Jacob C. N. Schuldt Kanta Matsuura Kenneth G. Paterson Proxy Signatures Secure Against Proxy Key Exposure. 141-161 2008 Public Key Cryptography http://dx.doi.org/10.1007/978-3-540-78440-1_9 conf/pkc/2008 db/conf/pkc/pkc2008.html#SchuldtMP08