Schneier on Security
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November 11, 2008
Reading a Letter from the Envelope it Was In
Paul Kelly and colleagues at Loughborough University found that a disulfur dinitride (S2N2) polymer turned exposed fingerprints brown, as the polymer reaction was initiated from the near-undetectable remaining residues.
Traces of inkjet printer ink can also initiate the polymer. The detection limit is so low that details of a printed letter previously in an envelope could be read off the inside of the envelope after being exposed to S2N2.
"A one-covers-all versatile system like this has obvious potential," says Kelly.
"This work has demonstrated that it is possible to obtain fingerprints from surfaces that hitherto have been considered extremely difficult, if not impossible, to obtain," says Colin Lewis, scientific advisor at the UK Ministry of Defence. "The method proposed has shown that this system could well provide capabilities which could significantly enhance the tools available to forensic scientists in the future."
Posted on November 11, 2008 at 7:55 AM
• 28 Comments
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> a disulfur dinitride (S2N2) ...
A rather dangerous reagent; it explodes if allowed to warm up to 30°C (86°F). It has to be generated in situ, and Kelly observes that the design of the generator "precludes portability."
> ... turned exposed fingerprints brown, as the polymer reaction ...
This polymer, by the way, is both an electrical conductor (a bit unusual), and also superconductor (but only at very low temperatures, colder than the boiling point of helium.)
One wonders if these properties could be used to detect the polymerisation at concentrations too low to leave a visible mark?
> The detection limit is so low that details of a printed letter previously in an envelope could be read off the inside of the envelope after being exposed to S2N2.
Wow. That is impressive. And not stated here, but described in the original article, the process is so sensitive they can even get an image through two sheets of paper -- so folding the letter so the ink faces inwards will not help much (although it will make reading harder, by superimposing more letters.)
One wonders, though, how much the image would be blurred if the letter could move around inside the envelope, instead of being held in place by a paperweight for the whole exposure period?
Great, now I have to shred my envelopes too.
Maybe wrapping your letter in black printed paper would be a good security measure.
Folding your letters with the writing on the inside might be enough to keep them secret, but that's hardly the point, is it?
Fold your letters on the inside of a sheet of newsprint first. Be sure to rub the newsprint sheet first to smear around the printing. Or, change to ink that is not sensitive to to the detection methond (how about laser printer toner?)
Would be funny if it reacted with the paper as well.
Just sayin ...
@a nonny bunny-
I'd try a random pattern, myself.
"Or, change to ink that is not sensitive to to the detection methond (how about laser printer toner?)"
Sorry no, Laser printer toner moves more easily from paper than dry InkJet ink.
Back before Aug 1980 I noticed that papers that had come from a photo copier when stored in a plastic A4 wallet and left for around six months under enough pressure for contact had transfered some of the toner across onto the plastic. So much so that it was easily visable to the naked eye and quite readable if you put a plain sheet of A4 in.
Further work by my then lady friend at University showed that not only did the toner come off onto plastic it would transfere to any surface it was left against immediatly.
Although the transfer process is slow the concentration builds up with time. With only moderate preasure (just enough to ensure contact) the image is sharp when viewed with appropriate equipment less so with only small spacing. She went on to find various methods of speeding the process up using various chemicals that have vapour tempratures around room temprature.
I developed simple equipment to see even very low concentrations of toner. It consisted of little more than an apropriate light source and fillters and tipping the contact surface through the light at an appropriate shallow angle so that it contrasted with the transfered toner. I discovered this trick by tipping other peoples unopened payslips throught the light in my first job...
Further I do know that the toner is easily visable at very very low concentrations using an Electron Microscope from mucking about with one a few years later.
[Urther super glue vapour helps bring it up just like it does other surface differences.
However the immediate practical use the students put my lady friends work to was transfering images to tee shirts for rag week...
The reason I know the date so well...
Well when she was looking for a project for her degree course I wrote to her (yes this was pre email/sms/mobile phone folks) and sent her an A4 wallet which showed the effect and it's original contents. She wrote back in the August returning the wallet and page along with a letter saying she'd got a new boy friend in the Student's Union who had been a rag week organiser...
I was partly responding to Roger's response, who in his second to last paragraph said: "they can even get an image through two sheets of paper -- so folding the letter so the ink faces inwards will not help much".
It's almost as if someone has been watching Beverly Hills Cop 2, and sounds like a more reactive version of the superglue trick.
It would have been nice to see the result of the 'paper inside the envelope' trick, as I have a feeling that messing with this stuff as a regular person might be a little dangerous. http://en.wikipedia.org/wiki/Disulfur_dinitride just sounds like an over all dangerous ingredient. So I would also imagine that there are lot of materials that it would destroy if it was allowed to react with it.
I wonder if you could hide traces of a message under a newly printed one so S2N2 becomes a key to unlock intentionally hidden text.
@Davi Ottenheimer - at worst, you could always write on the envelope (as opposed to the paper) with whatever substance you thought was secretive enough, yet could be picked up by your method (human sweat?)
