Gang Shen Feng Liu 0001 Zhengxin Fu Bin Yu 0003 Perfect contrast XOR-based visual cryptography schemes via linear algebra. 15-37 2017 85 Des. Codes Cryptogr. 1 https://doi.org/10.1007/s10623-016-0285-5 db/journals/dcc/dcc85.html#ShenLFY17