Proof Handling

Build and present zero-knowledge proofs in your Midnight DApp, from constructing witnesses to showing proof generation progress and handling privacy disclosures.

When to Use

•Building witness data for contract transactions
•Showing proof generation progress to users
•Implementing disclosure consent flows
•Handling proof timeouts and retries
•Explaining privacy guarantees to users

Key Concepts

Witness Construction

Witnesses are TypeScript functions that provide private data to ZK circuits. The data stays local - only the cryptographic proof goes on-chain.

typescript

// Witness provides private input - circuit proves you know it
const witnesses = {
  get_secret: ({ privateState }: WitnessContext<PrivateState>): bigint => {
    return privateState.secret; // Never leaves the client
  }
};

Client-Side Proofs

All proof generation happens locally on the user's device:

•Never on remote servers - Private data never leaves the browser
•Proof server runs locally - Docker container on port 6300
•Takes seconds, not milliseconds - Show progress UI to users
•Can timeout - Implement retry logic for reliability

Disclosure UX

When circuits use disclose(), users must understand what they're revealing:

typescript

// This reveals the exact value on-chain
const disclosedAge = disclose(userAge);

// User should see: "This will reveal your age (25) publicly"

References

Document	Description
witness-fundamentals.md	Witness patterns and type mapping
client-side-proofs.md	Proof server setup and performance
disclosure-ux.md	Privacy disclosure UI patterns
web3-comparison.md	Ethereum signing vs Midnight proofs

Examples

Example	Description
witness-builder/	Functions to construct witness objects
proof-progress/	Proof generation progress component
disclosure-modal/	Privacy disclosure consent modal

Quick Start

1. Define Witness Types

typescript

interface PrivateState {
  secretKey: Uint8Array;
  credentials: Map<string, Credential>;
  balance: bigint;
}

2. Implement Witnesses

typescript

import type { WitnessContext } from "@midnight-ntwrk/midnight-js-types";

const witnesses = {
  get_balance: ({ privateState }: WitnessContext<PrivateState>): bigint => {
    return privateState.balance;
  },

  get_credential: (
    { privateState }: WitnessContext<PrivateState>,
    credentialId: Uint8Array
  ): Credential => {
    const id = bytesToHex(credentialId);
    const credential = privateState.credentials.get(id);
    if (!credential) {
      throw new WitnessError("Credential not found", "NOT_FOUND");
    }
    return credential;
  }
};

3. Show Proof Progress

typescript

function TransferButton({ onTransfer }) {
  const { status, progress, error, generateProof } = useProofStatus();

  const handleClick = async () => {
    const result = await generateProof(async () => {
      return contract.callTx.transfer(recipient, amount, witnesses);
    });

    if (result.success) {
      onTransfer(result.transaction);
    }
  };

  return (
    <div>
      <button onClick={handleClick} disabled={status === "generating"}>
        {status === "generating" ? "Generating Proof..." : "Transfer"}
      </button>
      {status === "generating" && <ProgressBar value={progress} />}
      {error && <ErrorMessage error={error} />}
    </div>
  );
}

4. Handle Disclosure Consent

typescript

function DisclosureFlow({ disclosures, onConfirm, onCancel }) {
  return (
    <DisclosureModal
      disclosures={disclosures}
      onConfirm={onConfirm}
      onCancel={onCancel}
    >
      <p>This transaction will reveal the following information:</p>
      <ul>
        {disclosures.map((d) => (
          <li key={d.field}>
            <strong>{d.label}</strong>: {d.value}
          </li>
        ))}
      </ul>
    </DisclosureModal>
  );
}

Common Patterns

Witness with Validation

typescript

const witnesses = {
  get_credential: (
    { privateState }: WitnessContext<PrivateState>,
    credentialId: Uint8Array
  ): Credential => {
    const credential = privateState.credentials.get(bytesToHex(credentialId));

    if (!credential) {
      throw new WitnessError("Credential not found", "NOT_FOUND");
    }

    // Validate before returning
    if (credential.expiry < BigInt(Date.now())) {
      throw new WitnessError("Credential expired", "EXPIRED");
    }

    return credential;
  }
};

Async Witness for External Data

typescript

const witnesses = {
  get_oracle_price: async (
    { privateState }: WitnessContext<PrivateState>,
    tokenId: Uint8Array
  ): Promise<bigint> => {
    const response = await fetch(`/api/price/${bytesToHex(tokenId)}`);
    if (!response.ok) {
      throw new WitnessError("Price unavailable", "ORACLE_ERROR");
    }
    const { price } = await response.json();
    return BigInt(price);
  }
};

Proof Retry Logic

typescript

async function submitWithRetry(
  buildTx: () => Promise<Transaction>,
  maxRetries = 3
): Promise<string> {
  let lastError: Error | null = null;

  for (let attempt = 1; attempt <= maxRetries; attempt++) {
    try {
      const tx = await buildTx();
      const provenTx = await walletAPI.balanceAndProveTransaction(tx, newCoins);
      return await walletAPI.submitTransaction(provenTx);
    } catch (error) {
      lastError = error as Error;

      // Don't retry user rejection
      if (error.message.includes("rejected")) {
        throw error;
      }

      // Wait before retry with exponential backoff
      if (attempt < maxRetries) {
        await new Promise((r) => setTimeout(r, 1000 * Math.pow(2, attempt)));
      }
    }
  }

  throw lastError ?? new Error("Proof generation failed");
}

Security Considerations

•Never persist witness data - Keep private state in memory only
•Clear sensitive data - Zero out arrays after use when possible
•Validate all inputs - Check parameter validity in witnesses
•Handle errors gracefully - Don't leak information in error messages
•User consent for disclosures - Always show what will be revealed

Related Skills

•wallet-integration - Wallet connection required before proof generation
•state-management - Managing private state for witnesses
•transaction-flows - Submitting proven transactions
•error-handling - Proof error messages and recovery

Related Commands

•/dapp-check - Validates proof server configuration
•/dapp-debug proofs - Diagnose proof generation issues