Kazuhiro Ogata Kokichi Futatsugi Formal Analysis of the NetBill Electronic Commerce Protocol. 45-64 http://springerlink.metapress.com/openurl.asp?genre=article&issn=0302-9743&volume=3233&spage=45 2003 conf/isss2/2003 ISSS db/conf/isss2/isss2003.html#OgataF03