"Designing and implementing malicious hardware"

[Leichter, Jerry]

On Sat, 26 Apr 2008, Karsten Nohl wrote:
| 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.
I suspect the only heavy-weight defense is the same one we use against
the "Trusting Trust" hook-in-the-compiler attack:  Cross-compile on
as many compilers from as many sources as you can, on the assumption
that not all compilers contain the same "hook".  For hardware, this
would mean running multiple chips in parallel checking each others
states/outputs.  Architectures like that have been built for
reliability (e.g., Stratus), but generally they assume identical
processors.  Whether you can actually build such a thing with
deliberately different processors is an open question.  While in
theory someone could introduce the same "spike" into Intel, AMD,
and VIA chips, an attacker with that kind of capability is probably
already reading your mind directly anyway.

Of course, you'd end up with a machine no faster than your slowest
chip, and you'd have to worry about the correctness of the glue
circuitry that compares the results.  *Maybe* the NSA would build
such things for very special uses.  Whether it would be cheaper for
them to just build their own chip fab isn't at all clear.  (One thing
mentioned in the paper is that there are only 30 plants in the world
that can build leading-edge chips today, and that it simply isn't
practical any more to build your own.  I think the important issue
here is "leading edge".  Yes, if you need the best performance, you
have few choices.  But a chip with 5-year-old technology is still
very powerful - more than powerful enough for many uses.  When it
comes to "obsolete" technology, you may have more choices - and
of course next year's "5 year old technology" will be even more
powerful.  Yes, 5 years from now, there will only be 30 or so
plants with 2008 technology - but the stuff needed to build such a
plant will be available used, or as cheap versions of newer stuff,
so building your own will be much more practical.)

							-- Jerry

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