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

[Mark H. Wood]

On Wed, May 14, 2014 at 07:31:26PM -0400, Robert J. Hansen wrote:
> On 5/14/2014 6:11 PM, Leo Gaspard wrote:
[snip]
> > * You state it is a lower bound on the energy consumed/generated by 
> > bruteforcing. Having a closer look at the Wikipedia page, I just 
> > found this sentence: "If no information is erased, computation may
> > in principle be achieved which is thermodynamically reversible, and 
> > require no release of heat."
> 
> Yeah, adiabatic computing.  Give me a call as soon as we have an
> adiabatic computer: I'll be deeply fascinated.  Right now that's even
> more theoretical than quantum computing -- we've actually observed
> quantum computation in the lab on a small scale, while adiabatic
> computing is so far a complete no-go, AFAIK.
> 
> (Then again, it's been a few years since I've dived into the literature
> on it -- if you can find a paper demonstrating real-world adiabatic,
> energy- and entropy-free computing, I will be deeply fascinated.  I
> wasn't kidding about that.)
> 
> > information on each flipped bit. Actually, IIUC, flipping a bit is a
> >  reversible operation, and so the landauer principle does not apply.
> 
> Look!  A bit of information:  ___
> 
> That's what it was before.  Of course, it's now carrying the value '1'.
> So, tell me: you say bit flips are reversible, so what was the value
> before it was 1?  I promise, I generated these two bits with a fair coin
> (heads = 0, tails = 1).
> 
> "Reversible" means "we can recover previous state without guessing."
> Current computing systems are not reversible.

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

  http://www.brighthubengineering.com/thermodynamics/4616-what-are-reversible-and-irreversible-processes/

which gives the interesting summary: thermodynamically reversible
processes are theoretical and don't occur in the real world.  These
seem to live in the same realm with 100% frictionless surfaces and
insulation with infinite R-factor.

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

But I'm way, way out of my depth here so I'll shut up.

-- 
Mark H. Wood, Lead System Programmer   mwood at IUPUI.Edu
Machines should not be friendly.  Machines should be obedient.
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