There’s no kill switch for the Internet, no secret on-off button in an Oval Office drawer. Yet when a Senate committee was exploring ways to secure computer networks, a provision to give the president the power to shut down Internet traffic to compromised Web sites in an emergency set off alarms. Corporate leaders and privacy advocates quickly objected, saying the government must not seize control of the Internet. Lawmakers dropped it, but the debate rages on. How much control should federal authorities have over the Web in a crisis? How much should be left to the private sector? It does own and operate at least 80 percent of the Internet and argues it can do a better job. “We need to prepare for that digital disaster,” said Melissa Hathaway, the former White House cybersecurity adviser. “We need a system to identify, isolate and respond to cyberattacks at the speed of light.”