Cloud storage system provides storage to enterprises and end users, and it helps to reduce the cost of server maintenance. Unfortunately, this new type of data storage service also brings new security challenges as data owners do not have full control of their data. The existing research works cannot completely resolve these security issues efficiently or may cause other security problems. In this paper, a framework with the third party auditor is first proposed. Based on our system model, a key management scheme and an authentication process are proposed which can ensure the security of cloud storage. In addition, two types of key loading methods are proposed to improve the performance of key management. In order to create a system with balanced load, AES encryption algorithm is adopted to reduce the computation load on the user side. Through analyses and simulations, we have shown that the proposed system achieves both security and good overall performance.