Xiaoling Dai John C. Grundy Three Kinds of E-wallets for a NetPay Micro-Payment System. 66-77 http://springerlink.metapress.com/openurl.asp?genre=article&issn=0302-9743&volume=3306&spage=66 2004 conf/wise/2004 WISE db/conf/wise/wise2004.html#DaiG04