GPG's vulnerability to brute force [WAS: Re: GPG's vulnerability to quantum cryptography]

[Robert J. Hansen]

> I notice that the Wikipedia article refers here to "thermodynamically
> reversible" which is perhaps not the same thing as computationally
> reversible.  So I looked up "thermodynamically reversible" and found

At the level we're talking about, the distinction between  
thermodynamics and computational theory gets a little hazy.  Seriously  
-- we're talking about the deepest magics of math and physics, where  
in order to get peak efficiency from our CPUs we have to exploit the  
Bekenstein limits of the event horizons of black holes.  So, "perhaps  
not the same thing" is absolutely true.  Perhaps a lot more similar  
than we think.  At this level of theory, things get pretty wacky.

I studied quantum computation and quantum information theory some in  
grad school.  My takeaway from it was that by the standards of the  
field I am basically a dog who's learned to shake hands, sitting at a  
table listening to Deutsch and Witten and Susskind argue and thinking  
that I'm really smart just because I can recognize one word every few  
minutes.  Every now and again they look over my way, realize I'm  
paying attention, say "good boy!" and scratch my ears and I bark and  
think I'm making a real contribution to the discussion.

Yes, I am *that far* out of my depth here -- Scott Aronson I ain't.   
Please be careful about thinking I'm any kind of an authority here.

> which gives the interesting summary: thermodynamically reversible
> processes are theoretical and don't occur in the real world.

Yep.  Second Law again: entropy must always increase, meaning nothing  
is truly thermodynamically reversible.  But if adiabatic computing  
*did* exist, man, it would be cool -- as I said, if someone's able to  
demonstrate it I'll be deeply fascinated.  (And then I'll try to see  
if I can leverage it to travel back in time.  Because hey, once the  
Second Law no longer limits you, the world's your oyster.)

> That article seems confused as to whether a reversible process must be
> infinitely slow or infinitely fast, but Wikipedia says the former:
>
>   http://en.wikipedia.org/wiki/Reversible_process_%28thermodynamics%29

The closer you approach energy-free computing, the slower the process  
goes -- this is a consequence of several different things, including  
the Margolus-Levitin theorem, which says that a bit can't be flipped  
faster than h/4E seconds.  The less energy you put in, the slower the  
flip goes.