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Comment by ss95060 | Hacker News Reader

Suppose, somehow, you seed your super-duper PRNG with the same value on multiple servers. If you are not incorporating other forms of entropy into your unique IDs, you are hosed.

The cryptographic hash acts sort of like a blender for all your random/unique-ish information sources. Counters/MACs/urandom/times/IPaddrs/etc go in, a nicely mixed value comes out.

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Meh. You should just use all of that random unique-ish stuff to properly seed your PRNG. You're trying to improve the generator "randomly," which is generally dangerous and can be counterproductive. Better to rely on sound theory and good practice / keep it simple. Leave all that craziness to the kernel entropy pool and just use it as a seed.

You might want to read this:

http://security.stackexchange.com/questions/89813/managing-k...

Anyways, if you decide you want to learn more about secure hash functions and cryptography in general, you can't go wrong with Bruce Schneier's "Applied Cryptography".

Good luck!

Applied Cryptography is a good book. I first read it more than 15 years ago :). Schneier actually designed the CSPRNG that is used by OS X. It's called Yarrow and is SHA based. It does more or less what you suggest, but with many subtle improvements to mitigate attacks. It has features that you're unlikely to properly reproduce in user-space. Trying to improve its output by re-hashing in user-space is, even if done properly, unnecessary. If you screw something up and do it improperly you could easily reduce the quality of the generator.

I'm not sure how your link is relevant. If there's not enough entropy on the system for the kernel, there's not enough for you either.

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