There is no sensible way to encrypt those on a budget
To keep costs down those systems have a simplex wireless connection. With that you have no chance of securely exchanging a key.
If you want moderate security for wireless keyboards use Blutooth. It may not be very secure, but it's a _lot_ more than secure than this... but then again that's probably not an option for Microsoft as not all versions of Windows support it. And unlike USB HIDs the BIOS will not emulate a normal AT keyboard for you.