Abstract
A cloud computing environment is continuously changing and developing from single cloud to multi clouds, from fixed cloud to mobile cloud. But security problems have issued such as user authentication, access control, user privacy etc. A user authentication among of them is particularly important due to inherent characteristic called "pay-per-use". In this study, a unified service approach is used to provide a fast authentication service to mobile users. This method was designed for mobile users considering the convenience and scalability of services.