This article http://www.bbc.co.uk/news/business-22545804 appears to
vindicate what I have always said about contactless cards.

Not only are they vulnerable to fraud by villains with scanners who
don't need to get close enough to arouse suspicion, but it now seems
that they're also vulnerable to *accidental* use!

I'm very glad that I ditched my CapitalOne card when they insisted [1]
on replacing it with a contactless card.

More and more financial institutions are now using these things, making
it very difficult to avoid unless you do away with credit and debit
cards altogether. I've unfortunately still ended up with one or two, but
they never leave the house and are used only for on-line transactions.

The sooner the whole idea is re-thought, the better!

[1] Using the very dubious tactic of pretending that my previous card
had been "compromised" (which I'm damn sure it hadn't!) in order to
provide a pretext for needing to issue a new card long before the old
one had expired.
--
Cheers,
Roger
____________
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checked.

Can always re-purpose the tin foil from the hat to screen the card

On Monday, May 20, 2013 2:22:00 PM UTC+1, Lee wrote:
Can always re-purpose the tin foil from the hat to screen the card

You would need a ferrite-based card holder to deny magnetic field access.
Could be a business opportunity.


rusty

On 05/20/2013 02:13 PM, Roger Mills wrote:
This article http://www.bbc.co.uk/news/business-22545804 appears to
vindicate what I have always said about contactless cards.

Not only are they vulnerable to fraud by villains with scanners who
don't need to get close enough to arouse suspicion, but it now seems
that they're also vulnerable to *accidental* use!

I'm very glad that I ditched my CapitalOne card when they insisted [1]
on replacing it with a contactless card.

More and more financial institutions are now using these things, making
it very difficult to avoid unless you do away with credit and debit
cards altogether. I've unfortunately still ended up with one or two, but
they never leave the house and are used only for on-line transactions.

The sooner the whole idea is re-thought, the better!

[1] Using the very dubious tactic of pretending that my previous card
had been "compromised" (which I'm damn sure it hadn't!) in order to
provide a pretext for needing to issue a new card long before the old
one had expired.

There's several Youtube videos on disabling them but I suspect they're
using US cards pre-Chip and Pin and it looks like you'd damage that area
as well.
Andy C

On 05/20/2013 03:08 PM, Tim Streater wrote:
In article ,
Roger Mills wrote:

This article http://www.bbc.co.uk/news/business-22545804 appears to
vindicate what I have always said about contactless cards.

Not only are they vulnerable to fraud by villains with scanners who
don't need to get close enough to arouse suspicion, but it now seems
that they're also vulnerable to *accidental* use!

I'm very glad that I ditched my CapitalOne card when they insisted [1]
on replacing it with a contactless card.

I don't believe I've seen one of these. Is it obvious what they are? Or
might I find that the CC company sent me one when the current card
expired and I didn't know it?

Put "contactless card symbol" into Google.

On Mon, 20 May 2013 06:38:41 -0700 (PDT), therustyone
wrote:

On Monday, May 20, 2013 2:22:00 PM UTC+1, Lee wrote:
Can always re-purpose the tin foil from the hat to screen the card

You would need a ferrite-based card holder to deny magnetic field access.
Could be a business opportunity.


Mu-metal.

--
Frank

On 20/05/2013 14:38, therustyone wrote:
On Monday, May 20, 2013 2:22:00 PM UTC+1, Lee wrote:
Can always re-purpose the tin foil from the hat to screen the card

You would need a ferrite-based card holder to deny magnetic field access.
Could be a business opportunity.

Here's one company that has been offering products for several years. I
expect there are others.

http://difrwear.com/

Bernie

On 20/05/2013 15:42, Tim Streater wrote:
In article ,
Andy Cap wrote:

On 05/20/2013 03:08 PM, Tim Streater wrote:
In article ,
Roger Mills wrote:

This article http://www.bbc.co.uk/news/business-22545804 appears to
vindicate what I have always said about contactless cards.

