Ventzislav Nikov Svetla Nikova On Proactive Secret Sharing Schemes. 308-325 http://springerlink.metapress.com/openurl.asp?genre=article&issn=0302-9743&volume=3357&spage=308 2004 conf/sacrypt/2004 Selected Areas in Cryptography db/conf/sacrypt/sacrypt2004.html#NikovN04