> We found that the use of client re-encoding can make the image filtering more powerful in that it effectively results in an image’s hash representing all imag

> We found that the use of client re-encoding can make the image filtering more powerful in that it effectively results in an image’s hash representing all images with identical pixel values as opposed to merely a specific image encoding. The process of re-encoding extends the generalizability of hash-based filtering because any image containing the same pixel contents will be encoded to the same file and thus have an identical hash (see Figure 3). When this happens, any changes to the original image’s encoding or to its metadata will be ineffective at evading filtering, and some change to the image’s pixel values, if even a small one, will be required to change the resultant hash. When the client does not re-encode, then any change to an image’s file encoding, including to its metadata, is sufficient to change its hash. So, just add a random off-color pixel in your image and the system will fail.