Not only are they vulnerable to fraud by villains with scanners who
don't need to get close enough to arouse suspicion, but it now seems
that they're also vulnerable to *accidental* use!

I'm very glad that I ditched my CapitalOne card when they insisted [1]
on replacing it with a contactless card.

I don't believe I've seen one of these. Is it obvious what they are? Or
might I find that the CC company sent me one when the current card
expired and I didn't know it?

Put "contactless card symbol" into Google.

I see, thanks. Hmmm, looks like it's too late.


If you carry it around, keep it in a lead sheath!
--
Cheers,
Roger
____________
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checked.

On 20/05/2013 14:38, therustyone wrote:
On Monday, May 20, 2013 2:22:00 PM UTC+1, Lee wrote:
Can always re-purpose the tin foil from the hat to screen the card

You would need a ferrite-based card holder to deny magnetic field access.
Could be a business opportunity.


rusty

But if the same issue applies to NFC mobiles, the ferrite will play
merry hell with recpetion...

--
Rod

Roger Mills wrote:


If you carry it around, keep it in a lead sheath!

Silver is best, then copper, but aluminium foil seems quite good, as
tested with Oyster cards, which use the same near field communication
standards. Lead is likely to be relative non-conductive. Sandwiching
them between two Oyster cards may also trigger a multiple card error.

You can get foil lined card sheaths, and even foil lined card wallets
(e.g. google "rfid blocking wallet").

The foil probably detunes the circuit, as well as screening it.

On Monday 20 May 2013 14:38 therustyone wrote in uk.d-i-y:

On Monday, May 20, 2013 2:22:00 PM UTC+1, Lee wrote:
Can always re-purpose the tin foil from the hat to screen the card

You would need a ferrite-based card holder to deny magnetic field access.
Could be a business opportunity.


rusty

Or you could just tell your bank to replace them...

--
Tim Watts Personal Blog: http://squiddy.blog.dionic.net/

http://www.sensorly.com/ Crowd mapping of 2G/3G/4G mobile signal coverage

Reading this on the web? See:
http://wiki.diyfaq.org.uk/index.php?title=Usenet

On 20/05/2013 15:42, Tim Streater wrote:
In article ,
Andy Cap wrote:

On 05/20/2013 03:08 PM, Tim Streater wrote:
In article ,
Roger Mills wrote:

This article http://www.bbc.co.uk/news/business-22545804 appears to
vindicate what I have always said about contactless cards.

Not only are they vulnerable to fraud by villains with scanners who
don't need to get close enough to arouse suspicion, but it now seems
that they're also vulnerable to *accidental* use!

I'm very glad that I ditched my CapitalOne card when they insisted [1]
on replacing it with a contactless card.

I don't believe I've seen one of these. Is it obvious what they are? Or
might I find that the CC company sent me one when the current card
expired and I didn't know it?

Put "contactless card symbol" into Google.

I see, thanks. Hmmm, looks like it's too late.

I seem to recall that my bank wrote to me saying they were going to
issue these things, and to object if you did not want one. I did, and
later they sent one anyway. I wrote and pointed out I had requested not
to have one, and they said basically "tough, its policy now!"


--
Cheers,

John.

/================================================== ===============\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\================================================= ================/

On 20/05/2013 15:39, Frank Erskine wrote:
On Mon, 20 May 2013 06:38:41 -0700 (PDT), therustyone
wrote:

On Monday, May 20, 2013 2:22:00 PM UTC+1, Lee wrote:
Can always re-purpose the tin foil from the hat to screen the card

You would need a ferrite-based card holder to deny magnetic field access.
Could be a business opportunity.

Mu-metal.

Probably needs to be a sandwich of mu-metal inside copper inside silver
to get maximum screening in minimum thickness.

According to that R4 consumer programme some M&S terminals were
particularly aggressive about using contactless cards at a distance.

It is clear folly to have contactless unvalidated transactions. No-one
needs to be able to buy things whilst wizzing past on a water slide!

