> ---------- > From: Tim May[SMTP:[EMAIL PROTECTED]] > Sent: Wednesday, April 10, 2002 1:59 PM > To: [EMAIL PROTECTED] > Subject: Re: Detectable cash notes a fantasy > > On Wednesday, April 10, 2002, at 09:27 AM, Trei, Peter wrote: > >> For paper money failure rates will probably be high anyway. > > Perhaps, perhaps not. Remember, the primary app for this is > > anti-counterfeiting. > > "Sir: ALL your $20 bills are failing authentication. Please wait > > while I call Security." > > So, if in fact we _are_ talking about each $20 bill having such a > transponder, then why are our arguments about how easy it will be to > shield against remote probing not valid? Put the money in a foil packet, > or fold it over, or carry it in a stack, or in a standard metal > briefcase, and I _guarantee_ that detecting it from afar will be > extremely difficult. > The argument against shielding is that it is obnoxious that I (or anyone) should have to go even further than I already do to maintain even a fraction of the privacy which was naturally available to every person 150 years ago.
Folding the bill won't make any difference. stacking them might make a small difference, if the chips are close enough to detune each other. Some transponders (not the mu-tag, AFAIK) include anti-collision techniques, so many can be detected simultaneously. > If a stack of bills containing these transponders are supposed to be > read from afar, way beyond what a "valid bill detector" is likely to be > engineered to do, I'd like to see the physics worked out. > Detection range turns out to be function of antenna size - the reader's antenna, not the one on the transponder. So if you have a big (eg, doorframe size) antenna, you can do a lot better than the 'valid bill detector' on the countertop. There's actually a privacy win here for the passive tags - the returned signal strength falls with the fourth power of the distance. > (A stack of bills, or cards, will have extremely poor radiation patterns > from any but the top or bottom bill, and probably their patterns won't > be good either.) > There's a basic faq at http://www.ti.com/tiris/docs/customerService/faq.htm > --Tim May > Peter
