The BBC reports that the UK-based Internet Watch Foundation is sharing hash lists with Google, Facebook, and Twitter to prevent the upload of child abuse imagery:
Web giants Google, Facebook and Twitter have joined forces with a British charity in a bid to remove millions of indecent child images from the net. In a UK first, anti-abuse organisation Internet Watch Foundation (IWF) has begun sharing lists of indecent images, identified by unique "hash" codes. Wider use of the photo-tagging system could be a "game changer" in the fight against paedophiles, the charity said. Internet security experts said images on the "darknet" would not be detected.
The IWF, which works to take down indecent images of children, allocates to each picture it finds a "hash" - a unique code, sometimes referred to as a digital finger-print. By sharing "hash lists" of indecent pictures of children, Google, Facebook and Twitter will be able to stop those images from being uploaded to their sites.
(Score: 0) by Anonymous Coward on Monday August 10 2015, @07:46PM
No need for 4 separate hashes; just compute the hash of the uploaded image on the four possible rotations, and check if any one matches. 4 more operations on the image instead of a fast lookup in a database 4 times biggers, but I think Google has enough resources...
Anyway I hope their hash is not a basic binary md5sum where any changed pixel would defeat it; and more of a "perceptual hash" that stays invariant with scaling, noise, color corrections etc.