PhotoDNA is primarily used in the prevention of child pornography, and works by computing a unique hash that represents the image. This hash is computed such that it is resistant to alterations in the image, including resizing and minor color alterations. It works by converting the image to black and white, re-sizing it, breaking it into a grid, and looking at intensity gradients or edges.
PhotoDNA limits the spread of exploitative images circulated online. The software uses sophisticated “hash” matching technology to identify known illegal photos even if they have been altered. It also protects user privacy because it doesn’t look at images or scan photos; it simply matches a numerical hash against a database of known illegal images.