In mathematics, a true random number generator it's impossible, because any formula defines a process that, however complex, is not random.
A random event must be unrelated to any cause or condition, and therefore cannot be causal. It is a brute fact par excellence.
If I draw up a list of all the possible conditions, I can say that a random event is outside of it. But isn't this a rule that determines the conditions of a random event?
Edit: this question was wrong, I was confusing 'causal' with 'deterministic'.