I’ve spent a lot of time developing an operating system and working on my low level boot loader. But now I want to take some time off my operating system while not leaving the low-level environment and doing something involving security.
So I chose to build my own standard password utility following the pre-boot authentication scheme. Since I want the software to be at least a little portable I want it to use as little external support as is possible. I figured that I’d be best if I somehow managed to ‘hook’ into the bios somewhere between the self checks and the int 19 bootstrap from within a running real mode OS.
However finding information on how to modify the bios code proved to be impossible. I’ve found nothing on how to achieve the before mentioned. I have only found pages describing how to flash your bios.
Does anyone know how I can read/write bios code? Or can someone provide links to pages that describe this?
I know that it’s not only possible to brick my device but it is also likely, I’m aware of the risk and willing to take it.
Pinczakko’s articles on BIOS reverse engineering are a great place to start looking at this. There was also a book published by the same author but it is now out of print.
I’m not sure if this approach is the best approach towards a secure boot, but the articles on this site are very detailed and should point you towards a method for modifying your BIOS firmware.