PhotoDNA, developed in conjunction with the National Center for Missing and Exploited Children, calculates a mathematical hash based on an existing image that contains child pornography. After building up a library of several thousand images, the software can recognize a match anywhere that it's deployed, even if the photo has been altered. The hash also doesn't need to include identifying information, and the original photograph doesn't have to be given to law enforcement. While it won't catch new pictures, it can identify a huge number of photos that are traded online. Besides PhotoDNA, other versions of image recognition software have been used for anything from reverse image searches to filtering for copyrighted content.