Ivan Damgård Maciej Koprowski Generic Lower Bounds for Root Extraction and Signature Schemes in General Groups. 256-271 2002 conf/eurocrypt/2002 EUROCRYPT http://link.springer.de/link/service/series/0558/bibs/2332/23320256.htm db/conf/eurocrypt/eurocrypt2002.html#DamgardK02