Yesterday, the blogosphere crackled with news that ‘net surfers could use a website to generate fake boarding passes that would enable them to slip past airport security and gain access to airport concourses. The news provides a good opportunity to illustrate a credentialing (and identity) system, how it works, and how it fails.

It’s very complicated, so I’m going to try to take it slowly and walk through every step.

The Computer Assisted Passenger Prescreening System (CAPPS) separates commercial air passengers into two categories: those deemed to require additional security scrutiny — termed “selectees” — and those who are not. When a passenger checks in at the airport, the air carrier’s reservation system uses certain information from the passenger’s itinerary for analysis in CAPPS. This analysis checks the passenger’s information against the CAPPS rules and also against a government-supplied “watch list” that contains the names of known or suspected terrorists.

Flaws in the design and theory of the CAPPS system make it relatively easy to defeat. A group with any sophistication and motivation can test the system to see which of its members are flagged, or what behaviors cause them to be flagged, then adjust their plans accordingly.

A variety of flaws and weaknesses inhabit the practice of watch-listing. Simple name-matching causes many false positives, as so many Robert Johnsons will attest. But the foremost weakness is that a person who is not known to be a threat will not be listed. Watch-listing does nothing about people or groups acting for the first time.

In addition, a person who is known and listed can elude the system by using an alias. The use of a false or synthetic identity (and thus an inaccurate boarding card) could assist in this. But the simplest wrongful use of this fake boarding card generator would be to make a boarding card that allows a known bad person to receive no more security scrutiny than all the good people.

When CAPPS finds that a passenger should be given selectee status, this is transmitted to the check-in counter where a code is printed on the passenger’s boarding pass. At the checkpoint, the boarding pass serves as a credential indicating that the person is entitled to enter the concourse, and also indicating what kind of treatment the person should get — selectee or non-selectee. The credential is tied to the person bearing it by also checking a government-issued ID.

In a previous post, I included a schematic showing how identification cards work (from my book Identity Crisis). This might be helpful to review now because credentials like the boarding pass work according to the same three-step process: First, an issuer (the airline) collects information, including what status the traveler has. Next, the issuer puts it onto a credential (the boarding pass). Finally, the verifier or relying party (the checkpoint agent) checks the credential and accords the traveler the treatment that the credential indicates.

Checking the credential bearer’s identification, a repeat of this three-step process, and comparing the names on both documents, ties the boarding pass to the person (and in the process imports all the weaknesses of identification cards).

Each of these steps is a point of weakness. If the information is bad, such as when a malefactor is not known, the first step fails and the system does not work. If the malefactor is using someone else’s ticket and successfully presents a fake ID, the third step has failed and the system does not work.

The simple example we’re using here breaks the second step. A person traveling under his own name may present a boarding pass for the flight for which he has bought a ticket — but the false boarding pass he presents does not indicate selectee status. He has eluded the CAPPS system and the watch list.

The fake boarding pass generator does not create a new security weakness. It reveals an existing one. Though some people may want to, it’s important not to kill the messenger (who, in this case, is a Ph.D. student in security infomatics at Indiana University who created the pass generator to call attention to the problem). As I’ve said before, identity-based security is terribly weak. Its costs — in dollars, inconvenience, economic loss, and lost privacy — are greater than its security benefit.

Hopefully, the revelation that people can use fake boarding passes to elude CAPPS and watch-lists is another step in the long, slow process of moving away from security systems that don’t work well, toward security systems that do. Good security systems address tools and methods of attack directly. They make sure all passengers on an airplane lack the capacity to do significant harm.