If you are interested in secure backups you may want to take a look at the Tahoe Least Authority File System (http://allmydata.org/trac/tahoe), which has a much more useful security model than tarsnaps (for instance, you can share read-only access rights to a particular file with someone else). Tahoe is also entirely open source and the design is well documented, which in my view is two points up for Tahoe.