An Analysis of PhotoDNA

PhotoDNA is a popular hash used to combat CSAM. So far, only limited information has been provided about this hash in terms of its performance. In this paper, we provide an overview of robustness and false positives, as well as some basic observations about its structure. We show that it is robust against typical image processing such as lossy compression. On the other hand, it is only of limited robustness against cropping. We also give some suggestions for improving the algorithm or its use.