The Obnam mailing list archives sometimes have messages that look like they're encrypted. They aren't, it's actually just a base64 encoding, so it's easy enough to undo. The base64 encoding in the archives happens due to some bug in the mailing list software, which the people hosting the lists haven't been able to fix. It will hopefully be fixed in a future upgrade to a new version of Mailman.
Meanwhile, Gmane provides a much nicer interface to the mailing list than Mailman's archives, and doesn't suffer from this problem. Gmane is highly recommended. See the http://dir.gmane.org/gmane.comp.sysutils.backup.obnam group there for the Obnam support list.