Potassium nitrate is not illegal. Combining it into a pyrotechnic mixture without a factory license issued by these people
is. Also, good luck with your future should you burn up someone's property or, God forbid, injure someone. Got insurance?
Before trying your hand at making things that explode and burn, you should see what the consequences
are if you fuck up. And having been a professional pyrotechnician for over a decade, I have seen PLENTY of fuck ups.