Iam visiting Iran with my sister next month and I was curious what would happen if I publicly curse Mohammad?

This is a serious question, by the way. I am curious to see how I will be treated, since I am American and I believe have diplomatic immunity. I dont mean a night in jail or a fine but I do not want to be killed. If I say '*** Mohammad" with my sister on the street and point my middle finger to the sky, what will happen?