Tag! What is it?

A reader writes:

How do the little rectangular anti-theft tags work?

I get how the big anti-theft stickers work. They've got an obvious square spiral antenna that I presume collects enough microwatts from an incoming signal to run a little transmitter that sends another signal out.

But the little tags don't have any circuitry inside. I cut one open, and there are just some tabs of springy metal in there - two pieces next to each other, and a smaller piece separated from the other two by a clear plastic membrane.

The metal sticks to a magnet, but that's the end of my ability to figure out what it does.

Is there invisible nanotechnology in these things, or something? Hey, maybe they're a placebo!


Magneto-acoustic security tag innards

If they're a placebo, the alarm systems in shops seem to really believe that it works.

What you're looking at there (here's a more elegant cutaway picture on Wikipedia) is called a magneto-acoustic, or acousto-magnetic, tag. Which is one of those things that doesn't really sound as if it ought to work, but does.

The first two of the three tabs inside are, I think, a couple of pieces of amorphous metal - which is quite an exotic material to be stuck to commonplace consumer items just to stop people stealing them. Amorphous metal is, in a way, the opposite of nanotechnology; it's metallic glass, special because it lacks the microscopic crystal structure of normal metals.

The third tab is a piece of less exotic, medium-coercivity metal. When that third piece is magnetised, the two other strips, which are sitting loose in their little plastic coffin, become quite easily moved by external magnetic fields. (They're amorphous metal because that's already unusually easy for external fields to move.)

The security gateways as you leave the store emit a pulsed magnetic field up in the tens of kilohertz, at the resonant frequency of the amorphous-metal strips. When next to their mildly-magnetised buddy, this quite tiny field causes the amorphous-metal tags to buzz, and to continue to buzz for a very brief moment after each pulse of the external field. This very brief "ringing" period causes a tiny change in the magnetic field of the third strip, which an antenna in the security gateway, very implausibly, detects. And off go the sirens.

The thingy at the checkout that deactivates the tags is a degaussing coil. It more-or-less demagnetises the third strip, which both reduces the magnetic sensitivity of the other two strips, and removes the field which the other two strips modulate. So now the sirens don't go off.

I am entirely unable to think about any security system without immediately trying to figure out ways to defeat it. (I try to avoid airports nowadays. They make me feel like Jackie Chan in a deckchair factory.)

One obvious but impractical way to defeat magneto-acoustic tags would be to degauss them yourself; I don't know how strong the degausser needs to be to achieve this, though. You might be able to pinch stuff if you just smuggled a CRT-screen degaussing wand into the shop, and found somewhere to plug it in.

Swiping your own rare-earth magnet across the tag would, if anything, probably make it work better (by more strongly magnetising the third strip), but I wonder if leaving a magnet or three stuck to the tag, in a Halbach array if you're fancy, might silence it. Just chopping it bodily off with a potato peeler would probably do the job too, of course, but where's the fun in that?

(If you can magnetise tags yourself with a ten-cent eBay magnet, then you could pry them off things you've bought, reactivate them, and attach them inconspicuously to things which other people may innocently carry into shops. You could, is all I'm saying.)

The square-antenna type of tag, by the way, is also pretty simple. It doesn't actually have anything fairly describable as a transmitter in it, but is rather a tuned circuit that resonates somewhere in the low megahertz. This makes it detectable, if a nearby transmitter/receiver combo rapidly sweeps its output through the relevant frequency range and looks to see if something is managing to suck up some energy at the appropriate frequency.

This kind of tag is deactivated by, essentially, blowing out the capacitor essential to their resonance with a higher-powered signal. I think a shoplifter could probably defeat these tags by just dragging a knife across them a couple of times, though, breaking the circuit. I haven't actually tried this, though, because it'd mean missing out on all of the fun of a good old-fashioned armed robbery.

Perhaps someone who's worked in retail since fancy security tags came into vogue will enlighten us in the comments.

I would also like to hear from anybody who's successfully used the "just lob the item high over the security gate and into the hands of your partner in crime" technique.

15 Responses to “Tag! What is it?”

  1. Amstrad Says:

    Honestly, just pretending as though you didn't hear the alarm and continuing on your way just as easily defeats these. Cashiers routinely forget to degauss them and the vast majority of shops have policies in place the prevent them from chasing you down once you're out the door anyway.
    Though, your mileage may vary from shop to shop on this. It tends to be the big box stores that pay less head to the detectors going off from my observation.

  2. nubie Says:

    First, my mother worked at Walmart in the US for a bit, and the security team watching the security cameras was told to let anyone carrying less than $50 worth of stolen merchandise leave the store unhindered. Why I don't know. She was to "watch the dressing rooms" for thievery, but then not report or confront the offenders?

    Anyway, before the movie rental company Blockbuster, ironically, went bust I was given a movie with the locking tab installed by a new/busy employee. I found that with a pair of rare earth magnets you can easily retract the little plastic locks with small lengths of iron rod inside. One magnet on the thumb and another on the third finger is all it takes.

    I was demonstrating my new found knowledge to a friend in the blockbuster using my bare hands and the strip came right out because it hadn't been locked in!

