**2.1 App source and download**

Android application collection process includes gathering APK files from different Android marketplaces. The main application sources include Google Play, Anzhi, and AppChina. For every potential free app, the crawler script must ensure that the app has not been downloaded before and then calculate the app's hash using the SHA256 algorithm [30]. Once the app is downloaded, it can be archived for future use. Chrome APK Downloader, a desktop version of APK downloader tool, can be used to download the APK files of the free Android applications into desktop from Google Play marketplace [31]. For the paid applications, the Raccoon APK Downloader can be used to download APK files from Google Play Store [32].
