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offtopicofftopicPosted Dec 18, 2007 15:11 UTC (Tue) by njs (guest, #40338)In reply to: offtopic by ikm Parent article: On entropy and randomness
Okay, seriously, last crypto tutorial comment from me for a while, I swear... There's a lot of research of how you generate good cryptographic PRNGs. Some approaches use hashes, some don't. The analysis in general is utterly different from the analysis of scientific PRNGs, for the natural reason that you don't care so much about having provably insane period and provably insane equidistribution requirements; but you do care a huge amount about e.g. making sure that the internal state remains secret even when people can observe the PRNG's output, and that even if current state *were* revealed that this would not let you "run the PRNG backwards" and work out what earlier states were, and that you are clever about how you mix in new entropy from whatever external sources. Of course, you'd *like* a provably insane period too (to the extent that "period" is even meaningful when you are mixing in entropy as you go), but it's very hard to get an algorithm that resists all cryptographic analysis, *but* is still amenable to period analysis... Random posts on python-list are, unfortunately, pretty useless for grokking this stuff. If you really are curious to learn how CPRNGs are designed, one convenient place to start might be Schneier's various online-accessible writings on the subject: http://www.schneier.com/paper-prngs.html http://www.schneier.com/paper-yarrow.html http://www.schneier.com/blog/archives/2007/11/the_strange... (Yarrow in particular is a fun and interesting algorithm, though no longer quite considered state of the art. The last link is awesome for anyone who enjoys some geeky conspiracy, and has lots more links.)
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offtopic Posted Dec 19, 2007 3:39 UTC (Wed) by ikm (subscriber, #493) [Link] Yay! Thanks. You're right -- I've never thought too much about the use of RNGs outside Monte-Carlos and such, and the properties you've mentioned can mean a lot more than typical equidistribution/period requirements in other contexts indeed. I'll go dig the links you've provided. Thanks again, your help is much appreciated.
offtopic Posted Dec 20, 2007 17:57 UTC (Thu) by njs (guest, #40338) [Link] Cool, glad to help. Have fun! (If you really want to get into this, I also highly recommend Ferguson and Schneier's /Practical Cryptography/. /Applied Cryptography/ is better known, but it's all like "so in RSA you use modular arithmetic on primes in the following way" while /Practical Cryptography/ is all like "so if for some reason you want to reinvent SSL, here are the design trade-offs you have to decide about, and here are the subtle bugs you're going to put in that will break everything".)
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