Wow that is a pretty stupid mistake to make. I figured they forgot to remove their mod bias or something. One thing you do with big random numbers is use them to get smaller random numbers. For example, if you get the number 1000 it isn't so helpful by itself in telling you which position in a deck a given card should be in. So what you do is essentially 1000 mod 52 = 12 but imagine that you want a random number from 0 to 2 but your RNG gives you numbers up to 4. One thing you could do is rng_output mod highest_number_desired++ , which is guaranteed to give you a number that is no greater than the highest number you want. But: 0 mod 3 = 0 1 mod 3 = 1 2 mod 3 = 2 3 mod 3 = 0 4 mod 3 = 1 now you are more likely to select 0 or 1 than 2, even if the initial larger number is itself random. This is called mod bias, and if measures are not taken against it, it can be a critical security vulnerability in a system that generates smaller numbers from a range using a bigger random number as the initial seed.