Shoham Shamir Orna Kupferman Eli Shamir 2000 39 Electr. Notes Theor. Comput. Sci. 1 http://www1.elsevier.com/gej-ng/31/29/23/66/56/show/Products/notes/index.htt#005 db/journals/entcs/entcs39.html#ShamirKS00