Jung-Yeun Lee Jung Hee Cheon Seungjoo Kim An Analysis of Proxy Signatures: Is a Secure Channel Necessary? 68-79 2003 conf/ctrsa/2003 CT-RSA http://link.springer.de/link/service/series/0558/bibs/2612/26120068.htm db/conf/ctrsa/ctrsa2003.html#LeeCK03