You can keep a file secure by making the file inaccessible to other users. For example, a file with permissions of 600 cannot be read except by its owner and by superuser. A directory with permissions of 700 is similarly inaccessible. However, someone who guesses your password or who discovers the root password can access that file. Also, the otherwise inaccessible file is preserved on a backup tape every time that the system files are backed up to offline media.
The Solaris Cryptographic Framework provides digest, mac, and encrypt commands to protect files. For more information, see Chapter 13, Solaris Cryptographic Framework (Overview).