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Comment by philipkglass | Hacker News Reader

My guess, already voiced by "Ron" on Schneier's blog: they're looking for nitrocellulose. Nitrated paper is not obviously different in appearance from ordinary paper. Sheets of nitrocellulose have negligible vapor pressure and little dust so they could be missed by other means like the "puffer" detectors.

If they're putting these materials through a separate machine my guess would be another detector based on optical spectroscopy - maybe Raman? Something like that. It wasn't clear from the article whether the paper products in their separate bin just went through the same inspection chute as everything else.

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They're not putting them through a different detector, though. They take out the books and all papers the passenger has, put them in one of the normal trays, and run the trays through the xray.

My wife traveled out of Kansas City last weekend and was subject to this. She's a teacher, and traveled with grading, so they took out tests, her books, everything that was paper. The line agent said they were worried about papers being used to obfuscate other things in the xray machine. But that was the line agent, so he could have been (a) uninformed or (b) lying.

Trying to de-clutter the images by separating paper would fit the observed procedure too. It seems a little odd that it would start only now. Maybe screener testing recently showed better performance when paper is separated out.

Or perhaps there was some intel, or some cases where luggage was searched post scanner that came up with items that should have been caught. That might trigger a revision in procedures to prevent or reduce future cases, which is how I would expect and want a program that was actually meant to reduce danger to operate.

Stacks of paper may have densities similar enough to certain restricted liquids and polymers that distinguishing one from the other via x-ray is difficult.

i.e. plastic explosive sheets sharing space in a file folder, or bound with surrounding paper into what looks like a legitimate book.

It may have involved some original research on the spectral properties of the material their looking for; in relation to the radiant energy they have available; and the spectral sensitivities of the sensors (radiant energy source spectra, object spectra, sensor spectra = information to derive a reflectance/absorption spectra for the object and match it with whatever you're looking for). But this research also requires any software changes and testing, and possibly changes to the light source (to change the wavelength(s) available), changes to filters on the light source or the sensor or both.

I think the limiting factor is the spectral response of this specific material, given a limited radiant energy source.

Another way to do this, would be via resolution. Maybe they aren't doing spectra matching, they might be able to differentiate the physical pattern of the material, sorta like a scanning microphotograph.

EDIT: One of the comments says, "They went through the Xray machine in a separate bin, much like your laptop does."

They say it is a test program. Maybe they are not testing if it is effective but rather how much it slows down the screening?

If this is the case (and I agree that it sounds likely) then what's the appropriate response from a bad guy? You don't take an action without considering a reaction...

Could this method be applied to fabric? Perhaps certain types of plastics? What if the bad guy hints that this is their plan with no actual intent other than misdirection. This could overwhelm screening resources, frustrating passengers, and stressing security agents while the actual threat could travel through another means, if it exists at all.

I traveled this weekend and can't help performing threat analysis against these security models. It's never made me feel better.

It doesn't need to be nitrocellulose, you can impregnate ordinary paper with military high explosive using an appropriate solvent. But I doubt that is the purpose here because that could be handled by normal explosive detection mechanisms.

Agreed. IMO the other plausible theory is disguising a bladed weapon of some kind in stacks of paper. The rest of the speculation appears to be based on misinterpretations of the headlines (scanning vs X-ray).

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