With the rapid development of cloud storage, more and more people prefer to store their data in online storage systems. However, recent researches indicate that security problems still remain in current online storage systems, and some recent data leakage accidents of online storage systems also prove the existence of these vulnerabilities. Such security problems seriously hinder the development of online storage system. To address the issue, this study designs and implements a secure online storage system called CorsBox. CorsBox proposes a data synchronous protocol based-on directory trees for fast synchronization between data plaintext and ciphertext, designs a three-level key management scheme to enhance the security of user's data, and presents an effective method to maintain system eventual consistency. The paper finally conducts a set of intensive experiments on modern servers and the result shows that the security mechanisms only incur a little extra performance expenses, indicating that CorsBox can provide enhanced security for user's data while maintaining good performance.