Liqun Chen 0002 Paul Morrissey Nigel P. Smart Bogdan Warinschi Security Notions and Generic Constructions for Client Puzzles. 505-523 2009 ASIACRYPT https://doi.org/10.1007/978-3-642-10366-7_30 conf/asiacrypt/2009 db/conf/asiacrypt/asiacrypt2009.html#ChenMSW09