Knowledge Brings Fear

RFID in money errors

There are reports on various sites in the net that US $20 bills contain RFIDs. The "evidence" presented was that the bill caused anti-theft systems to sound the alarm and got burn-marks in the microwave.

It is rather obvious that this is nonsense. A bit of knowledge on how microwaves, RFIDs and anti-theft-systems work comes useful here. Lets begin with the anti-theft systems.

The simplest class of anti-theft systems works by measuring the drain on a HF field generator that is caused by introducing a resonant antenna into the field. The antenna is in the small tag thats placed on the protected products. There are various other kind of anti-theft systems, but these are quite frequent. The alarm goes invariably on if something suffiently resonant is in the field. Resonance can also be caused by a simple strip of metal, wire, metallized foil or pattern of conductive ink that has the right measurements.

The new $20 bills apparently does not contain a metal strip like the Euro bills. At least thats what I gather from the US Bureau of Engraving and Printing . The thread on Slashdot suggests that the metal components in the ink of the new bills cause a sufficiently good antenna when stacked. Single bills got no burnmarks.

For research purposes I microwaved a 5 Euro note and got the expected effect. Within a few seconds little sparks where visible on the metal stripe and on the metal printing on the right side and caused the expected burnmarks. From analyzing the note under a microscope it is clear that no RFID is in the Euro note. So much for debunking the "when microwaving makes burnmarks, it must be RFID" myth.

Now, for the "what if" department. Suggested reading for understanding the RFID-field is the RFID Handbook . RFIDs in money would need to solve one of the most critical problems of deploying tags into flexible materials in rough environments: how to bond the antenna to the actual chip. As far as I know this has not yet been done with sufficient robustness.

For destroying RFIDs in money, microwaving is not the brightest of all ideas. As we have seen above, it may damage the money visibly. The microwave frequency of 2.4 GHz is not optimally suited for coupling most of the radio energy into the antenna of lower frequency RFID tags. Of course some energy is coupled in, possibly enough to destroy the tag. But you risk damaging the money because a lot of energy is coupled in where you don't want it. Also, for food processing and other applications, microwave resistant tags are under development. So it cant be ruled out that the RFID in money will be microwave resistant at least to a certain point.

A more clever idea would be to use precisely the same frequency (or a half-wave of it) as the reader devices and just use much more power to couple enough energy into the tag to destroy the bond wires from chip to antenna. This will most probably not result in burn marks and cause no visible damage.

If the tag is placed always on the same position, just stamp it out with a hollow needle (like from a hypodermic syringe). This low-tech technique just leaves a small hole.

The interesting aspect is that RFID has already made it so much into the collective mind that the story was not rejected as bogus out of hand. This is somehow good, as public awareness tends to dampen the most excessive technology intrusions into real life.

What amused me to no end was the news that the US Department of Defense mandates RFID in all goods in purchases. I just wait for the news that some chinese or russian ammunitions vendor announces RFID-aware landmines. Would make the friend/foe distinction much simpler. Just explode if an enemy combat boot RFID tag is detected...

Also, placing RFID tags into personal identification documents like passports enables a completely new method of high-tech terrorism. Assassinating someone just requires replacing the doorsteps of some public places the person visits from time to time. The doorstep just waits for the right passport to come by before exploding. But thats probably collateral damage that must be tolerated so we all can feel more secure.

Posted by frank at March 3, 2004 12:46 AM | TrackBack