Why does it have to be a "folder" (directory..)?
tar(1) +
openssl(1) are all you need to achieve what you want in a mostly portable way.. though, adding/removing entries from archives might be an issue, another option would be using the
archivers/zip port. (
As a substitute for tar, not openssl.. zip encryption is poor.)
http://www.daemonforums.org/showpost.php?p=14487
Another option would be creating a dedicated partition, encrypted, owned by your user account with proper permissions preventing read/write access from other users.