Law Enforcement have been using the hashes of known child pornography images to find pedophiles for probably a decade. This technique is no different than what anti-virus vendors do to detect malicious software. I'm starting to hate Google in lieu of their new campaign on killing privacy and anonymity but I don't see the privacy concerns here. Server side hashing of images and comparing them to a database isn't exactly gives metadata to the NSA. I think it's good that Google is doing this. Odds are more image hosting providers should be doing. It's low hanging fruit but at least it could protect a child.