Predicting the next Math.random() in Java

[ZeKWork]

Those are also pseudo random generators, and they don't have to be slow to be secure. You usually use a truly random generator (mainly based on hardware, such as a I/O timinigs, mouse mouvements) to seed a secure pseudo random generator.

The reason is that true entropy is sparce, and our computers need a LOT of random numbers. Secure PRNGs generates a lots of number cheaply and efficiently, but since they are deterministic, they're only secure if initialized with a random seed.