Re: A _good_ and valid use for TPM

[Jan Alsenz]

Alex Besogonov wrote:
[skip]
>>> As far as I understand - no.
>> Actually - it is.
>> Check the "TCG PC Client Specific Implementation Specification for 
>> Conventional
>> Bios" or "TCG PC Specific Implementation Specification" at
>> https://www.trustedcomputinggroup.org/specs/PCClient/
>> and look for CRTM (Core Root of Trust for Measurement)
> Yes, BIOS is a root of trust, but not the Core Root. BIOS itself is
> checked before execution (pages 20 and onwards in the "TCG PC Client
> Specific Implementation Specification for Conventional Bios" spec),
> even before dynamic memory is initialized.
Well on page 32 they list two options, how to implement the CRTM:
BIOS Boot Block or entire BIOS
Since the BIOS is usually updateable, it seems that most manufacturers opt for
BIOS Boot Block, which I assume will be something like: "lets put the first
sector of the BIOS in ROM"
(of course it might be something else completely, but I doubt it)

>>> First, I don't think it's possible to implement SHA-1 hashing in MBR -
>>> there's probably just not enough space left in 512-byte code segment
>>> for that.
>> I am very sure of that.
> Well, I spoke phcoder on Jabber - there might be a way to do this.
> He's going to investigate it.
Sounds interesting.

>>> Second, the only safe action non TPM-aware MBR can perform if it
>>> detects tampering is just shutting down hard. Everything else is
>>> dangerous.
>> Yeah, but an attacker could patch that out too.
> Not if we first measure the MBR. It can be done without any
> TPM-specific code in the MBR if I'm not very mistaken.
Could you elaborate on that?
E.g. where do you measure the MBR from?

> PS: thanks for detailed explanation!
Sure, glad I could help!

Greets,

Jan

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