# Re: combining entropy

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On Sep 29, 2008, at 5:13 AM, IanG wrote:```
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```If I have N pools of entropy (all same size X) and I pool them
together with XOR, is that as good as it gets?

My assumptions are:

* I trust no single source of Random Numbers.
* I trust at least one source of all the sources.
* no particular difficulty with lossy combination.
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It's within epsilon for a good many epsilon.

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I'm presuming you want the resultant size to be X, as well. Otherwise, the suggestion that Ben has, concatenation is obviously better, and you can solve obvious problems.
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Another solution is to hash the N pools together with a suitably secure function. (Most the available algorithms are suitably secure for this purpose.) The downside of this is that you are capping your entropy at the size of the hash function. It's better than XOR because it's not linear, blah, blah, blah.
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However, if you had three pools, each relatively large, it doesn't hurt anything to XOR them together. It's pretty easy to prove that the result does not decrease entropy, but I think it's impossible to prove that it increases it. XORing is really taking the max of the N pools.
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You have to realize that XOR is bad if there's a chance to leak the entropy pool, XOR is a bad function. If whoever produced pool X sees X^Y, then they know Y. But you know that, too.
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Jon

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