Google explains digital fingerprints that caught Gmail offender

Google's decision to notify law enforcement of a Gmail user sharing indecent photos of children reignited questions around what privacy the company provides, but Google has stepped forward with new details on how, exactly, its monitoring system worked. A Houston man was arrested last week after Google informed police in Texas that he had been sharing child pornography with his Gmail account; that content, Google says now, has a special digital fingerprint which distinguishes it from other materials.

"Each child sexual abuse image is given a unique digital fingerprint," a spokesperson for the company told AFP, "which enables our systems to identify those pictures, including in Gmail."

However, the same fingerprint is not present in other images, even if they're showing otherwise nefarious activities. The Google spokesperson says that even if Gmail was being used to plot a burglary, for instance, pictures used for that would not be flagged.

The assumption could be that the photos the arrested suspect shared were already known to authorities and thus had been given the fingerprinting treatment. Whether it would have identified new images is unclear, since Google has not shared technical details of the process.

Roughly a year ago, Google frustrated privacy advocates when a court filing confirmed its belief that Gmail users should have no expectation of secrecy for their content. Indeed, for some time Microsoft made an advertising campaign of that, accusing Google of putting data mining for advertising above its users privacy.