In recently, due to fast development of the cloud computing, a lot of cloud applications have been developed continuously. However, users of cloud applications can easily and quickly access and share their information by their terminal devices which merely own the web-browsing ability. Such the operation convenience has brought some data security issues in the cloud computing. Thus, according to top threats to cloud computing V1.0 proposed by Cloud Security Alliance, there are several issues of intensity of information security with the providers of the cloud applications. Thus, we proposed to apply the AAA certification mechanism to enhance the Authentication security control and to utilize the watermark and RSA algorithms for increasing security of file-sharing in the clouds.