# Webhook signature verification

The payment notification arrives at an address only your store and our service know. But the address can leak—from logs, from configuration, from development history. To tell a genuine notification from a forgery, every request is signed with the webhook secret.

Verify the signature before fulfilling the order or changing its status.

## Signature headers

| Header        | Value                                            |
| ------------- | ------------------------------------------------ |
| `X-Timestamp` | Send time, unix time in seconds                  |
| `X-Signature` | The `sha256=` prefix and the HMAC-SHA256 in hex  |

The signed string is the send time, a dot, and the request body:

```
1756901234.{"wallet":{...},"project":{...},"invoice":{...},"payment":{...}}
```

The signing key is the Webhook secret field in the project settings. You can reissue it there if the value is compromised.

## Verification steps

1. Take the raw request body, before parsing the JSON.
2. Concatenate the `X-Timestamp` value, a dot, and the body.
3. Compute the HMAC-SHA256 with the webhook secret.
4. Compare the result with `X-Signature` using a constant-time comparison.
5. Reject the request if `X-Timestamp` deviates too far from the current time.

## Implementation examples

**PHP**

```php showLineNumbers
<?php
$secret = 'webhook secret from your project settings';

$body      = file_get_contents('php://input');
$timestamp = $_SERVER['HTTP_X_TIMESTAMP'] ?? '';
$signature = $_SERVER['HTTP_X_SIGNATURE'] ?? '';

// Guards against a replayed request
if (abs(time() - (int)$timestamp) > 300) {
    http_response_code(400);
    exit;
}

$expected = 'sha256='.hash_hmac('sha256', $timestamp.'.'.$body, $secret);

// hash_equals compares in constant time
if (!hash_equals($expected, $signature)) {
    http_response_code(403);
    exit;
}

$data = json_decode($body, true);

// Handle the order

http_response_code(200);
```

**TypeScript**

```typescript showLineNumbers
import { createHmac, timingSafeEqual } from 'node:crypto';
import express, { type Request, type Response } from 'express';

const app = express();
const secret = 'webhook secret from your project settings';

// The body must stay raw, so express.raw instead of express.json
app.post('/webhook', express.raw({ type: 'application/json' }), (req: Request, res: Response) => {
    const timestamp = req.get('X-Timestamp') ?? '';
    const signature = req.get('X-Signature') ?? '';
    const body = (req.body as Buffer).toString('utf8');

    if (Math.abs(Date.now() / 1000 - Number(timestamp)) > 300) {
        return res.sendStatus(400);
    }

    const expected = 'sha256=' + createHmac('sha256', secret)
        .update(`${timestamp}.${body}`, 'utf8')
        .digest('hex');

    const a = Buffer.from(expected);
    const b = Buffer.from(signature);

    // Check the length first: timingSafeEqual needs buffers of equal size
    if (a.length !== b.length || !timingSafeEqual(a, b)) {
        return res.sendStatus(403);
    }

    const data = JSON.parse(body);

    // Handle the order

    res.sendStatus(200);
});
```

**JavaScript**

The same handler without types—for a plain JavaScript project.

```js showLineNumbers
import { createHmac, timingSafeEqual } from 'node:crypto';
import express from 'express';

const app = express();
const secret = 'webhook secret from your project settings';

// The body must stay raw, so express.raw instead of express.json
app.post('/webhook', express.raw({ type: 'application/json' }), (req, res) => {
    const timestamp = req.get('X-Timestamp') ?? '';
    const signature = req.get('X-Signature') ?? '';
    const body = req.body.toString('utf8');

    if (Math.abs(Date.now() / 1000 - Number(timestamp)) > 300) {
        return res.sendStatus(400);
    }

    const expected = 'sha256=' + createHmac('sha256', secret)
        .update(`${timestamp}.${body}`, 'utf8')
        .digest('hex');

    const a = Buffer.from(expected);
    const b = Buffer.from(signature);

    // Check the length first: timingSafeEqual needs buffers of equal size
    if (a.length !== b.length || !timingSafeEqual(a, b)) {
        return res.sendStatus(403);
    }

    const data = JSON.parse(body);

    // Handle the order

    res.sendStatus(200);
});
```

**Python**

```python showLineNumbers
import hmac
import hashlib
import time

from flask import Flask, request

app = Flask(__name__)
secret = 'webhook secret from your project settings'

@app.post('/webhook')
def webhook():
    timestamp = request.headers.get('X-Timestamp', '')
    signature = request.headers.get('X-Signature', '')
    body = request.get_data(as_text=True)

    if abs(time.time() - int(timestamp or 0)) > 300:
        return '', 400

    expected = 'sha256=' + hmac.new(
        secret.encode(),
        f'{timestamp}.{body}'.encode(),
        hashlib.sha256,
    ).hexdigest()

    if not hmac.compare_digest(expected, signature):
        return '', 403

    data = request.get_json()

    # Handle the order

    return '', 200
```

## Things to keep in mind

**The raw body is signed.** If you parse the JSON and rebuild it, the key order and spacing change—the signature will not match. Compute the HMAC before parsing.

**Every retry has its own signature.** The send time is new on each retry, so the signature differs too. Do not store a signature to compare against future requests.

**The secret can be reissued.** The old secret stops working immediately after reissue, so update the value in your store at the same time.

**Signature verification does not replace idempotency.** The signature proves the request is authentic, not that you are seeing it for the first time. A retry arrives with a valid signature—check against `invoice.id`.