September 18, 2020

Rescorla on Airport ID Checks

Eric Rescorla, at Educated Guesswork, notes a flaw in the security process at U.S. airports – the information used to verify a passenger’s ID is not the same information used to look them up in a suspicious-persons database.

Let’s say that you’re a dangerous Canadian terrorist, bearing the clearly suspicious name “Guy Lafleur”. Now, the American government is aware of your activities and puts you on the CAPPS blacklist to stop you from boarding the plane. Further, let’s assume that you’re too incompetent to get a fake ID….

You have someone who’s not on the blacklist buy you a ticket under an innocuous assumed name, say “Babe Ruth”. This is perfectly legitimate and quite easy to do…. Then, the day before the flight you go onto the web and get your boarding pass. You print out two copies, one with your real name and one with the innocuous fake name. Remember, it’s just a web page, so it’s easy to modify When you go to the airport, you show the security agent your “Guy Lafleur” boarding pass and your real ID. He verifies that they match but doesn’t check the watchlist, because his only job is to verify that you have a valid-looking boarding pass and that it matches your ID. Then, when you go to board the plane, you give the gate agent your real boarding pass. Since they don’t check ID, you can just walk onboard.

What’s happened is that whoever designed this system violated a basic security principle that’s one of the first things protocol designers learn: information you’re using to make a decision has to be the information you verify. Unfortunately, that’s not the case here. The identity that’s being verified is what’s written on a piece of paper and the identity that’s being used to check the watchlist is in some computer database which isn’t tied to the paper in any way other than your computer and printer, which are easy to subvert.

In a later post, he discusses some ways to fix the problem.