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Re: TPM unusable for DRM

From: Colbus Emmanuel
Subject: Re: TPM unusable for DRM
Date: Thu, 9 Nov 2006 14:04:37 +0100

> 3. Local software can perform these operations, refusing to decrypt
> content unless an acceptable endorsement is provided by the TPM.

That's one of the points I never understood... So far I know, on x86,
there is no instruction which makes it possible to call the tpm device
from userspace.

Well, the software could use the IOPL field in the EFLAGS register to
check if its in/out instructions are not trapped by an aggressive
kernel, but a) a complete hardware emulator (bochs-like) could lie about
the content of eflags, and b) if the software has to be able to use
direct I/O, then it turns any software which requires TPM attestation
into a very big potential security hole.

So my first question is : how does the local software has to proceed, in
order to check that the endorsement key he got from the kernel was
really provided by the TPM?

My other question is : how does the software knows that this endorsement
key is acceptable? Well, if the software was installed by the hardware
provider, this would be no problem, but otherwise : a) if it had to
carry with itself any possible valid endorsement key value, or even only
their signed hash value, it would make it very easy to break one of
them, and then all the security would be defeated; and b) if it has to
rely on a remote attestation, since it can't call directly the network
card to ask the TPM-provider about the validity of this key (for the
very same reason it can't call the tpm device directly), how should he
behave in order to check that the answer he got wasn't simply faked?

Emmanuel Colbus

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