Jacobus A. Ophoff Stephen Perelson Reinhardt A. Botha A Model for Secure Value-Added Service Subscriptions in Cellular Networks. 1-9 2006 ISSA http://icsa.cs.up.ac.za/issa/2006/Proceedings/Full/60_Paper.pdf conf/issa/2006 db/conf/issa/issa2006.html#OphoffPB06