I don't quite see the application on letters. Unless people routinely destroy letters, but not the envelopes? If so, use laserprint and fold printing to the inside. Laser toner should not be able to travel through paper, it is far to coarse and chemically inert. Remember that laser print sits on the surface, while ink sinks into the paper.
> I don't quite see the application on letters. Unless people routinely destroy letters, but not the envelopes?
Or, take the letter with them, or securely file it, but discard the envelope. Sure. Why not. Seems likely and of great value for security services, or more covert surveillance (rifle through trash, but not break into offices) for organized crime and corruption investigations, where paper trails add up.
In many cases, I can see bringing back things like the example, envelope, to a lab where the equipment could be made available.
@ Fred P, Davi Ottenheimer,
" ...you could always write... ...with whatever substance you thought was secretive enough, yet could be picked up by your method (human sweat?)"
From what I remember the search for secret ink finding reagents gave rise to thousands of tests some of which worked against each other.
Then somebody did some lateral thinking and realised that what they should be looking for was broken or disturbed fibers in the paper.
This gave rise to the tincture iodine vapour test, the oblique lighting test and later the ESDA test.
All of which appeared to drive another stake into reagent testing.
I guess this one has just revived the corpse...
Now, people cram lots of paper into their wallet, and unless they're George from Seinfeld they clean it out every once in a while.
If people have been following Bruce's advice to keep their passwords written down in their wallet (assuming they printed it out for legibility), you could conceivably recover them from any other bit of paper that happened to be wedged into the wallet next to them for some time...
Vanishingly small probability from a real-world security standpoint, but I see a movie plot thread in there somewhere...
@ Pat Cahalan,
" ...Bruce's advice to keep their passwords written down in their wallet... you could conceivably recover them from any other bit of paper that happened to be wedged into the wallet next to them for some time..."
To make it easier you need to first develop a reliable test for the "smell of money" to find the right bits of paper. ;)
Could someone please post the link to where Bruce suggests writing passwords down? It sounds like a counter intuitive security measure to me.
Nothing a security-envelope can't solve. The printing and ink use inside security-envelopes should distort anything like this to the point that its worthless anyway.
> Dave at November 11, 2008 9:21 AM:
> Great, now I have to shred my envelopes too.
What!!? You mean to say you weren't already? I shred all refuse (to mix in decoy matter with the confidential stuff), bag it, and incinerate. Can't be too safe.
On a positive note, I will be making invisible fingerpaint goatse's on all my outgoing envelopes from here out. I wonder if I could get finger oil in an inkjet cartridge....
"The printing and ink use inside security-envelopes should distort anything like this to the point that its worthless anyway."
Probably not for two reasons,
1, The print ink used in most security envelopes is likley to be considerably different to that used in your inkjet printer.
2, As noted by Roger, "This polymer, by the way, is an electrical conductor ".
Either or both of these will relativly easily alow you to distinquish between the security printing in the original envelopes and the polymer formed by either finger prints or inkjet printer ink and the disulfur dinitride (S2N2).
Very interesting exposure... I ask few expert chemistry people, but they didn't heard about this. Very helpful discover.
As the author of the original paper I'm glad to see it sparked some debate and generated some perceptive questions!
@ PK / Paul,
"I'm glad to see it sparked some debate and generated some perceptive questions!"
Any answers to them you'd care to share?
Well first a quick background to it. If you look here
this does report things before we realised all its possibilities. The fingerprints were noted first and then I had the idea of the inkjet work. God knows what prompted me to do that, though I had the idea in the bath!! I actually went back into work (pausing to get dressed of course!) and printed a test faint inkjet image and then put it into the apparatus we had set up with a S2N2 sample running. Next morning it became apparent we had an interesting effect on our hands. Initially we assumed it was a nucleation effect ie inkjet particles were causing it - until we realised that images were appearing on the opposite sides of exposed paper ie it had to be liquid component seeping through.
To address some of the issues raised (in order of appearance):
Yes, S2N2 is friction sensitive but the key feature of this kind of work is that you’re not manipulating it after you’ve made it - it is trapped out at liquid nitrogen temperatures and then as it warms it volatilises onto the samples in question, under vacuum.
The point about using the conductivity for detection is a good one; having said that, the polymer builds upon itself once formation has started - and so you really are magnifying/multiplying/however you want to put it very small amounts of the initial material that induced polymerisation in the first place. That’s the key point really - the amount of inkjet residues involved must be minuscule. So whether you’d need to go to even higher detection levels I’m not sure.
Of course there would be issues involving blurring etc in real life. But the effect is genuine and we even tested it in a “blind” trial - we got someone to pick an element symbol at random, print it out, put it in an envelope, leave it overnight, and then take the paper out. All we were left with was the envelope - but we were able to use this to correctly indicate his choice was rubidium.
Of course aside from anything involving inkjet, the fingerprint side of things is key as well - and we seem to be able to pick them off just about any medium.
So the bottom line is, if you receive a letter which has been printed in inkjet and was flat to the envelope, then we can take the discarded envelope and both read the text and lift your prints - at the same time!
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