An unexpected visitor gave my team the evidence we needed to prove that the government was secretly wiretapping Americans. Room 641A, which housed the “Big Brother machine,” inside AT&T’s Folsom Street building in San Francisco, California. Source: Mark Klein / The Electronic Frontier Foundation . On January 20, 2006, the front doorbell rang at the Electronic Frontier Foundation’s offices on Shotwell Street in the Mission District of San Francisco. At the time, Shotwell Street wasn’t the glamorous part of the Mission. Our offices sat between two auto repair shops, across the street from a utility substation. The sidewalk was often dotted with homeless people’s tents. At one point, San Francisco did a survey, and our block of Shotwell Street had the highest reported amount of human feces in the whole city. We had many people down on their luck ring that doorbell. Some were just lost.…