Cloud storage such as Dropbox and Bitcasa is one of the most popular cloud services. Currently, with the prevalence of mobile cloud computing, users can even collaboratively edit the newest version of documents and synchronize the newest files on their smart mobile devices. A remarkable feature of current cloud storage is its virtually infinite storage. To support unlimited storage, the cloud storage provider uses data deduplication techniques to reduce the data to be stored and therefore reduce the storage expense. Moreover, the use of data deduplication also helps significantly reduce the need for bandwidth and therefore improve the user experience. Nevertheless, in spite of the above benefits, data deduplication has its inherent security weaknesses. Among them, the most severe is that the adversary may have an unauthorized file downloading via the file hash only. In this article we first review the previous solutions and identify their performance weaknesses. Then we propose an alternative design that achieves cloud server efficiency and especially mobile device efficiency.