如何在Objective c中添加“In app Purchase”中的“Touch Id”？

I have successfully implemented "In App Purchase" in my project and now i am trying to implement a "Touch Id" to buy a product except asking a "Sign in to iTunes" popup. Here in my code it asks "touch id" and "Sign in to iTunes" popup both one after another, why this one wired both things i want only "touch id" to buy product. I have gone for IAP I have successfully implemented "In App Purchas