key bit lengths

[Mark M.]

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On Sun, 14 Apr 1996, Jack Mott wrote:

> In Applied Crypto, it talks about thermodynamic limitations of brute
> force attacks.  I did some calculations and it looks like it will take,
> given a perfectly effecient computer, the combined energy of 509,485,193
> average supernovas to brute force a 256 bit key. I was just wondering if
> there are any theoretical ways around this. I am just talking about
> plain brute force here, not attacking other weaknesses.

I doubt it.  This calculation is based on the minimum amount of energy
needed to invert a bit.  The amount of energy is a function of the temperature,
so a brute force attack might take much less energy several billion years
hence, since the universe will cooled off more.  There only way for there to
be any way around this, is if a way was found to lower the termperature to
near absolute zero consuming a very little amount of energy, or if some way
was found to invert a bit using less energy than is currently believed (very
doubtfull).  Of course, if P=NP, then brute-force attacks will be pointless.

- -- Mark

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