# Security

#### Privacy Guarantees

**What Observers See**:

```
Transaction on Blockchain:
├─ Sender: <HIDDEN via ZK proof>
├─ Recipient: 0xMerchant... (public)
├─ Amount: <ENCRYPTED with ElGamal>
├─ Timestamp: 2024-10-29 12:34:56
└─ Proof: <Valid ZK proof data>
```

**What You See:**

```
Your Private View:
├─ Sender: You (+1234567890)
├─ Recipient: Merchant (+0987654321)
├─ Amount: 0.10 USDC (decrypted)
├─ Privacy: FULL (ZK + Encryption)
└─ Status: Confirmed ✓
```

#### Security Best Practices

1. **Never Expose Private Keys**

```typescript
// ❌ WRONG
const privateKey = "0x123abc...";
console.log(privateKey);

// ✓ CORRECT
const privateKey = process.env.PRIVATE_KEY;
// Never log or expose
```

2. **Always Verify Backend-Side**

```typescript
// ❌ WRONG - Client says payment is valid
if (req.body.paymentValid) {
  grantAccess();
}

// ✓ CORRECT - Verify with NumPay
const verified = await numpay.verifyPaymentToken(token);
if (verified) {
  grantAccess();
}
```

3. **Prevent Replay Attacks**

```typescript
// Store used payment tokens
const usedTokens = new Set();

if (usedTokens.has(paymentToken)) {
  throw new Error('Payment already processed');
}

usedTokens.add(paymentToken);
```

4. **Rate Limit Proof Generation**&#x20;

```typescript
// Client-side: Prevent spam proof generation
const lastProofTime = localStorage.getItem('lastProof');
if (Date.now() - lastProofTime < 10000) {
  throw new Error('Please wait before generating another proof');
}
```


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