lkml.org 
[lkml]   [2003]   [Aug]   [14]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
From
SubjectRe: [RFC][PATCH] Make cryptoapi non-optional?
Date
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/

\
 
 \ /
  Last update: 2005-03-22 13:47    [W:0.189 / U:0.260 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site