Build Privacy-focused Web Apps with Zero-knowledge Proofs: A Comprehensive Guide
Tags: privacy, webdev, ethereum
Published: 2023-05-15 16:34:16

## How to Develop a Privacy-focused Web App Using Zero-knowledge Proofs

In today's data-driven world, protecting user privacy has become increasingly crucial. As web developers, we need to ensure that the applications we create are secure and maintain the confidentiality of users' data. One technique that has gained traction in recent years for achieving this is the use of Zero-knowledge Proofs (ZKPs). In this blog post, we will explore what ZKPs are, how they work, and how to develop a privacy-focused web app using ZKPs, complete with example code and real-life use cases.

### What are Zero-knowledge Proofs?

Zero-knowledge Proofs are cryptographic protocols that allow one party (the prover) to prove to another party (the verifier) that they possess specific knowledge or information without revealing any details about that information. In other words, the verifier can be convinced of the truthfulness of the claim without learning anything about the underlying data. This concept is essential for maintaining privacy in applications that deal with sensitive information.

### How Zero-knowledge Proofs Work

Zero-knowledge Proofs can be achieved through various cryptographic techniques, such as interactive proofs, non-interactive proofs, and zk-SNARKs (Zero-knowledge Succinct Non-Interactive Argument of Knowledge). We will focus on zk-SNARKs as they are more efficient and practical for web applications.

zk-SNARKs work by transforming a statement into a mathematical equation that can be proven true or false without revealing the actual values involved. To implement zk-SNARKs, we need to use a specific cryptographic library, such as [libsnark](https://github.com/scipr-lab/libsnark) or [ZoKrates](https://github.com/Zokrates/ZoKrates).

Developing a Privacy-focused Web App Using Zero-knowledge Proofs

Step 1: Install and Set up ZoKrates

For this tutorial, we will use ZoKrates, a toolbox for zk-SNARKs on Ethereum. First, install ZoKrates by following the instructions in the [official documentation](https://zokrates.github.io/gettingstarted.html).

Step 2: Define Your Zero-knowledge Proof

Let's create a simple example to demonstrate the use of Zero-knowledge Proofs. We will prove that we know the pre-image of a hash without revealing the pre-image itself.

Create a file called `proof.zok` and write the following code:

```
import "hashes/sha256/512bit" as sha256

def main(private field a, private field b) -> (field[2]):
  h = sha256([a, b])
  return h
```


This code defines a simple zk-SNARK that takes two private inputs (`a` and `b`) and returns the hash as the public output.

Step 3: Compile the Zero-knowledge Proof

Compile the `proof.zok` file using ZoKrates:

```
zokrates compile -i proof.zok
```


This command generates a set of files, including `out`, `proving.key`, `verification.key`, and `variables.inf`. These files are essential for generating and verifying proofs.

Step 4: Generate the Proof

Now that we have compiled our proof, we can generate a specific instance of the proof by providing the private inputs:

```
zokrates compute-witness -a <input_a> <input_b>

```

Replace `<input_a>` and `<input_b>` with your chosen values. This command generates a `witness` file that contains the public output of the zk-SNARK (the hash).

Next, generate the actual proof:

```
zokrates generate-proof

```

This command creates a `proof.json` file, which contains the proof for the given inputs.

Step 5: Verify the Proof

The verifier can now check the validity of the proof without knowing the actual input values. To do this, use the following command:

```
zokrates verify

```

If the verification is successful, you should see an output indicating that the proof is valid.

Step 6: Integrating Zero-knowledge Proofs in Your Web App

Now that we have a working zk-SNARK, let's integrate it into a web application. We will use a simple web server (e.g., Node.js with Express) and a frontend (e.g., React or Vue.js).

On the server side, implement an API endpoint that receives the proof and verifies it using ZoKrates. For example, you could have a `/verify` endpoint that accepts a JSON object containing the proof:

```
const express = require('express');
const app = express();

app.post('/verify', (req, res) => {
  const proof = req.body.proof;

  // Call ZoKrates CLI to verify the proof
  // ...

  // Send the verification result back to the client
  res.json({ success: true, message: 'Proof is valid' });
});

app.listen(3000, () => {
  console.log('Server is running on port 3000');
});
```


On the frontend, create a form that allows users to input their private data and generate a proof using ZoKrates. Once the proof is generated, send it to the server for verification:

```
async function handleSubmit() {
  // Generate the proof using ZoKrates
  // ...

  // Send the proof to the server for verification
  const response = await fetch('/verify', {
    method: 'POST',
    headers: { 'Content-Type': 'application/json' },
    body: JSON.stringify({ proof: proof }),
  });

  const data = await response.json();
  if (data.success) {
    alert(data.message);
  } else {
    alert('Proof is invalid');
  }
}
```


That's it! You now have a privacy-focused web app that leverages Zero-knowledge Proofs to ensure user data confidentiality.

### Real-life Use Cases and Examples

1. **Privacy-focused voting systems**: Using zk-SNARKs, you can build a voting system that allows users to prove they have the right to vote without revealing their identity.

2. **Secure authentication**: ZKPs can be used to create authentication systems that do not require users to reveal their passwords or other sensitive information.

3. **Private transactions in cryptocurrencies**: Cryptocurrencies like Zcash use zk-SNARKs to enable private transactions, allowing users to transact without revealing the transaction details.

In conclusion, Zero-knowledge Proofs provide a powerful mechanism to maintain privacy in web applications. By understanding and utilizing ZKPs, developers can build more secure and privacy-focused applications that protect users' sensitive data from unauthorized access.

---

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