&gt; Cryptographically random isn&#x27;t the aim here<p>We are agreeing here.<p>&gt; A seedable black-box RNG that guarantees what seed corresponds<p>If you can control when the blackbox gets updated, or the blackbox carries a guarantee of stable output for any given seed over the while time of its existence, yes. But if you are using OS provided RNGs, as a for instance, their behaviour is not defined (well they are, but those definitions are not set in stone) and may change as kernel updates happen.<p>That is why I suggest &quot;making your own simple PRNG&quot;. This could be as simple as picking a known documented algorithm as implemented by a particular library and using that in your test cases - it doesn&#x27;t need to be as much as writing your own function even.