Bluetooth Low Energy: listening for notifications/

Question

How is it possible to authenticate using Bluez 5.31 and GattTool?

I want to listen on a notification.

I've connected using gatttool -b <addr> -I and I can run the primary command etc.

I sniffed the device and I know that handle 0x0024 is where I will find a notification.

When I try char-write-req 0x0024 0100 I get Error: Characteristic write Request failed: Attribute requires authentication before read/write

So I try sec-level medium and then sec-level high but I still get the same error.

Any ideas?