On Sun, Jun 17, 2012 at 1:34 PM, Sascha Cunz <sascha...@babbelbox.org> wrote: > > Given the fact that the keys in the BIOS must somehow get there and it must > also be able to update them (how to revoke or add keys else?).
Based on what I've read the keys are stored in flash. The flash module itself is protected. There are a number of ways to implement something like this fairly securely. The simplest is to have an unprotected area of the flash that the OS can write to. Upon bootup the firmware looks in this area for a signed message. If the message is signed by a trusted source, then the firmware interprets it as instructions to update itself, add/remove keys, or whatever. Then before booting the OS the firmware sets the protect flag on the protected area of flash that is only unset by a hardware reset. The only software exploit against something like this is to find a bug in the code that inspects the flash (overflow/etc) to trick it into running an unsigned blob. There are also hardware attacks, like bypassing flash protection hardware, directly accessing flash, or controlling what shows up on the data bus when the CPU tries to read the firmware. Any of these can be made fairly difficult, and extreme case being how modern gaming consoles work (flash embedded in CPU, and so on). > > Unless this is completely done in hardware, there must be a software doing it. > Software can - by design - be reverse engineered; in some countries even > legally without any further agreement or license. With the scheme above no software need be distributed that contains any information useful for anything other than a replay attack. The blob would be signed prior to distribution. You could read the code that loads it into the flash, but that need not be kept secret. You can load whatever you want into the flash and it won't matter unless it is signed. If the hardware is fancy enough you could even update settings without having to reboot, and again unless the hardware isn't done right you're not going to be able to get around it without tapping busses/etc. Rich
