Google Play In App Billing. How to perform signature verification step?

The framework of our app is corona. In my android app, I used google play's in app billing. I can purchase the items in my app and my credit card has paid. Then , I need to perform signature verification. However the value of 'event.transaction.recepit'(http://developer.anscamobile.com/reference/in-app-purchases) was like that 64382835091xx96 or 90328327062xx88, (xx are two numbers) and the server telled me that they can't handle that data.

How can I get the data that the server need to perform signature verification step through corona's IAP api?

Thanks very much!

http://developer.android.com/guide/market/billing/billing_overview.html#billing-security

http://blog.anscamobile.com/2012/03/getting-started-with-android-in-app-billing/

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