Security s***w - preventing disassembly - don't...

Question

The fan has come loose on the motor so I want to remove the bottom casing - there are three normal small posidrive machine screws and one tamper-proof s***w - like a slot head with a raised portion in the centre to prevent a normal s***w driver entering the slotsI want to find out the name of this type of s***w to order a suitable s***w driver - then the repair should be very simple