Efficient Hash Lookup for PhotoDNA

PhotoDNA is a widely used method for generating robust image hashes. It is widely used today for the detection of CSAM. This results in large numbers of images that need to be compared. This is done over a Euclidean distance, which requires relatively expensive computations. We present an approach that allows the comparison of these images to be performed significantly more efficiently. We also show that both robustness and resistance to false positives are not compromised. Our approach is based on converting the PhotoDNA hash from 144 bytes to 300 bits, which can be compared using Hamming distance. An advantage is that the existing hashes can be converted directly, so no new calculation of hashes from reference images is necessary.