Explaining the Use of Cryptographic API in Android Malware

Cryptography allows for guaranteeing secure communications, concealing critical data from reverse engineering, or ensuring mobile users’ privacy. Android malware developers extensively leveraged cryptographic libraries to obfuscate and hide malicious behavior. Various system-based and third-party libraries provide cryptographic functionalities for Android, and their use and misuse by application developers have already been documented. This paper analyzes the use of cryptographic APIs in Android malware by comparing them to benign Android applications. In particular, Android applications released between 2012 and 2020 have been analyzed, and more than 1 million cryptographic API expressions have been gathered. We created a processing pipeline to produce a report to reveal trends and insights on how and why cryptography is employed in Android malware. Results showed that the usage of cryptographic APIs in malware differs from that made in benign applications. The different patterns in the use of cryptographic APIs in malware and benign applications have been further analyzed through the explanations of Android malware detectors based on machine learning approaches, showing how crypto-related features can improve detection performances. We observed that the transition to more robust cryptographic techniques is slower in Android malware than in benign applications.