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Comment by mox1 | Hacker News Reader

So I think the most up-voted response does a great job of explaining the "why it matters" part of the question, but he never really touched on the "whats the difference"... IE the fact the pseudo-random basically means generating statistically random data, using a deterministic process.

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That's really interesting, in that case I've understood it wrong all this time! So you are saying pseudo-random really is honest-to-god random, following all the accepted rules for random distributions and such, with the critical difference simply being that the output of the pseudo-random function can be predicted if you understand the generating process?

That's a good way to think about it. A quality PRNG tells an undetectable lie.

As I understand it, yes your basically correct.

Imagine a function like this:

  def init_prng(int x):
    state = x

  def prng():
    state = md5(state) 
    return state

So while this is a good, simple PRNG its very recoverable if you know the initial x (or if the space of possible x values is small, ie a 32-bit number). One of the goals of a PRNG function is to extend the usefulness of truly random data, because fully random data is hard to come by.

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