Microsoft and NetClean, a software company focused on tools for stopping the spread of child pornography online, have announced that they are partnering to give law enforcement agencies access to Microsoft's PhotoDNA image-matching technology at no cost to help in the investigation of child sex abuse cases. The software can be used to comb through collections of digital images to identify copies of known child porn images, speeding up the forensics work of investigators.

Developed by Microsoft in cooperation with Dartmouth College, PhotoDNA uses an approach similar to facial recognition and other biometric systems to mathematically create a signature for a particular image. In an e-mail exchange with Ars Technica, a Microsoft spokesperson said that PhotoDNA uses a mathematical approach called robust hashing, "calculating a unique signature into a 'hash' that represents the essence of a particular photo." The hash can't be used to recreate the image or identify individuals within the photo—meaning that law enforcement and others investigating child pornography don't have to retain copies of the offending images on their systems. But it can be used to match copies of images even if they have been resized or altered in other ways, the spokesperson said.

The technology is already in use by Microsoft and Facebook in cooperation with the National Center for Missing and Exploited Children. Microsoft is offering the technology through NetClean's free tool for law enforcement, NetClean Analyze, as well as through direct licensing of source code to agencies who want to integrate PhotoDNA into their own tools. Microsoft is also building the techology into the Child Exploitation Tracking System, software that the company originally developed in cooperation with Canadian law enforcement and is now managed and used by a consortium of US and international agencies, including the Department of Homeland Security and FBI.