What Is a Receipt?

The receipt consists of a single file in the app bundle. The file is in a format called PKCS #7. This is a standard format for data with cryptography applied to it. The container contains a payload, a chain of certificates and a digital signature. You use the certificate chain and digital signature to validate that Apple produced the receipt.

The payload consists of a set of receipt attributes in a cross-platform format called ASN.1. Each of these attributes consists of a type, version and value. Together, these represent the contents of the receipt. Your app uses these attributes to both determine the receipt is valid for the device and what the user purchased.

This tutorial uses OpenSSL libraries to do much of the work of verifying the cryptography and decoding the ASN.1 data provided in the receipt. OpenSSL isn’t very Swift-friendly, so you’ll be creating a Swift wrapper during this tutorial.

Compiling OpenSSL for the iPhone isn’t a simple process. You can find scripts and instructions on GitHub if you want to do it yourself. The starter project includes OpenSSL 1.1.1, the newest version, in the OpenSSL folder. It’s compiled as static libraries to make modification more difficult. This includes the folder as well as the C header files. The project also includes the bridge header to use the OpenSSL libraries from Swift.