Fix BLE ECDH handshake: wait for CCCD write before sending pubkey

BleRelayManager.kt: Android's gatt.writeCharacteristic() returns false when a
previous GATT operation (CCCD writeDescriptor) is still pending. Split
startEcdhHandshake() into two phases: CCCD setup (with ecdhCccdPending flag)
and sendEcdhHandshake() triggered from onDescriptorWrite callback.

main.cpp: After SET_ID over USB, update BLE advertising scan response
manufacturer data so Android can find the server by its new UUID prefix.

android/app/src/main/java/nl/eddydevink/sylda/BleRelayManager.kt

@@ -81,6 +81,7 @@ class BleRelayManager(
     private var ecdhDone = false
     @Volatile private var ecdhDeferred: CompletableDeferred<Boolean>? = null
     private var ecdhNotifyBuf: ByteArray = ByteArray(0)
+    private var ecdhCccdPending = false
 
     private class CompletableDeferred<T> {
         private var result: T? = null
@@ -314,6 +315,7 @@ class BleRelayManager(
                 ecdhCharacteristic = null
                 aesKey = null
                 ecdhDone = false
+                ecdhCccdPending = false
                 seq = 1
                 triedAddresses.clear()
                 handler.postDelayed(scanRunnable, RECONNECT_DELAY_MS)
@@ -413,6 +415,19 @@ class BleRelayManager(
                 Log.d(TAG, "ECDH notify received (${value.size} bytes)")
             }
         }
+
+        @SuppressLint("MissingPermission")
+        override fun onDescriptorWrite(gatt: BluetoothGatt, descriptor: BluetoothGattDescriptor, status: Int) {
+            if (ecdhCccdPending && descriptor.characteristic.uuid == ECDH_UUID) {
+                ecdhCccdPending = false
+                if (status == BluetoothGatt.GATT_SUCCESS) {
+                    sendEcdhHandshake(gatt)
+                } else {
+                    Log.w(TAG, "ECDH: CCCD write failed (status=$status)")
+                    gatt.disconnect()
+                }
+            }
+        }
     }
 
     @SuppressLint("MissingPermission")
@@ -425,18 +440,30 @@ class BleRelayManager(
             return
         }
 
+        val cccd = char.getDescriptor(OTA_CCCD_UUID)
+        if (cccd == null) {
+            Log.w(TAG, "ECDH: no CCCD descriptor")
+            gatt.disconnect()
+            return
+        }
+
         if (!gatt.setCharacteristicNotification(char, true)) {
             Log.w(TAG, "ECDH: setCharacteristicNotification failed")
             gatt.disconnect()
             return
         }
 
-        val cccd = char.getDescriptor(OTA_CCCD_UUID)
+        ecdhCccdPending = true
-        if (cccd != null) {
         cccd.value = BluetoothGattDescriptor.ENABLE_NOTIFICATION_VALUE
-            gatt.writeDescriptor(cccd)
+        if (!gatt.writeDescriptor(cccd)) {
+            ecdhCccdPending = false
+            Log.w(TAG, "ECDH: writeDescriptor for CCCD returned false")
+            gatt.disconnect()
+        }
     }
 
+    @SuppressLint("MissingPermission")
+    private fun sendEcdhHandshake(gatt: BluetoothGatt) {
         val myPsk = psk ?: run {
             Log.w(TAG, "ECDH: no PSK")
             gatt.disconnect()
@@ -462,9 +489,9 @@ class BleRelayManager(
             System.arraycopy(sig, 0, handshake, ECDH_PUBKEY_LEN, 32)
 
             ecdhDeferred = CompletableDeferred()
-            char.value = handshake
+            ecdhCharacteristic!!.value = handshake
-            char.writeType = BluetoothGattCharacteristic.WRITE_TYPE_NO_RESPONSE
+            ecdhCharacteristic!!.writeType = BluetoothGattCharacteristic.WRITE_TYPE_NO_RESPONSE
-            if (!gatt.writeCharacteristic(char)) {
+            if (!gatt.writeCharacteristic(ecdhCharacteristic!!)) {
                 Log.w(TAG, "ECDH: write pubkey failed")
                 gatt.disconnect()
                 return

src/main.cpp

@@ -617,6 +617,15 @@ void loop() {
                     preferences.end();
                     uuidProvisioned = true;
                     pUuidChar->setValue(deviceUuid, ESP_UUID_LEN);
+
+                    uint8_t mfgPayload[6];
+                    mfgPayload[0] = 0xFF;
+                    mfgPayload[1] = 0xFF;
+                    memcpy(mfgPayload + 2, deviceUuid, 4);
+                    NimBLEAdvertisementData scanResp;
+                    scanResp.setCompleteServices(NimBLEUUID(SERVICE_UUID));
+                    scanResp.setManufacturerData(std::string((char*)mfgPayload, 6));
+                    NimBLEDevice::getAdvertising()->setScanResponseData(scanResp);
                     Serial.println("OK");
                     Serial.println("UUID stored in NVS");
                 } else {