributing to EIPs related to identity will help shape the future of private on-chain authentication.