How did the cops catch up with the Prince George's County coach and substitute teacher accused of possessing and sharing child pornography?

Turns out Christopher Speights was busted after a tip from his file sharing service. So how does a massive media service sort through millions of hours of video find child porn?

Think about the scale of this for a minute. Dropbox has 500 million users uploading billions and billions of files.
It's not saying how it finds the files containing child porn. But we think we know.

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Experts suspect Dropbox is scanning it's billions of user files with an amazing bit of software developed my Microsoft and computer scientists at Dartmouth.

"PhotoDNA is an image matching technology used to break down a photo into a single mathematical fingerprint in order to find copies of that image," said co-creator Hany Farid on a promotional video uploaded to YouTube.

The developers of PhotoDNA took the millions of images of child porn collected at the National Center for Missing and Exploited Children, assigned each pixel a numeric value and then created a digital fingerprint that the software can find in any image uploaded not just to Dropbox, but to dozens of other file sharing services, including Facebook and YouTube.
But if police are right, Charles Speights did very little to hide what he was uploading. The names on some of the files police say he uploaded make pretty clear what's in them. And that's before the authorities even looked at the video inside.

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Companies that use PhotoDNA report hits for child porn to the National Center for Missing and Exploited Children in Alexandria. The National Center then alerts law enforcement, in this case, the Maryland State Police.
Troopers said they had very little trouble tracking down Speights with his email address and internet service provider. And when they confronted him, they said in court documents that he admitted he was sharing child porn.
So who was he sharing it with? We'll see if there are more arrests to come.
Scores of companies are already using PhotoDNA. Supporters are urging even more companies to use it. Although some privacy advocates are a worried about where this kind of trolling through people's files might end.