Точечный выбор (Pointed Selection)

hackerlab

Task: a PNG of a black-dot grid (DotCode 2D barcode) whose IHDR height is falsified to hide a second symbol below the declared image. Solution: re-decode IDAT past the fake height to reveal the hidden symbol, then build a DotCode decoder by reversing zint's dotcode.c; the visible top symbol is a decoy, the hidden bottom symbol holds the real flag.