Contactless bank cards

Peter Tomlinson pwt at
Fri Nov 19 09:30:21 GMT 2010

Roland Perry wrote:
> ps A paywave and an Oyster presumably don't interfere, because 
> Barclays have a card with both.
To labour the point a little, the interference discussed here has two 

(1) When more than one card responds to the terminal's RF field . There 
is a provision in the 14443 standard for anti-collision processing, but 
the characteristics of the interface between card and terminal are such 
that anti-collision may fail, or the terminal may not even implement the 
function. (And the rule breaking blocker card would no doubt be designed 
to start shouting continuously as soon as it has enough power to start 
up, rather than simply indicate that its there and waiting.)

(2) Having two cards in very close proximity to each other (e.g. in a 
closed wallet) affects the electrical characteristics of the RF 
circuitry of both cards, so that data comms may well be compromised, or 
it may even compromise the ability of the cards to collect enough power 
to operate.

Where there is more than one application hosted on one card, then that 
card will wait for an application select - and then only one app will 
become active - or the card may be programmed to pre-select one app, 
leaving the terminal to ask if it wants to see if another app is 
present. (TfL is now in the position where it will soon have to cope 
with 3 different on-card apps: Oyster, ITSO, EMV. Oyster and ITSO are 
both just data areas, but in the DESFire cards currently being issued 
these require app select; EMV is a microprocessor-based application.)

The carrier frequency used in the power transfer and data comms is 13.56 
MHz, but the aerial coil in the card is NOT tuned to that frequency. It 
is resonant at a rather higher frequency. The aerial coil in the 
terminal is tuned to something very near 13.56 MHz - but the presence of 
the card changes the exact resonant frequency of the terminal's aerial 
coil. I don't know anyone who has been able to create an exact 
mathematical model for this somewhat approximate technology. As I wrote 
in a July 2003 paper, having (as expected) found that different 
designers make different decisions about both card and terminal aerial 
coil characteristics:

    (I) have realised that those developing 14443 made unwarranted
    assumptions about the circuitry used around the two coils involved:
    card aerial coil and terminal aerial coil. One statement made to me
    is that it is assumed that the card’s coil will be tuned to around
    15 to 17 MHz, and the terminal’s coil to 13.56 MHz. Another
    statement is that adjusting the tuning of the terminal’s aerial coil
    changes the characteristics - you tune for maximum range but accept
    a blind spot close to the terminal, or you tune for no blind spot
    and the range reduces.

In practice it sems that card manufacturers try to make the resonant 
frequency of the aerial coil as high as possible (over 20 MHz), while 
maintaining power collecting performance. Some cards indeed use as large 
an area of coil as is reliably possible from a durability point of view, 
others have a very small coil (which is why we can have adequate 
performance from very small form factors).


More information about the ukcrypto mailing list