[jeremyw]

He always seems to fail to recognize the principles of defense-in-depth...

That's a broad claim given that Schneier's built his reputation/business around defense-in-depth. It's practically his hobby horse and I don't read any such misunderstanding in his essays on the TSA.

[ghshephard]

Of course he has built his reputation on it - I've probably learned half of what I know about defense-in-depth from Schneier. The thing is, when it comes to aviation security, he has a tendency to discount the importance of preventing bad things from getting on airplanes, and over emphasize the importance of preventing bad people from getting on airplanes.

I think they are both important. The TSA clearly thinks the "prevent bad things from getting on airplanes" is a more containable problem. We have the NSA/CIA/FBI and other intelligence services that will work on the bad-people problem, which in no way means we can't simultaneously dedicate resources to the "bad things on airplane problem."

As an aside, our last line of defense is clearly "Prevent Bad people from doing bad things on airplanes" - which is how both the christmas underwear bomber and the shoe bomber were stopped.