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Re: Broken dream of mine :(

From: Michal Suchanek
Subject: Re: Broken dream of mine :(
Date: Tue, 22 Sep 2009 00:42:17 +0200

2009/9/22 Sam Mason <address@hidden>:
> On Mon, Sep 21, 2009 at 05:32:07PM +0200, Michal Suchanek wrote:
>> 2009/9/21 Sam Mason <address@hidden>:
>> > Yup, I wasn't trying to protect against the admin.  Just noting that it
>> > will help to tell them when things are getting out of date.
>> You can send them an email or show a warning message on the terminal
>> until they upgrade without any need for TPM.
> Yes, the machines could be blocked from the network and the admin
> emailed.
>> > But you can't be sure that a remote attacker hasn't put a rootkit in
>> > somewhere.  AFAIU, TPM should allow you to detect this.
>> As should any other comparison with previous checksums which can be,
>> for example, stored on a readonly boot media together with a
>> bootloader that checks them.
> There are a boatload of attacks against this aren't there?  If it's
> just a checksum, the attacker can remember the checksum from before and
> send it back every time.  It could also simulate the entire machine and
> insert bad data when it wants.

Yes, the trivial attack is to replace the boot medium but that's local
access attack, not remote attack. The remote attacker cannot overwrite
it, and the boot loader and initial environment can enforce any policy
you wish (get the list of valid checksums from your server using SSL
for example). So decide against which attack you are trying to defend.

>> > I'd agree, I'm struggling to think of any use cases outside of high
>> > assurance that would want anything to do with TPM.
>> It doesn't give high assurance. It only gives assurance in combination
>> with physical security in which case it is just one of many options,
>> and not particularly appealing.
> Physical security does nothing about remote/software attacks though.

Yes, and TPM does no more for remote attacks than a boot CD and a BIOS
with flash protection.

>> You have to rely on the TPM
>> manufacturer quite a bit because the devices come as blackboxes with
>> unknown internals.
> Yes, but they're implementing a public spec and the economic incentives
> all seem to be pointing the right way with this.  If the manufacturer
> screws up their implementation they're going to look bad to the people
> who matter.

But it will break your system.

>> Then netboot the machines. No need for reimaging and users staring at
>> broken machines.
> An attacker can modify the bios so that it points to somewhere it
> controls.  Again, this isn't for normal PCs.

Attacker from where? If it's the user you cannot allow users near the
PC. If it's remote then this can be prevented with and without TPM.



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