Read this paper as well <a href="https://www.usenix.org/system/files/conference/usenixsecurity14/sec14-paper-mowery.pdf" rel="nofollow">https://www.usenix.org/system/files/conference/usenixsecurit...</a> -- the researchers who wrote it obtained (on the surplus market) the same model of backscatter X-ray body scanner that was deployed in US airports and proceeded to evaluate and validate various schemes for undetectable concealment of firearms, knives, and explosives. Other research was done, including a software implant that would selectively strip indications of contraband from the image (when a specific signal pattern was in the image), and a firmware implant that would disable all interlocks and deliver an elevated radiation dose. If it is possible for a few university researchers to obtain this sort of security scanner without resorting to crimes, it is folly to assume that actual adversaries cannot.<p>We need better methods for detecting energetic materials (and not just suspicious shapes) in luggage (detecting it concealed inside humans is harder and would require transmission x-ray which is not currently acceptable for airport security screening) that don't crumble when the adversary has unlimited access to the security scanner to test concealment methods.<p>Concealing knives is kinda easier than concealing energetic materials -- it is, for example, possible to machine a blade that fits perfectly inside a blunt, innocuous "sheath" so precisely (hi, wire EDM) that they appear as a single piece of metal. However, this is not so much of an issue nowadays, as no amount of threatening or stabbing with a knife will make a pilot open the cockpit door.