midnight-dapp:proof-handling

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.

// 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:

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

Disclosure UX

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

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

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

References

Examples

Quick Start

1. Define Witness Types

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

2. Implement Witnesses

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

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

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

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

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

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

1. Never persist witness data - Keep private state in memory only
2. Clear sensitive data - Zero out arrays after use when possible
3. Validate all inputs - Check parameter validity in witnesses
4. Handle errors gracefully - Don't leak information in error messages
5. 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