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Voice assistants can detect typing on nearby devices, which could potentially be used to work out what a person is writing on their phone from up to half a metre away.

Ilia Shumailov at the University of Cambridge and his colleagues built a machine-learning system that could recognise the sound of tapping on a touchscreen and combined it with other artificial intelligence tools to try to determine what people were typing.

Shumailov and his team asked three volunteers to type randomly displayed 5-digit PIN or English words on a touchscreen device while audio was …