Ahmad-Reza Sadeghi Markus Schneider 0002 Electronic Payment Systems. 113-137 http://springerlink.metapress.com/openurl.asp?genre=article&issn=0302-9743&volume=2770&spage=113 2003 conf/drm/2003 Digital Rights Management db/conf/drm/drm2003.html#SadeghiS03