Messages in this thread | | | Date | Sun, 17 Aug 2003 22:25:38 -0600 | From | Val Henson <> | Subject | Re: [RFC][PATCH] Make cryptoapi non-optional? |
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On Sat, Aug 16, 2003 at 06:27:44AM +0000, David Wagner wrote: > Val Henson wrote: > >If entropy(x) == entropy(y), then: > > > >entropy(x) >= entropy(x xor y) > >entropy(y) >= entropy(x xor y) > > No, that's still wrong. Please see my earlier email with a counterexample. > That counterexample disproves not only the earlier claim, but also this more > recent revised claim.
Sigh. Yes, I was thinking of the case where x and y already have maximum entropy, in which case
entropy(x) >= entropy(x xor y)
For any y. The point I was really trying to make (badly) is that xoring won't increase entropy on average in this particular case (folding the output of SHA-1). In other words, xoring won't make anything better, and has the possibility of making things worse. In any way I evaluate it, folding and truncating are just as good for this particular case, except that folding costs more computationally. (The computational cost was significant enough to have a measurable effect on throughput, so I'm not arm-chair optimizing here.)
I sacrifice any earlier points I attempted to make on the altar of hasty mathematics.
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