I saw this awesome presentation on why rand() is considered harmful. When you need a random number, don’t call rand() and especially don’t say rand() % 100! This presentation will explain why that’s so terrible, and how C++11’s header can make your life so much easier.
If you need uniqueness and non-deterministic, especially on the context of security or crypto then you need to think about a few things. For example the frequency, non-uniform distribution, and not using a pseudo random number generator (such as Mersenne twister) and not a linear congruential generator.