Your epapers, please?

Thomas Shaddack shaddack at ns.arachne.cz
Sun Apr 3 07:54:19 PDT 2005


On Thu, 31 Mar 2005, Major Variola (ret) wrote:

> At 10:08 PM 3/31/05 +0200, Eugen Leitl wrote:
> >   government plan to insert remotely readable chips in American
> >   passports, calling the chips [2]homing devices for high-tech
> >   muggers,
> 
> So the market for faraday-cages for your passport will grow to
> equilibrium.  A cage will cost less than a buck in parts, easily
> affordable by the clueful.  The damage to the clueless will
> quickly be the best advertising for the product.  Since we
> have been wearing conductive mesh burkhas for some time,
> the only inconvenience will be for the terahertz voyeurs
> employed by the TSA.

Beware of one gotcha. Faraday cage will shield only the electrical 
component. Low-frequency tags (125 kHz, typically) are magnetically 
coupled. Experiments shown that such tag is readable, even if entirely 
wrapped in aluminum foil. Laying a tag on top of a feromagnetic surface 
(iron sheet) does not help (probably only diminishes the range, didn't do 
the exact measurements yet); the sheet has to be between the tag's coil 
and the reader coil to be effective.

Putting the tag into an enclosure made of a feromagnetic material helps, 
though. Altoids can proved to be a pretty effective shielding.





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