Harvey appeared to deny that pre-emption and counter-warfare might be a much more routine element of cyber-defense than these principles have been in the world of conventional war.

"I don't think that the existence of a new domain will, in itself, make us any more offensive than we are in any other domain. The legal conventions within which we operate are quite mature and well established," Harvey told the Guardian.

"The consequences of a well planned, well executed attack against our digital infrastructure could be catastrophic. With nuclear or biological weapons, the technical threshold is high. With cyber the finger hovering over the button could be anyone from a state to a student," he said.