Babak Sadighi Firozabadi Marek J. Sergot 96-103 http://springerlink.metapress.com/openurl.asp?genre=article&issn=0302-9743&volume=2845&spage=96 2002 conf/spw/2002 Security Protocols Workshop db/conf/spw/spw2002.html#FirozabadiS02