Hacking a Computer Keyboard?

[PICMICRO]

I need a keyboard for my Electronic Password Door System. Instead of making my own awkward looking keyboard by soldering a button array, I have decided to use numeric computer keyboard. Like this.
**broken link removed**
The problem is all of them come with USB interface and not PS/2.
So I ripped open it in the hope that I can fit my PIC inside it and implement matrix button reading sequence.
There are 20 keys, but the controller there seems to be decoding all of them using only 8 lines.
I thought, the maximum no. of keys that you can decode by using (4+4 = 8) lines is just 4X4 = 16 keys.

So, certainly there is something I am missing regarding reading button arrays. What might be it?
I also need to decode the keys using 8 lines.

 

[binzer]

How about this: **broken link removed**
or: https://www.digole.com/index.php?productID=397

 

[PICMICRO]

Nice keypads. Thanks, binzer.
In the mean time I further Ripped the numeric keypad, and saw familiar 5X4 matrix arrangement. So, it seems 9 lines (5+4) are being used, 1 line being hidden from my sight.