Stewart Baker, former director of policy at the Department of Homeland Security, the parent agency of the TSA, took me to task for my recent posting about the new TSA boarding pass scanners being installed at airport security checkpoints.

My observation was that the ID checkpoint is insufficient and in the wrong place; fixing the Schneier/Soghoian attack requires that a strong ID check be performed at the boarding gate, which the new system still doesn't do. Stewart says that the TSA security process doesn't care what flight someone is on as long as they are screened properly and compared against the "no fly" list.

Maybe it doesn't; the precise security goals to be achieved by identifying travelers have never been clearly articulated, which is an underlying cause of this and other problems with our aviation security system. But the TSA has repeatedly asserted that passenger flight routing is very much a component of their name screening process. For example, the regulations governing the Secure Flight program published last October in the Federal Register [pdf] say that "... TSA may learn that flights on a particular route may be subject to increased security risk" and so might do different screening for passengers on those routes. I don't know whether that's true or not, but those are the TSA's words, not mine.

Anyway, Stewart's confusion about the security properties of the protocol, and about my reasons for discussing them notwithstanding, the larger point is that aviation security is a complex (and interesting) problem in the discipline that I've come to understand as "human-scale security protocols".

I first wrote about human scale security as a computer science problem back in 2004 in my paper Toward a broder view of security protocols [pdf]. Such protocols share much in common with the cryptographic authentication and identification schemes used in computing: they're hard to design well and they can fail in subtle and surprising ways. Perhaps cryptographers and security protocol designers have something to contribute toward analyzing and designing better systems here. We can certainly learn something from studying them.