# Unpredictable Randomness Definition

Let's consider randomness as the outcome of an experiment, two extremal cases are of interest. The first is the truly random  number generator, which is based on a non-deterministic process. In that case, without condition, you cannot guess the outcome before it actually happens.

The second extreme case is  pseudo-randomness. The process takes as input a (short) sequence of numbers (seed). It is called a pseudo-random number generator  if, by considering only its outputs, it is computationally infeasible to distinguish it from a truly random number generator. By ``computationally infeasible'', we mean non polynomial time and memory and by ``distinguish'', we mean that the probability to take the good decision is significantly greater than 1/2.

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