Re: Timing Cryptanalysis Attack

[Tom Weinstein <tomw@netscape.com>]

Perry E. Metzger wrote:
> 
> Tom Weinstein writes:
>
> > The problem with that approach is that if the system is heavily
> > loaded, it can take an arbitrarily large amount of user time.
> 
> Totally untrue. The process can take an arbitrary amount of wall clock
> time, not user time.

Whoops.  You are absolutely correct.  Pardon my brain-damage.  I
was thinking wall clock time, as you indicated.

> > Somewhat better is to sleep for a random amount of time after you're
> > done.
> 
> I don't think so. First of all, you can still extract some
> information. If you have been gone as long as the maximum computation
> plus the maximum random fudge, you know that you had to have conducted
> the maximum computation. This means that some bits are indeed
> leaking. Your approach also has the disadvantage that it is hard to
> produce good random numbers -- you are perhaps familiar with that
> problem?

Yes, you are correct.  It's better than taking a fixed amount of wall
clock time, but definitely not better than a fixed amount of user
time.

As Paul mentions in his extended abstract, there is actually an easy way
to fix the problem without hurting either latency or throughput much. 
If you blind and and unblind around the modular exponentiation, it
appears impossible to perform this attack.  Because you don't know the
inputs to the exponentiation operation, you can't make any predictions
based on those inputs.

-- 
Sure we spend a lot of money, but that doesn't mean | Tom Weinstein
we *do* anything.  --  Washington DC motto          | tomw@netscape.com