In general, the subject matter described in this disclosure can be embodied in methods, systems, and program products. A computing device provides a user interface of a third-party application program. The user interface includes an interface element that causes purchase of a product. The computing device receives an indication that the user has selected the interface element. The computing device identifies a first identity certificate that is assigned to the third-party application program from among multiple identity certificates that are assigned to multiple respective third-party application programs. The computing device sends the first identity certificate to a server system so as to cause the server system to determine whether the first identity certificate matches a second identity certificate. The computing device receives an indication that the third-party application program is able to legitimately purchase the product. The computing device sends a request for purchasing the product.