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We use face detection to try to censor material which may be offensive but this does not work perfectly so you should read the following before using OPTIC NERVE: It is possible to handle and display undesirable images

GCHQ installed face recognition software that excluded shots in which faces had not been detected, but warned staff that this might still enable pornography to seep through.

The document obtained by the Guardian said, “We use face detection to try to censor material which may be offensive but this does not work perfectly so you should read the following before using OPTIC NERVE: It is possible to handle and display undesirable images. There is no perfect ability to censor materials which may be offensive. Users who feel uncomfortable about such material are advised not to open them.”

But the document then went on to warn staff that those who do view porn and distribute it afterwards could face action.

“You are reminded that under GCHQ’s offensive material policy, the dissemination of offensive material is a disciplinary offence.”

The Guardian, reporting on documents provided by fugitive intelligence leaker Edward Snowden, said that under the Optic Nerve program GCHQ collected and saved millions of still images from Yahoo users globally between 2008 and 2010. The program was still running in 2012.

The program did not collect webcam chats in their entirety, but took screenshots every five minutes.

The system was an experiment in “automated facial recognition” to monitor GCHQ targets and discover new ones, reported the paper.

GCHQ said all of its programs were legal, but Yahoo was incensed at the revelations and condemned the breach of privacy of its users.