iPhone - Did the user purchase my app?

I'm trying to figure out how to verify that the user purchased my app before allowing them to register an account on my server. I failed with my web search - a开发者_StackOverflow社区ll I found was references to in-app purchase.. Is there some kind of receipt on the phone? Is it safe to send to my server etc..

EDIT - You can't query Apple for information on the users who have purchased the app? An email? anything??

EDIT2 -

Heres the deal, you can use the app all you want. But theres an optional ability to work with my server. This is expensive, if pirates start using my server I can't handle it. So I need to be able to authorize the user at my server. I'm guessing in app payment is what I'll have to use as that allows me to verify a purchase against a user/password on my site, I just didn't want the user to have to go through the hassle of the in-app payment and allow them to purchase the app upfront.

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I don't think that's possible.

You could check to make sure your app hasn't been cracked instead? - (see top answer Reducing piracy of iPhone applications)

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You need some kind of registration with backend from within the app.