Microsoft Wireless Comfort Desktop 5000 Keyboard special keys
Solution 1:
I think the kernel is the lowest rational layer.
The kernel will identify keys using scan codes which you can see in sudo showkey -s by pressing that key. The kernel also converts these keys to key codes which you can see in sudo showkey -k. If you want to remap some keys, you can alsodo it with the setkeycodes utility.
If you want to go even lower, wireshark can intercept USB traffic.