Two important topics related to the cloud security are discussed in this chapter: the authentication of logical users accessing the cloud, and the security of data stored on public cloud servers. A real cloud platform is used as example; it is designed and implemented to support basic web applications, and to be shared by small and medium companies. Such platform is built using the OpenStack architecture. The user authentication is based on an original biometric approach exploiting fingerprints and open to multimodal improvements. The platform guarantees secure access of multiple users and complete logical separation of computational and data resources, related to different companies. High level of protection of the data, stored in the cloud, is ensured by adopting a peculiar data fragmentation approach. Details are given about the authentication process and of the service modules involved in the biometric authentication. Furthermore are discussed the key issues, related to the integration of the biometric authentication, in the cloud platform.