Jacob Appelbaum wrote:
Perry E. Metzger wrote:
A pretty scary paper from the Usenix LEET conference:

http://www.usenix.org/event/leet08/tech/full_papers/king/king_html/

The paper describes how, by adding a very small number of gates to a
microprocessor design (small enough that it would be hard to notice
them), you can create a machine that is almost impossible to defend
against an attacker who possesses a bit of secret knowledge. I suggest
reading it -- I won't do it justice with a small summary.

It is about the most frightening thing I've seen in years -- I have no
idea how one might defend against it.


"Silicon has no secrets."

I spent last weekend in Seattle and Bunnie (of XBox hacking fame/Chumby)
gave a workshop with Karsten Nohl (who recently cracked MiFare).

In a matter of an hour, all of the students were able to take a
selection of a chip (from an OK photograph) and walk through the
transistor layout to describe the gate configuration. I was surprised
(not being an EE person by training) at how easy it can be to understand
production hardware. Debug pads, automated masking, etc. Karsten has
written a set of MatLab extensions that he used to automatically
describe the circuits of the mifare devices. Automation is key though, I
think doing it by hand is the path of madness.

If we could convince (this is the hard part) companies to publish what
they think their chips should look like, we'd have a starting point.

Perhaps,
Jacob

Silicon has no secrets, indeed. But it's also much too complex for exhaustive functionality tests; in particular if the tests are open ended as they need to be when hunting for backdoors.

While a single chip designer will perhaps not have the authority needed to significantly alter functionality, a small team of designers could very well adopt "their" part of a design and introduce a backdoor.

Hardware designs currently move away from what in software would be open source. Chip obfuscation meant to protect IP combined with the ever increasing size of chips makes it almost impossible to reverse-engineer an entire chip.

Bunnie pointed out that the secret debugging features of current processors perhaps already include functionality that breaks process separation. The fact that these features stay secret suggest that it is in fact hard to detect any undocumented functionality.

Assuming that hardware backdoors can be build, the interesting question becomes how to defeat against them. Even after a particular triggering string is identified, it is not clear whether software can be used to detect malicious programs. It almost appears as if the processor would need a hardware-based virus-scanner or sorts. This scanner could be simple as it only has to match known signatures, but would need have access to a large number of internal data structures while being developed by a completely separate team of designers.

        -Karsten

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