I use the createInsecureRfcommSocketToServiceRecord method from BluetoothDevice to pair devices without user confirmation.
So far I've assumed that the method uses the Just Works Protocol because it is the only one that does not require any user action. But this protocol was only introduced with Bluetooth 2.1, so I am wondering how this method works with pre Bluetooth 2.1 devices.
Anyway the documentation only says that the link key will not be encrypted but there is no hint how it is generated.