"Cloud computing" has been popular in recent years, more and more service provider proposed cloud services especially cloud storage service. However, one of the worrying problems is data security. in this thesis, we proposed a whole service named SSTreasury+ which includes encryption application and cloud storage service. the user's data before uploading to the cloud could be encrypted first to prevent the data to be stolen during transmission or in the cloud storage. in addition, the decryption key which generated by our system can be portable to increase flexibility and convenience. in the back-end storages we use existing cloud storage as a backup storage in order to reduce constructing costs. We expected by the above methods to achieve a safe and flexible cloud storage service.