At the Aspen Security Forum conference on Thursday, Gen. Alexander revealed that there's been a 17-fold increase in cyberattacks against U.S. infrastructure between 2009 and 2011, .

The hacker community has built many of the tools that are needed to protect cyberspace and should continue to build even better ones, he said during his keynote at Defcon. He gave the example of Metasploit and other penetration testing tools.

"Sometimes you guys get a bad rap," he said. "From my perspective, what you're doing to figure out vulnerabilities in our systems is great. We have to discover and fix those. You guys hold the line," he said.

Gen. Alexander's presence at Defcon was a rare event. Before introducing him to the stage, Defcon founder Jeff Moss, who is the chief security officer of ICANN and a member of the U.S. Homeland Security Advisory Council, revealed that he has tried for the past 20 years to get a high-ranking NSA official to speak at the conference.

"Like magic, on our 20th anniversary and NSA's 60th anniversary it's all come together," Moss said. "For me it's really eye-opening to see the world from their [NSA's] view."