Contactless bank cards

Ian Mason ukcrypto at sourcetagged.ian.co.uk
Thu Nov 18 13:39:03 GMT 2010

```On 18 Nov 2010, at 00:46, Tom Thomson wrote:

>> I've never actually found a machine to try my card out upon. But the
>> Oyster pads in London require the card to be pretty much out of a
>> wallet
>> and touching the surface. Similarly the RFID cards used on the
>> buses in
>> Nottingham. The technology is dozens of order of magnitude away from
>> scanning the bus pass in the passenger's pocket as he gets on board.
>> --
>> Roland Perry
>
> Well Roland, I usually understand "orders of magnitude" as decimal
> ones,  But even if I were to assume that you meant binary orders of
> magnitude (which would usually suggest to me an intention to
> mislead, but let us assume that although you meant binary you had
> no such intention) "dozens" means a factor of at least 2 to the
> 24th, or something a bit bigger that 2 times 10 to the 7. If I
> guess that there's an inverse square law in there somewhere, I get
> something over 1.4 times 10 cubed on the distance - and I have
> happily waved my oyster card at more than a centimetre from the
> reader on buses in London (it seemed to need to be closer on tubes,
> I guess because the S/N ration is lower in tube stations). So I
> guess that dozens of orders of magnitude technology change suggests
> at readability at something like 14 metres, which is quite a bit
> more than the distance from my shirt pocked to the reader when I
> board a bus.  If your orders of magnitude are honest decimal ones,
> and not binary ones as I have cynically assumed, it comes to such
> an enormous distance that I don't want to contemplate it.  So I
> think you are probably wrong in your assertion.
>
> Mìcheal
>

If we want to put this into perspective with some real world results
there's a paper here <http://www.eng.tau.ac.il/~yash/kw-usenix06/
index.html> where a reading range of 25 cm was achieved with USD 110
worth of hardware. This paper is 4 years old and may not even
represent the state of the art at the time.

Ian
```