Build a secure client / server file system storage

The goal of this project is to implement individually a secure client / server file system storage, such as the ones availlable in NAS systems or in FTP clients. Considering the main aspect of this project is security, apprioriate techniques have to be used. Free to choose protocols and languages that you find appropriate. From a general point of view, the server allows :
• new clients to register to the system;
• clients to log in the system;
• authenticated clients to store files where there are allowed to;
• authenticated clients to download files if they are allowed to;
• authenticated clients to delete files if they are allowed to.