Steven D. Galbraith Wenbo Mao Invisibility and Anonymity of Undeniable and Confirmer Signatures. 80-97 2003 conf/ctrsa/2003 CT-RSA http://link.springer.de/link/service/series/0558/bibs/2612/26120080.htm db/conf/ctrsa/ctrsa2003.html#GalbraithM03