Toru Nakanishi Nobuaki Haruna Yuji Sugiyama Unlinkable Electronic Coupon Protocol with Anonymity Control. 37-46 1999 conf/isw/1999 ISW http://link.springer.de/link/service/series/0558/bibs/1729/17290037.htm db/conf/isw/isw1999.html#NakanishiHS99