Re: Can a program prove it's own integrity?

From: AE (hidden_at_nospam.com)
Date: 05/20/04 

Date: Thu, 20 May 2004 10:09:12 +0200

sounds good to me :-)

It's a slow and fairly complex procedure, but since it's done only in
case the boot floppy is broken and has to be replaced it can be done.

Kiuhnm wrote:

> AE wrote:
>
>>This is where the idea comes from:
>>
>>I wanted to protect a complete harddisk - encrypting disk driver,
>>modified bootloader to run the operating system from the encrypted
>>disk.
>>
>>The weak point is the bootloader: If it is changed an attacker can
>>read the password as I'm typing it and all security is gone.
>
>
> Write a self-decrypting pre-bootloader that, in order to work properly and
> eventually decrypt the real bootloader on the hard disk, needs some random
> data.
> Put the pre-bootloader and the random (or compressed cryptographic) data on
> a floppy disk. Your floppy disk must be *full* and the data nearly
> incompressible (just use a state-of-the-art compressor).
> The encrypted part of the pre-bootloader, once decrypted and executed, must
> enter the highest privilege level (e.g. Ring 0), use time-stamp instructions
> and execute *many* difficult-to-emulate operation (the code could test the
> "protected mode" functionalities..., execute pmode BIOS extension, etc...).
> The code will also check the floppy disk hash and print "expected secret
> strings" on the screen.
>
> You could use a low-density floppy disk (it's more efficient because you
> have less data).
>
> You should:
> 1) (physically) disconnect the hard disk,
> 2) (physically) write-protect the floppy and insert it,
> 3) turn on your computer,
> 4) insert the password,
> 5) wait :-)
> 6) Examine the screen and decide if the floppy disk is genuine.
> If you think it's genuine:
> 7) turn off your computer,
> 8) reconnect the hard disk,
> 9) turn on your computer,
> 10) etc...
>
> Notes:
> - your hard disk must be disconnected or the attacker could copy partial
> data of the floppy to the hard disk (even if it's encrypted);
> - your pre-bootloader should even check hashes of your BIOS, etc...
> - Once your software enters Ring 0, no one can completely control it. One
> can only run that program in a virtual machine and still it's very difficult
> to emulate perfectly all the functionalities and handle accurately all the
> time-stamp instructions. Anyhow, a virtual machine requires a lot of code
> and it's practically impossible to compress the data in the floppy in order
> to create enough space for the virtual machine.
> - When you find out that the floppy is compromised, your password could have
> just been saved somewhere in the BIOS, etc...
> This is not a problem: just create another disk and use another password.
> Your true bootloader can't be revealed while you execute the pre-bootloader
> with the hard disk disconnected.
>
> Kiuhnm
>
>

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