    Also come to find out my young sister had been collecting the locking strips because of the pretty yellow color and there was a large open box of them by the door. I suppose now that I should have started locking the DVD's we returned to confuse the store staff :)

  3. farnz Says:

    You miss the really easy way to take goods from shops that use the tags; the majority of stores have a policy that says that staff should not confront anyone; a few extend that to add "unless the staff are certain that a theft is in progress", and virtually no big store permits staff to act on their own initiative.

    Additionally, it's common for the tag disabling device to fail; this leads to two different methods to steal items, both of which would probably work (based on my experience of trying to leave stores with purchased items where the tag disabler hadn't worked). For both of them, you need to have something with a tag that you've purchased, as well as the item you're stealing.

    1) Just walk out confidently - you have a legitimate purchase with a tag that might not have been disabled, and thus a receipt. It's not worth the risk of trouble to challenge you on the alarm, as store management is likely to get upset with you for upsetting a customer.

    2) Hide the item you've purchased. When the alarm goes off, immediately find a member of staff. Wave the receipt (don't let them see what it says) and the tagged item at them, and complain that the tag hasn't been disabled. Chances are good that in the interests of avoiding a nasty scene on the shop floor, the employee will simply disable the tag and send you on your way.

    In both cases, you need enough chutzpah to cope if you're pulled up on it, and a plan for coping if they notice. I'm assuming, though, that any thief confident enough to try and either just walk out or social engineer an employee into helping with the theft also has enough talent to hand to cope if they're caught.

  4. Stoneshop Says:

    The little, magneto-acoustic, tags can be armed again after being disarmed, so they tend to be used for applications like libraries.

    Back whe I was in University I was involved in a record library, and with the introduction of CDs we were faced with the possible, ahem, unauthorized borrowing of said items (vinyl records are a bit harder to conceal, and were less attractive as well, so we didn't have as big a need to protect them). So we investigated the various systems available, and because of the required reversability the M-A was chosen. Disarming was done by what was essentially a big degausser, and arming by, indeed, a permanent magnet; both were part of a device we had to install in the desk.

  5. Stoneshop Says:

    Oh, forgot to mention the desk unit was about as big as a mid-tower PC case, with the degaussing components taking up well over 80% of the space). When activated it emitted a fairly loud *THUMP*, so all in all not exactly easy to bring a comparable-powered one with you and use it without being noticed. It also still failed to fully disarm about one in ten items, mostly multi-CD albums, so it would be far from a sure way to remain undetected.

    A mu-metal lunchbox would be the better bet.

    We did have a rather large-scale nicking of discs once, about 50 items over the course of two months, which we started to cotton on to after finding empty jewel cases hidden in a rarely-used section of the building. A video cam, and then some log analysis as to which persons had visited on all the days we saw activity in that section (covering a 5x40m area with a single BW cam in VHS resolution isn't likely to yield positive identification of someone we didn't know) yielded a list of just three suspects, and by carefully checking those three for the modus operandi as we had figured out made it possible to collar the perp.

  6. JsD Says:

    Isn't the obvious attack to quietly slip one of the teensy, unobtrusive acousto-magnetic tags inside, under or next to the gate, so the alarm wigs out constantly for no readily discernable reason until somebody turns it off, THEN wander out with the loot? :D

    • Fallingwater Says:

      Every gate I've ever seen was a sensible distance away from anything that might intrude. Any tags applied to or near it would have been easy to identify and remove, and anyone tired of the alarm sounding all the time would surely perform a cursory inspection before switching it off.

      The best attack is probably to simply find the tag and excise it from the item one wishes to steal. A pair of strong scissors (of the kind you can buy to make short work of plastic packaging) and a wire clipper oughta be all you'd need.

      This comes from someone who's never done it (seriously, all I've stolen in my life are plastic bags from the supermarket and a few packs of gum), but it sounds logical enough.

      This wouldn't work with tamperproof tags, of course, but those do typically include electronics and one assumes they'd be a huge pain to deal with. Homework for next week: build a tiny portable EMP generator to fry tamperproof tags... :P

      • Stoneshop Says:

        "build a tiny portable EMP generator to fry tamperproof tags..."

        Starting with a small flash unit (out of a disposable camera, for instance) would be my take on one.

  7. hagmanti Says:

    The ones at the Circuit City I used to work out (6 years ago, so grain of salt time) didn't sound when the stuff was too close to them (maybe it had to see both sides of the gate?), so that wouldn't have worked.

    On the other hand, just getting the side of the box that had the tag to brush up against one of the gates pretty much guaranteed walking out without setting off the alarm...


  8. whengreg Says:

    Whatever method you use, walk out at the same time as another shopper, to give the employees another target.

  9. monomer Says:

    The real question is whether all these magnets and radio waves at the entrances are harming us, or healing us.

  10. Stark Says:

    Yeah... I couldn't help myself so I replied to him.

  11. jul Says:

    Wouldn't a low tech Faraday cage enough to do the trick of preventing the activation of the tag ?
    Wrapping the tag in a conductor that has enough electrons able to move freely will normally automagically prevent the circuit to resonnate. (Faraday cage is basically an antenna that voids the electromagnetic field inside its volume).

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