Not until I walked with Bruce Schneier toward the mass of people unloading their laptops did it occur to me that it might not be possible for us to hang around unnoticed near Reagan National Airport’s security line. Much as upscale restaurants hang mug shots of local food writers in their kitchens, I realized, the Transportation Security Administration might post photographs of Schneier, a 48-year-old cryptographer and security technologist who is probably its most relentless critic. In addition to writing books and articles, Schneier has a popular blog; a recent search for “TSA” in its archives elicited about 2,000 results, the vast majority of which refer to some aspect of the agency that he finds to be ineffective, invasive, incompetent, inexcusably costly, or all four.

As we came by the checkpoint line, Schneier described one of these aspects: the ease with which people can pass through airport security with fake boarding passes. First, scan an old boarding pass, he said—more loudly than necessary, it seemed to me. Alter it with Photoshop, then print the result with a laser printer. In his hand was an example, complete with the little squiggle the T.S.A. agent had drawn on it to indicate that it had been checked. “Feeling safer?” he asked.

Ten years ago, 19 men armed with utility knives hijacked four airplanes and within a few hours killed nearly 3,000 people. At a stroke, Americans were thrust into a menacing new world. “They are coming after us,” C.I.A. director George Tenet said of al-Qaeda. “They intend to strike this homeland again, and we better get about the business of putting the right structure in place as fast as we can.”

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