Secure provides private single key encryption to user files. It can be used either as a standalone or Services menu application.
Installation...
To use Secure as a Services application, install in ~/Apps (or somewhere in your ApplicationPaths) and either Logout/Login or select Workspace menu "View -> Update Viewers". Consequently, applications which can provide Filenames to the pasteboard will dynamically have a "Services -> Secure" menu available. Some of the applications which can provide pasteboard Filenames include Workspace, Librarian, and Edit.