Friday, January 07, 2005

Microwaves, Faraday Cages, and the Xenaverse

In a recent CNN article, Passport chips raise privacy concerns, there are a number of disturbing quotes made by the author that question whether the author really was ready to publish.

The worst of them is:
Wrapping your passport in aluminum foil actually works. It is called a "Faraday Cage," and it's the same thing that protects you from the microwaves as you watch your popcorn pop. The foil blocks electromagnetic waves so a nearby chip reader can't force your passport chip to perk up and say "howdy."

Now, let's put aside the fact that it is completely unnecessary to mention Faraday Cages here. It would have been sufficient to just say that wrapping a passport in aluminum foil prevents remote access (though that's not necessarily true, but we'll get to that). Let's put aside the fact that it is COMPLETELY UNNECESSARY to talk about electronics PERKING UP TO SAY HOWDY. After that, we're left with the fact that the thing that protects you from microwaves while popping popcorn is NOT so much a Faraday Cage but the SKIN EFFECT. Faraday Cages work great at low frequencies. However, at high frequencies the time it takes for the charge carriers within the metal to compensate for the oscillations of the field is not negligible! Any lag and the Faraday Cage will cease to work! So for high frequencies, it is the "skin effect" that prevents radiation from escaping. Microwaves attenuate very quickly in a lossy material like copper, and thus very few microwaves can escape the unit. For copper, the "skin depth" can be as little as micrometers (millionths of a meter). All of that energy gets dissipated in the resistance of the copper metal surrounding the microwave.

So those are her big strikes. She brings up technical detail that's not needed. She makes reference to electronics saying "howdy," and finally she makes a completely inappropriate reference to Faraday Cages.

But we're lucky enough to spot some other problems with this article. In particular:

The passport chips will hold much more data, from 64 kilobytes to eventually 514 kilobytes, as much as the first personal computers. They will hold the same information as a paper passport plus a digitized photo and face template for the still-unproven facial recognition software, which also is supposed to identify you from a distance, unnoticed.

Now, that's not supposed to be 514 kilobytes. That's supposed to be 512 kilobytes. Thus, her editors also are to blame here. But then she goes on to say how that is "as much as the first personal computers." This statement is not necessary. It could have just as equally been applied to "64 kilobytes," and it is completely irrelevant. It is meant to imply that this is a LOT of data. However, the first personal computers were not storing RECOGNIZABLE IMAGES. The amount of information entropy in a recognizable image is going to take up most of that space.

And on top of all of this, her language is very poor. For example:

The fact that passport data can be read unencrypted, with no physical contact, from up to 30 feet away, according to the American Civil Liberties Union, upsets privacy advocates for two reasons.

This is a one sentence paragraph. I would argue that nearly every word could be reordered, but then I would feel petty.

And at the end of the article, after that Faraday Cage mess, we get:

Try it out with your work I.D. card or a toll-booth pass. I wouldn't recommend wrapping your micro-chipped dog in Reynolds Wrap, however. Neighbors might think you were planning a barbecue.


So I decided to look up this woman to see if she had any contact information available. It looks like she does. She has HER OWN WEBSITE at Personally, I think the domain name is very appropriate. On her site, she has links to her doctoral dissertation. The title of her doctoral dissertation involves the Xenaverse. Sounds like some interesting research.

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