Why A XOR B Reveals Concealed Flags
XOR has a unique mathematical property: X ^ X = 0. If a CTF puzzle provides two images with identical background pixels, XORing the two frames completely zeros out the background to solid black.
Any pixel that was modified—even by a single RGB value—evaluates to a non-zero value, causing the hidden flag to shine brightly against the black background.
Key Investigative Takeaway
Always run A XOR B whenever a challenge distributes two seemingly identical images or an image alongside a mask file.