So a hash matches, then what?
The threat of this approach might deter the computer illiterate but then Darwin was already looking after people who try sharing kiddie porn via facebook.
First off, how is this hash to be generated? Will google et al calculate a hash for every image before it can be uploaded and simply not accept (sight unseen) anything that produces a hash they don't like? The first time you cant upload your holiday snaps will be the last time you use their service, so that is not a runner. Any hash will have to be calculated after upload, which means the company is now in possession of the suspect image. In most jurisdictions, possession of kiddy porn (knowingly or otherwise) is a serious criminal offence and I am not sure if safe harbour rules apply if the company is aware of the content.
What happens when a matching hash is detected? Do they send the 'suspect' image to someone else to verify? In which case they will be knowingly participating in the transport and distribution of what they believe to be kiddy porn across state and national boundaries! Try explaining that to the company lawyers.
Perhaps they have a human verify the image before they alert the authorities? In which case they must have paid employees looking at kiddy porn on company computers, on company time, with the companies knowledge and worse, consent! I wonder how HR will fill that vacancy. "Wanted: child porn expert, equal opportunity employeer"
If you are serious about stopping child exploitation, then stop this techno bullshit and actively support genuine child protection organisations.
Biting the hand that feeds IT
