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Comment by sliverstorm | Hacker News Reader

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?

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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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