WeChat Pay Webhooks Skill

When to Use This Skill

• How do I receive WeChat Pay webhooks (APIv3 notifications)?
• How do I verify the Wechatpay-Signature header?
• How do I decrypt the encrypted resource in a WeChat Pay notification?
• How do I handle TRANSACTION.SUCCESS or REFUND.SUCCESS events?
• Why is my WeChat Pay signature verification failing?

How WeChat Pay Notifications Work

WeChat Pay APIv3 does not use HMAC or the Standard Webhooks spec. Each notification is:

1. Asymmetrically signed (SHA256withRSA) — verify with the WeChat Pay platform public key, matched by the Wechatpay-Serial header, over the message "{timestamp}\n{nonce}\n{body}\n".
2. Separately encrypted — the resource object is AEAD_AES_256_GCM ciphertext. Decrypt resource.ciphertext with your 32-byte APIv3 key to recover the transaction/refund JSON.

The signed body is the raw request bytes (the ciphertext envelope), so verify first, then decrypt. Always use the raw request body — never JSON.parse before verifying.

Verification (core)

const crypto = require('crypto');

// 0. Select the platform public key by Wechatpay-Serial. WeChat publishes new
//    certificates ~24h before signing with them, so an unpinned rotation must
//    fail with its own error, not a generic "invalid signature".
const PLATFORM_KEYS = JSON.parse(process.env.WECHAT_PAY_PLATFORM_KEYS || '{}');

function selectPlatformKey(serial) {
  const key = PLATFORM_KEYS[serial];
  if (!key) {
    throw new Error(
      `No platform key configured for serial ${serial} — ` +
      'fetch the current certs via GET /v3/certificates and add it'
    );
  }
  return key;
}

// 1. Verify the RSA-SHA256 signature over "{timestamp}\n{nonce}\n{body}\n"
function verifySignature(timestamp, nonce, rawBody, signatureB64, platformPublicKey) {
  const message = `${timestamp}\n${nonce}\n${rawBody}\n`;
  const verifier = crypto.createVerify('RSA-SHA256').update(message, 'utf8');
  try {
    return verifier.verify(platformPublicKey, signatureB64, 'base64');
  } catch {
    return false; // malformed key/signature
  }
}

// 2. Decrypt resource.ciphertext (AEAD_AES_256_GCM) with your 32-byte APIv3 key
function decryptResource({ ciphertext, nonce, associated_data }, apiV3Key) {
  const buf = Buffer.from(ciphertext, 'base64');
  const decipher = crypto.createDecipheriv('aes-256-gcm', apiV3Key, nonce);
  decipher.setAuthTag(buf.subarray(buf.length - 16));           // last 16 bytes = auth tag
  if (associated_data) decipher.setAAD(Buffer.from(associated_data));
  const plain = Buffer.concat([decipher.update(buf.subarray(0, -16)), decipher.final()]);
  return JSON.parse(plain.toString('utf8'));
}

Also reject notifications whose Wechatpay-Timestamp is more than 5 minutes from now (replay protection).

For complete handlers with route wiring, event dispatch, and tests, see:

• examples/express/
• examples/nextjs/
• examples/fastapi/

Common Event Types

This skill targets the Global (English) APIv3 endpoint, which defines only these three events. The mainland-China-only REFUND.ABNORMAL event is not part of the global endpoint.

Acknowledging Notifications

Respond with HTTP 200 or 204. A success body is optional, but the documented form is:

{ "code": "SUCCESS", "message": "OK" }

On any failure (bad signature, processing error) return a non-2xx status. WeChat Pay retries on a schedule (~15s, 15s, 30s, 3m, 10m, 20m, 30m … up to ~24h), so handle notifications idempotently and re-verify the order amount before fulfilling.

Environment Variables

# Recommended: platform public keys (PEM) keyed by certificate serial, as JSON
WECHAT_PAY_PLATFORM_KEYS='{"serial_a":"-----BEGIN PUBLIC KEY-----\n...\n-----END PUBLIC KEY-----"}'
# 32-character APIv3 key used to decrypt resource.ciphertext (AES-256-GCM)
WECHAT_PAY_API_V3_KEY=your_32_character_apiv3_key_here

# Single-key alternative — folded into the map above when both are set
WECHAT_PAY_PUBLIC_KEY="-----BEGIN PUBLIC KEY-----\n...\n-----END PUBLIC KEY-----"
WECHAT_PAY_PLATFORM_SERIAL=your_platform_cert_serial

The platform public key / certificate is downloaded and rotated by serial number via GET /v3/certificates (itself AES-GCM encrypted). WeChat publishes new certificates ~24h ahead of use, so a single pinned key rejects every notification the moment a rotation lands — key your store by Wechatpay-Serial and refresh it (e.g. every 12h). See references/setup.md.

Local Development

# Start tunnel (no account needed)
npx hookdeck-cli listen 3000 wechat --path /webhooks/wechat

Reference Materials

• references/overview.md - WeChat Pay webhook concepts, events, payload structure
• references/setup.md - notify_url configuration, APIv3 key, platform cert rotation
• references/verification.md - Signature verification and resource decryption details