Microsoft has released a new, free version of its child porn image-detection software PhotoDNA to enable all websites and photo-sharing service providers to be able to spot illegal images.

PhotoDNA was created by Microsoft in collaboration with Dartmouth College back in 2009 to aid the work of the National Center of Missing and Exploited Children (NCMEC) in the US.

The software giant says that 1.8bn pictures are uploaded to the internet every day, and 720,000 of those images are child pornography.

Over 70 companies are already using PhotoDNA to detect these illegal images and aid law enforcement around the world, including Facebook, Twitter and Flipboard, but they have to host the software on their premises and have the money and technical expertise to keep it running and up-to-date.

Moving PhotoDNA into the cloud

"Finding these known child sex abuse images in that huge universe is like finding a needle in a haystack," said Courtney Gregoire, a senior attorney at Microsoft's Digital Crimes Unit.

"We needed an easier, more scalable way to identify and detect the worst of the worst images ... and that's how the concept for PhotoDNA in the cloud was born."

Microsoft has launched the PhotoDNA Cloud Service so that smaller companies, online communities and other organisations that enable users to upload content can detect and identify these images for free.

Although the original suspects might have long since been caught and their victims freed, these illegal images continue to be traded online by other paedophiles.

Using hashing technology to detect images

PhotoDNA works by using "hashing" technology to tag known child sexual abuse images held in databases by Interpol, NCMEC and other law enforcement agencies around the world, so that the system flags up duplicate copies of the images found elsewhere without humans having to view them.

When an illegal image is run through the software, it converts the image into a greyscale format with a grid, assigning a numerical value to each tiny square. The software then scans images from content uploaded by websites to see if they match that numerical hash.

Although paedophiles now try to alter images by cropping them, changing the file extension or altering a few pixels, PhotoDNA is still able to detect the files and flag them.

"At its core, this is really one of the most heinous crimes that can happen to a child during some of their most vulnerable years," said Gregoire.

"My hope is much wider-scale deployment of this important technology to better protect these victims of sexual abuse."