Most file systems including those in MS Windows, traditional UNIX systems, and secure file systems [19,22,30] do not store files encrypted on the server. Of course, the user may decide to encrypt files before storage but this overwhelms the user with the manual encryption/decryption and sharing the file with other users - while trying to minimize the amount of computation. This is precisely the problem that Plutus addresses.
Though MacOS X and Windows CIFS offer encrypted disks, they do not allow group sharing short of sharing a password.