Twitter to introduce PhotoDNA system to block child abuse images

Microsoft-developed system may be introduced this year once complication of handling pictures posted alongside billions of tweets can be overcome.
Twitter is to introduce a tagging system to prevent child abuse images being posted on its service, which now sees millions of pictures posted among the 2bn tweets every five days.
The intention is to introduce the system, which uses a Microsoft-developed industry standard called "PhotoDNA", later this year if possible.
The move was revealed exclusively to the Guardian as the prime minister, David Cameron, steps up pressure on internet providers and particularly on search engines to block access to images of child abuse online.
Twitter's move has come independently of UK pressure. Microsoft and Facebook already use PhotoDNA to monitor images posted to the social network, Microsoft's Skydrive service and accessible via its Bing search engine.
PhotoDNA works by producing a "hash" – a single number generated from the binary data of a picture or video, and some biometric information in the picture. The method still works even if the image is resized or altered.
When an image is posted, its hash is compared against known images of child abuse which have been flagged by operations such as the UK's Internet Watch Foundation and the US's National Center for Missing & Exploited Children's (NCMEC) Child Victim Identification Program. The latter found 17.3m images of abuse in 2011 – and since 2002 has reviewed more than 65m images and videos of child sexual exploitation reported by the authorities. However, the "hash" database is reckoned to be much smaller.

Source: https://www.guardian.co.uk/technology/2013/jul/22/twitter-photodna-child-abuse