Targeted mobile advertising has become a key economic force in recent years with the growing mobile app markets. However, the fact that it requires continuous collection of users' profiles raises severe privacy concerns. In this research, we propose to explore a secure and practical mobile advertising architecture, which aims to enable targeted advertising over encrypted mobile user preferences. The proposed idea is based upon a cryptographic primitive — private stream searching (PSS). We show how PSS can be leveraged in this new context, and discuss a number of solution directions to make our design practical and suitable for mobile devices.