Directly subscribe to characteristic that doesn't follow RFC spec?

[SleepyNinja0o]

I am having an issue with a Zengge BLE Mesh Light. I'm trying to subscribe to a characteristic that doesn't seem to follow the RFC spec. In order to enable notifications, you have to send byte 0x01 to the Notify UUID (00010203-0405-0607-0809-0a0b0c0d1911). Once this request is sent, you can see the Handle Value Notifications inside a Bluetooth packet capture. When I use the bluepy library, it works just fine but it seems Bleak sets up the notifications/callback in a different manner.

If I call start_notify in Bleak, I get an error message stating I can't write to the attribute although the attribute has Read/Write/Notify permissions. I have tried this in Windows and Linux.

Is there a way to override the start_notify method or something else in order to directly subscribe to a UUID?

Code:

def callback(sender: BleakGATTCharacteristic, data: bytearray):
    print(f"CALLBACK:   {sender}: {data}")


async def connect():
    async with BleakClient("08:65:F0:05:25:65",winrt=dict(use_cached_services=False)) as client:
        print(f"Connected: {client.is_connected}")
        for service in client.services:
            if service.uuid == "00010203-0405-0607-0809-0a0b0c0d1910":
                print(f"Service UUID: {service.uuid}")
                char = service.get_characteristic("00010203-0405-0607-0809-0a0b0c0d1911")
                print(f"Char: {char.uuid}")
                print(f"Char Prop: {char.properties}")
                await client.write_gatt_char(char, bytes([0x1]), True)
                await client.start_notify(char, callback)
                await asyncio.sleep(10)

asyncio.run(connect())

Output:

Service UUID: 00010203-0405-0607-0809-0a0b0c0d1910
Char: 00010203-0405-0607-0809-0a0b0c0d1911
Char Prop: ['read', 'write', 'notify']


2023-03-30 13:13:09,386 bleak.backends.winrt.client MainThread DEBUG: Disconnecting from BLE device...
2023-03-30 13:13:12,494 bleak.backends.winrt.client Dummy-2 DEBUG: session_status_changed_event_handler: id: <_bleak_winrt_Windows_Devices_Bluetooth.BluetoothDeviceId object at 0x000001EB3B72A970>, error: 0, status: 0
2023-03-30 13:13:12,494 bleak.backends.winrt.client MainThread DEBUG: closing requester
2023-03-30 13:13:12,510 bleak.backends.winrt.client MainThread DEBUG: closing session
Traceback (most recent call last):
  File "<stdin>", line 32, in connect
  File "C:\Users\Austin\AppData\Local\Programs\Python\Python311\Lib\site-packages\bleak\__init__.py", line 710, in start_notify
    await self._backend.start_notify(characteristic, wrapped_callback, **kwargs)
  File "C:\Users\Austin\AppData\Local\Programs\Python\Python311\Lib\site-packages\bleak\backends\winrt\client.py", line 946, in start_notify
    await winrt_char.write_client_characteristic_configuration_descriptor_async(
OSError: [WinError -2140864509] The attribute cannot be written

[dlech]

To work around this, I think we would need to add new APIs that separate registering value changed callbacks and enabling notifications.

[SleepyNinja0o]

That sounds like a great idea. I've searched everywhere and haven't seen anyone else with this type of issue.
I can control my Zengge lights otherwise (On/Off/Change Colors) but notifications would also be nice. Trying to integrate some BLE Mesh lights into Home Assistant.

[dlech]

haven't seen anyone else with this type of issue

Sounds like #972.

[SleepyNinja0o]

@dlech
This has been solved by 'cocoto'. After pairing with the Zengge device and obtaining the session key, he sent the command '0x01' with no data to the Notify UUID of the device USING AN ENCRYPTED PACKET and then called start_notify().

Here is a code example:

async def send_packet(self, command, data, dest=None, withResponse=True, attempt=0, uuid=UUID_CONTROL):
    """
    Args:
        command: The command, as a number.
        data: The parameters for the command, as bytes.
        dest: The destination mesh id, as a number. If None, this lightbulb's
            mesh id will be used.
    """
    assert (self.sk)
    if dest == None: dest = self.mesh_id
    packet = pckt.make_command_packet(self.sk, self.mac, dest, command, data)
    try:
        print(f'[{self.mesh_name}][{self.mac}] Writing command {command} data {repr(data)}')
        return await self.client.write_gatt_char(uuid, packet)
    except Exception as err:
        print(f'[{self.mesh_name}][{self.mac}] Command failed, attempt: {attempt} - [{type(err).__name__}] {err}')
        if attempt < 2:
            self.connect()
            return self.send_packet(command, data, dest, withResponse, attempt+1)
        else:
            self.sk = None
            raise err


async def enable_notify(self):
    await self.send_packet(0x01,bytes([]),self.mesh_id,uuid=UUID_NOTIFY)
    await self.client.start_notify(UUID_NOTIFY, self.notification_handler)

If anyone needs further assistance, they can check out my repo here for Zengge BLE mesh devices:
https://github.com/SleepyNinja0o/ZenggeBLEMeshBleak