Pim Tuyls Henk D. L. Hollmann Jack H. van Lint Ludo M. G. M. Tolhuizen XOR-based Visual Cryptography Schemes. 169-186 2005 37 Des. Codes Cryptography 1 http://dx.doi.org/10.1007/s10623-004-3816-4 db/journals/dcc/dcc37.html#TuylsHLT05