Kaoru Kurosawa Koji Okada Keiichi Sakano Wakaha Ogata Shigeo Tsujii Nonperfect Secret Sharing Schemes and Matroids. 126-141 http://link.springer.de/link/service/series/0558/bibs/0765/07650126.htm 1993 EUROCRYPT db/conf/eurocrypt/eurocrypt93.html#KurosawaOSOT93