The organizers provide us with two files: a grayscale picture of Lena, and a dithered black-and-white version.

The solution is to apply Floyd-Steinberg dithering to reduce lena.tif to a black-and-white image, then XORing the result with embed.tif.

More details and code samples are available in the attached link.

Original writeup (https://github.com/wwong/ctf/blob/master/writeups/2018/volgactf-quals/blackandwhite/Writeup.ipynb).