Webhooks

const crypto = require('crypto');

/**
 * Verify webhook signature and timestamp
 * @param {string} payload - Raw request body as string
 * @param {string} signatureHeader - X-Hook0-Signature header value  
 * @param {string} secret - Secret from metadata.secret in subscription creation
 * @param {Object} headers - HTTP headers from webhook request
 * @param {number} maxAgeMinutes - Max age for webhook (default: 5 minutes)
 * @returns {boolean} true if webhook is authentic and within allowed time window
 */
function verifyWebhookSignature(payload, signatureHeader, secret, headers, maxAgeMinutes = 5) {
    try {
        // Parse signature header: t=timestamp,h=headers,v1=signature
        const elements = signatureHeader.split(',');
        const timestamp = elements.find(e => e.startsWith('t=')).split('=')[1];
        const headerNames = elements.find(e => e.startsWith('h=')).split('=')[1];
        const providedSignature = elements.find(e => e.startsWith('v1=')).split('=')[1];
        
        // Build header values string
        const headerNameList = headerNames.split(' ');
        const headerValues = headerNameList.map(name => headers[name] || '').join('.');
        
        // Build signed payload
        const signedPayload = `${timestamp}.${headerNames}.${headerValues}.${payload}`;
        
        // Compute expected signature
        const expectedSignature = crypto
            .createHmac('sha256', secret)
            .update(signedPayload, 'utf8')
            .digest('hex');
        
        // Compare signatures securely
        const signaturesMatch = crypto.timingSafeEqual(
            Buffer.from(expectedSignature, 'hex'),
            Buffer.from(providedSignature, 'hex')
        );
        
        // Verify timestamp to prevent replay attacks
        const webhookTime = parseInt(timestamp) * 1000; // Convert to milliseconds
        const currentTime = Date.now();
        const ageMinutes = (currentTime - webhookTime) / (1000 * 60);
        
        if (ageMinutes > maxAgeMinutes) {
            console.error(`Webhook timestamp exceeds maximum age: ${ageMinutes.toFixed(1)} minutes > ${maxAgeMinutes} minutes`);
            return false;
        }
        
        return signaturesMatch;
        
    } catch (error) {
        console.error('Webhook verification error:', error);
        return false;
    }
}

const express = require("express");
const app = express();

// Important: Get raw body for signature verification
app.use(express.raw({ type: "application/json" }));

app.post("/webhook", (req, res) => {
    const payload = req.body.toString(); // Raw string needed for signature
    const signature = req.headers["x-hook0-signature"];
    const secret = process.env.WEBHOOK_SECRET; // Store securely from metadata.secret
    
    if (verifyWebhookSignature(payload, signature, secret, req.headers)) {
        console.log("✅ Authentic webhook");
        
        // Parse and process the event
        const event = JSON.parse(payload);
        
        // Handle your transaction event here
        
        res.status(200).send("OK");
    } else {
        console.log("❌ Invalid webhook - rejected");
        res.status(400).send("Invalid signature");
    }
});