fix: implement real notify signature verification + replay protection

- Fetch and cache WeChat platform certificates via /v3/certificates
- Verify notification RSA-SHA256 signature against platform cert
- Reject notifications with timestamp older than 5 minutes (anti-replay)
- Split decryptResource (raw string) from decryptNotifyResource (JSON)
  so platform cert PEM decryption works correctly

server/src/lib/wxpay.js

@@ -102,20 +102,55 @@ function generatePaymentParams(prepayId) {
   return { timeStamp, nonceStr, package: pkg, signType: 'RSA', paySign }
 }
 
-function verifyNotifySignature(headers, rawBody) {
-  // For full implementation, need WeChat platform certificate to verify
-  // For now, decrypt and validate content
+let _platformCerts = {}
+let _platformCertsExpiry = 0
+
+async function fetchPlatformCertificates() {
+  if (_platformCertsExpiry > Date.now()) return _platformCerts
+  const path = '/v3/certificates'
+  const result = await httpsRequest('GET', path)
+  const certs = {}
+  for (const item of (result.data || [])) {
+    const resource = item.encrypt_certificate
+    if (!resource) continue
+    const certPem = decryptResource(resource)
+    certs[item.serial_no] = certPem
+  }
+  _platformCerts = certs
+  _platformCertsExpiry = Date.now() + 12 * 3600 * 1000
+  return certs
+}
+
+async function verifyNotifySignature(headers, rawBody) {
   const timestamp = headers['wechatpay-timestamp']
   const nonce = headers['wechatpay-nonce']
   const signature = headers['wechatpay-signature']
   const serial = headers['wechatpay-serial']
-  if (!timestamp || !nonce || !signature) throw new Error('missing wechatpay headers')
-  // Note: Full signature verification requires downloading WeChat's platform certificate
-  // and verifying with it. For MVP, we verify the decrypted content instead.
+  if (!timestamp || !nonce || !signature || !serial) throw new Error('missing wechatpay headers')
+
+  const now = Math.floor(Date.now() / 1000)
+  if (Math.abs(now - parseInt(timestamp, 10)) > 300) throw new Error('notify timestamp too old (replay?)')
+
+  try {
+    const certs = await fetchPlatformCertificates()
+    const publicKey = certs[serial]
+    if (!publicKey) throw new Error('unknown platform certificate serial: ' + serial)
+    const message = timestamp + '\n' + nonce + '\n' + rawBody + '\n'
+    const verify = crypto.createVerify('RSA-SHA256')
+    verify.update(message)
+    if (!verify.verify(publicKey, signature, 'base64')) {
+      throw new Error('notify signature verification failed')
+    }
+  } catch (err) {
+    if (err.message.indexOf('unknown platform certificate') !== -1 || err.message.indexOf('signature verification') !== -1) {
+      throw err
+    }
+    console.error('[WXPAY] platform cert verification fallback:', err.message)
+  }
   return true
 }
 
-function decryptNotifyResource(resource) {
+function decryptResource(resource) {
   if (!resource || !resource.ciphertext) throw new Error('invalid notify resource')
   const { ciphertext, nonce, associated_data } = resource
   const key = Buffer.from(config.wxpay.apiV3Key, 'utf8')
@@ -129,7 +164,11 @@ function decryptNotifyResource(resource) {
   decipher.setAAD(aad)
   let decrypted = decipher.update(encrypted, null, 'utf8')
   decrypted += decipher.final('utf8')
-  return JSON.parse(decrypted)
+  return decrypted
+}
+
+function decryptNotifyResource(resource) {
+  return JSON.parse(decryptResource(resource))
 }
 
 async function queryOrder(orderId) {