--
Regards,
Martin Brown

On 20/05/2013 15:08, Tim Streater wrote:
In article ,
Roger Mills wrote:

This article http://www.bbc.co.uk/news/business-22545804 appears to
vindicate what I have always said about contactless cards.

Not only are they vulnerable to fraud by villains with scanners who
don't need to get close enough to arouse suspicion, but it now seems
that they're also vulnerable to *accidental* use!

Advantage is they can probably only grab £20 at a time.

I'm very glad that I ditched my CapitalOne card when they insisted [1]
on replacing it with a contactless card.

I don't believe I've seen one of these. Is it obvious what they are? Or
might I find that the CC company sent me one when the current card
expired and I didn't know it?

Depends how good you are at playing "guess the icon".

Contactless cards icon is "))))" with each one getting bigger.

If you have a Barclaycard chances are it as this functionality now.

--
Regards,
Martin Brown

In uk.d-i-y Andy Cap wrote:
There's several Youtube videos on disabling them but I suspect they're
using US cards pre-Chip and Pin and it looks like you'd damage that area
as well.

A carefully placed drill hole should do the trick.

If anyone asks, you drilled it to keep cards on your keychain like those
Tesco Clubcard thingies.

Theo

On 20/05/2013 18:00, Martin Brown wrote:

Probably needs to be a sandwich of mu-metal inside copper inside silver
to get maximum screening in minimum thickness.

Looking at BS ISO/IEC 14443-2 reveals that the operating frequency for
these cards is 13.56 MHz - one of the ISM 'free-radiation' freqs.

Hence low-frequency screening techniques such as mu-metal aren't
particularly relevant. Wrapping the card in aluminium foil ought to
work fine. Ideally all the seams should make good contact, but you're
not looking for a particularly high screening factor and more or less
any sort of overall wrap ought to do the trick.

--
Andy

On 20/05/2013 18:03, Martin Brown wrote:
On 20/05/2013 15:08, Tim Streater wrote:
In article ,
Roger Mills wrote:

This article http://www.bbc.co.uk/news/business-22545804 appears to
vindicate what I have always said about contactless cards.

Not only are they vulnerable to fraud by villains with scanners who
don't need to get close enough to arouse suspicion, but it now seems
that they're also vulnerable to *accidental* use!

Advantage is they can probably only grab £20 at a time.

But they can probably do it multiple times before being asked for a PIN
- and rack up quite a bill in the process.
--
Cheers,
Roger
____________
Please reply to Newsgroup. Whilst email address is valid, it is seldom
checked.

Roger Mills explained on 20/05/2013 :
This article http://www.bbc.co.uk/news/business-22545804 appears to vindicate
what I have always said about contactless cards.

Not only are they vulnerable to fraud by villains with scanners who don't
need to get close enough to arouse suspicion, but it now seems that they're
also vulnerable to *accidental* use!

I'm very glad that I ditched my CapitalOne card when they insisted [1] on
replacing it with a contactless card.

More and more financial institutions are now using these things, making it
very difficult to avoid unless you do away with credit and debit cards
altogether. I've unfortunately still ended up with one or two, but they never
leave the house and are used only for on-line transactions.

The sooner the whole idea is re-thought, the better!

[1] Using the very dubious tactic of pretending that my previous card had
been "compromised" (which I'm damn sure it hadn't!) in order to provide a
pretext for needing to issue a new card long before the old one had expired.

A sheet of cooking foil, in with the notes of your wallet, would
probably be enough to defeat it.

--
Regards,
Harry (M1BYT) (L)
http://www.ukradioamateur.co.uk

On 20/05/2013 17:44, John Rumm wrote:


I seem to recall that my bank wrote to me saying they were going to
issue these things, and to object if you did not want one. I did, and
later they sent one anyway. I wrote and pointed out I had requested not
to have one, and they said basically "tough, its policy now!"



CapitalOne didn't provide an option of any kind - they just sent out the
new cards. I had several irate phone conversations with their CS staff -
all to no avail - so I told them to stuff their card!

