On the sixth floor of an AT&T building on Folsom Street in San Francisco, a locked room labeled 641A held the hardware that gave the NSA a copy of every byte of internet traffic passing through. The man who figured this out was a retired AT&T technician named Mark Klein, and in January 2006 he walked into the lobby of the Electronic Frontier Foundation a few blocks away and started talking. Cindy Cohn, then EFF's legal director, recounts the encounter in The MIT Press Reader , in an excerpt from her book Privacy's Defender . Klein told EFF's executive director Shari Steele that he could prove the NSA was tapping the internet backbone at scale. Since 2003, he had run the seventh-floor room on Folsom Street where AT&T's fiber-optic backbone connected to the wider internet. Those cables also ran a flight down, into 641A — a room only employees with NSA security clearances could enter. Beside the secret room sat a splitter cabinet.…