Messages in this thread | | | From | (David Wagner) | Subject | Re: [RFC][PATCH] Make cryptoapi non-optional? | Date | Fri, 15 Aug 2003 01:45:45 +0000 (UTC) |
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Val Henson wrote: >On Thu, Aug 14, 2003 at 07:40:25PM +0000, David Wagner wrote: >> I don't see where you are getting this from. Define >> F(x) = first80bits(SHA(x)) >> G(x) = first80bits(SHA(x)) xor last80bits(SHA(x)). >> What makes you think that F is a better (or worse) hash function than G? > >See Matt Mackall's earlier post on correlation, excerpted at the end >of this message. Basically, with two strings x and y, the entropy of >x alone or y alone is always greater than or equal to the entropy of x >xored with y. > >entropy(x) >= entropy(x xor y) >entropy(y) >= entropy(x xor y)
Sorry; that's not accurate. Here's a counterexample. Let x and y be two 80-bit strings. Assume that x is either 0 or 1 (equal probability for both possibilities). Assume y is either 0 or 2 (equal probability for both possibilities), and is independent of x. Then entropy(x) = 1 bit entropy(y) = 1 bit entropy(x xor y) = 2 bits
The difference between F and G is very small, and there is not much basis for choosing one over the other. - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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