How to make a fancy border

Hello, I'm looking for advise on how I might achieve the border attached in the image. In case it doesn't display very well. Its a dashed border, but has little squares placed in corner. Pretty confident there is a CSS solution to this. So the dashed line is simple enough, and I know I have the before and after pseudo ready to be used. Using before and after I can place the square in the top left and bottom left. But what about top right and bottom right. Advise would be great