The lights go out; the internet goes down. Banks close; cash machines fail. Radio and television stations stop broadcasting. Airports and railway stations bar their doors. City streets are jammed with traffic. After a long night of uncertainty, power and communications are still blacked out – in fact, they might not come back for months. People start to panic and, as looters emerge, police are unable to restore order. With savings out of reach, the only things of value are fuel, food and water.

This is what an attack by a cyber weapon could look like, according to testimony to the US’s House homeland security committee in April from Sami Saydjari, president of Professionals for Cyber Defense, a non-profit organisation set up to alert the government and public to the dangers of threats from cyberspace. It would, he told the committee, take the US “from being a superpower to a third world nation overnight”. “We are a nation unprepared to properly defend ourselves and recover from a strategic cyber attack,” he said.