Amir Herzberg Markus Jakobsson Stanislaw Jarecki Hugo Krawczyk Moti Yung Proactive Public Key and Signature Systems. 100-110 1997 ACM Conference on Computer and Communications Security http://doi.acm.org/10.1145/266420.266442 db/conf/ccs/ccs1997.html#HerzbergJJKY97