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Re: awareness + flexibility + security


From: Jonathan S. Shapiro
Subject: Re: awareness + flexibility + security
Date: Sun, 13 Nov 2005 18:41:20 -0500

On Sat, 2005-11-12 at 17:04 +0100, Michal Suchanek wrote:

> That attestaion function is not really that strong. There are a few
> registers for hashes, and these have to be actively filled by some
> software.
> So in case of GNU/Linux, the BIOS  would store its own checksum, the
> checksums of  option roms (if any), the checksum of the bootsector,
> and load it. Grub then would store its
> checksum, checksums of the stages it loads, the kernel, the initrd, and load 
> it.
> You can later read these checksums. But what do they prove?

First, you are missing a step: the checksum of grub is computed as it is
loaded by the boot sector.

Second: the cryptographic hash is computed **by the hardware**. You can
certainly compute something else, but the hardware won't believe it.

Third: once the hash is computed, the TC chip can be asked to generate a
signed message containing the hash. The TC chip will only sign this
message for hashes that it has computed. The signature is generated
using a key known only to the TC chip. The crypto involved is
asymmetric; the signature can be checked, but it cannot be
cost-effectively forged.

So the attestation itself is pretty strong, but there is no requirement
that the host OS ever asks the TC chip to sign anything. You can choose
not to respond, but if you do respond you cannot lie effectively.

Note that "choose not to respond" is a very feasible option for the
forseeable. There is no way for a remote party to tell the difference
between a legacy machine (that is not TC capable) and a TC capable
machine that declines to answer.

It is also possible for the OS to place the user in control of which
challenges will receive responses. For example, I might elect to have my
computer respond to challenges from my bank but not challenges from
RIAA.

shap





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