Maximum Black Subsquare(Hard)

Imagine you have a square submatrix where each cell is either black or white. How can you design an algorithm to find the maximum subsquare such that all four borders are filled with black pixels?--有一个正方形矩阵，里面的每一个小格子要么被涂上黑色要么被涂上白色。设计算法，找到四条边都是黑色格子的最大子正方形。