is it illegal to hide your face in public?

I know if I were to dress up as a KKK member and walk down the street with my face covered, I would be beaten and poured in to the back of a police car for hiding my face in public.

Then again I see women walking around in these arab dresses with just a slit to see out of and no one bats an eye at it. I would like to know of the exact law that makes it illegal to hide your face in public.

can some one point me in the direction PLEASE?