1 Introduction
In the past few years, smartphone users are increasing explosively. It is predicted that such growth will keep speedy in the near future. According to a recent study by Ericsson [1], the number of smartphone subscribers worldwide is 2.7 billion in 2014 and is expected to reach 6.1 billion by 2020. With such a rapid growth of users, smartphone is becoming a new domain for various hackers to steal sensitive data of users, such as leaking contacts [2], sending messages to contacts furtively [3], and stealing credit card information [4].