REDMOND -- It's supposed to pick a needle from a virtual haystack, and according to Microsoft's Digital Crimes Unit, PhotoDNA has yet to make a mistake when it comes to tracking down child porn.

Approximately two billion images, saidsenior program manager Sue Hotelling, and we've had no false positives.

Now Facebook, the world's largest social network -- which, incidentally, also is the largest photo sharing site on the Web -- says it will be using the same technology to filter photos uploaded by members.

The program, calledPhotoDNA, can take an image and break it down into what essentially is a fingerprint called a hashtag. It was developed by researchers at Microsoft and Dartmouth College who were looking ways to identify images even afterthey've been altered in color, orientation and size from the original.

Because it is based on numbers rather than graphics, the technology can search faster and more accurately through photos than the human eye.

Microsoft unveiled PhotoDNA in 2009, then freely licensed it to the National Center for Missing and Exploited Children. They've already matched thousands of photos against a database of 10,000 photos they call the worst of the worst.

It's really nasty stuff, saidSue Hotelling, Microsoft senior program manager, It's prepubescent children who are being violated in the most graphic ways, in an image.

The goal is to ensure that these images of children being sexually assaulted aren't continually redistributed and these children aren't victimized again and again and again, said Ernie Allen, NCMEC president.

Part of the problem, they said, is even after an arrest is made, once an image posts online, it's very difficult to keep that photo from proliferating.

Some of the images that we found matches to have been around since the 90's, Hotelling said.

There are at least 50,000 of these kinds of images circulating on the Internet every day, Allen said.

Privacy advocates have asked how the technology distinguishes between child porn and say, baby photos or risque photos of adults.

Microsoft's response -- PhotoDNAis not facial or object recognition software, and the photos it scans must be compared to images already collected by NCMEC. At this point, NCMEC said it has provided a database of 10,000 photos that meet specific criteria.

We have purposely defined it very, very narrow so that no one can think this is protected speech, Allen said.

Though Facebook is just debuting the software on their site, PhotoDNA beta testing has already scanned almost two billion images through Microsoft's Bing search engine and SkyDrive photo-sharing site.

Hotelling said Facebook's involvementincreases the scope of what they can accomplish.

That's where people go, and that means the good and the bad are there, shesaid. This will really help to stop the prevalence of this type of activity.

Hotelling saidthey're uncovering an average of ten child porn photos a day, all of which have been removed based on violations of terms of service.

A NCMEC spokesperson said PhotoDNA has already led toseveral investigations and arrests, including one in New Zealand.

Facebook is the first social networking platform to adopt the program, though NCMEC is apparently in talks with others to sub-license the software.

The Microsoft Digital Crimes Unit said in the future, they hope to even be able to create a version of the program that can also fingerprint video files.

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