In one of his final Capitol Hill appearances, General Keith Alexander, the US National Security Agency's director, called for a stronger strategy to deter cyberattacks, saying the line that would prompt a US response against an adversary "does not yet exist".
Alexander, who retires next month after nearly 40 years in the US Army and almost nine at the helm of the NSA, said on Thursday that his greatest concern was a terrorist attack against the United States or Europe.
He also addressed the ongoing controversy over NSA surveillance, saying he was open to some proposed reforms. One of the most controversial disclosures involved the NSA's collection of US telephone call metadata. The programme gathers information on billions of phone calls from several US phone companies so that the agency can sift it for clues to terrorist plots. President Barack Obama, while saying that he believed the programme was useful and legal, has called for ending it in its current form to help restore trust in the agency.
One option that Alexander called feasible involves sharing what amounts to a watch list of suspected terrorists' phone numbers with phone companies. The companies would search for links to other numbers, returning that data to the government.
He said if the government could work out a system in which it could share those "terrorist selectors" in a classified manner, "it sets the case in precedent" for sharing data with industry for cybersecurity purposes.
Alexander, who is also head of the US Cyber Command, said deciding what was an "act of war" in cyberspace was a political or policy decision that was based on an attack's impact. "I would submit that if it destroys government or other networks to a point that it impacts our ability to operate, you've crossed that line," he said.