I assume they believe that if transactions are easier to carry out,
people will use them for small purchases in preference to cash, and they
(the card companies) will get lots of extra revenue.
--
Cheers,
Roger
____________
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checked.

On 20/05/2013 18:53, Theo Markettos wrote:
In uk.d-i-y Andy wrote:
There's several Youtube videos on disabling them but I suspect they're
using US cards pre-Chip and Pin and it looks like you'd damage that area
as well.

A carefully placed drill hole should do the trick.

If anyone asks, you drilled it to keep cards on your keychain like those
Tesco Clubcard thingies.

Theo

Do you have a template for where it needs to be drilled? [I assume that
it's not as simple as drilling out the contactless icon].
--
Cheers,
Roger
____________
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checked.

In article ,
Roger Mills wrote:
On 20/05/2013 18:53, Theo Markettos wrote:

Do you have a template for where it needs to be drilled? [I assume that
it's not as simple as drilling out the contactless icon].


If it's like our cards at work (mifare classic) then a small crack at the
edge is enough to break the antenna and stop it working :-(

Darren

On 05/20/2013 08:10 PM, Roger Mills wrote:


Advantage is they can probably only grab £20 at a time.

But they can probably do it multiple times before being asked for a PIN
- and rack up quite a bill in the process.

I believe it's £50/day max and they promise to refund any fraudulent
transactions.

Andy Wade wrote:
On 20/05/2013 18:00, Martin Brown wrote:

Probably needs to be a sandwich of mu-metal inside copper inside silver
to get maximum screening in minimum thickness.

Looking at BS ISO/IEC 14443-2 reveals that the operating frequency for
these cards is 13.56 MHz - one of the ISM 'free-radiation' freqs.

Hence low-frequency screening techniques such as mu-metal aren't
particularly relevant. Wrapping the card in aluminium foil ought to
work fine. Ideally all the seams should make good contact, but you're
not looking for a particularly high screening factor and more or less
any sort of overall wrap ought to do the trick.

At 13.56 Mhz the wavelength is something around 20 metres so any sort of
mesh with holes much smaller than that (hardly difficult!) would screen
it very effectively.

--
Chris Green
·

On 20/05/2013 20:31, D.M.Chapman wrote:
In article ,
Roger Mills wrote:
On 20/05/2013 18:53, Theo Markettos wrote:

Do you have a template for where it needs to be drilled? [I assume that
it's not as simple as drilling out the contactless icon].


If it's like our cards at work (mifare classic) then a small crack at the
edge is enough to break the antenna and stop it working :-(

Darren


Surely *someone* has access to an X-ray machine so that we can map how
to disable them without shafting the chip!

newshound wrote:

Surely *someone* has access to an X-ray machine so that we can map how
to disable them without shafting the chip!

By shining a bright LED torch though various MiFARE type cards I can
easily see the aerial wires running round the cards, but on a Barclays
contactless card I can't see anything other than the chip itself ...

On 20/05/2013 20:59, Andy Cap wrote:
On 05/20/2013 08:10 PM, Roger Mills wrote:


Advantage is they can probably only grab £20 at a time.

But they can probably do it multiple times before being asked for a PIN
- and rack up quite a bill in the process.

I believe it's £50/day max and they promise to refund any fraudulent
transactions.

So how do you prove that *you* didn't make the transaction?
--
Cheers,
Roger
____________
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checked.

On 20/05/2013 21:58, Huge wrote:
On 2013-05-20, Roger wrote:
On 20/05/2013 18:03, Martin Brown wrote:
On 20/05/2013 15:08, Tim Streater wrote:
In ,
Roger wrote:

This article http://www.bbc.co.uk/news/business-22545804 appears to
vindicate what I have always said about contactless cards.

Not only are they vulnerable to fraud by villains with scanners who
don't need to get close enough to arouse suspicion, but it now seems
that they're also vulnerable to *accidental* use!

Advantage is they can probably only grab £20 at a time.

But they can probably do it multiple times before being asked for a PIN

4

- and rack up quite a bill in the process.

Therefore £80



Not insignificant!
--
Cheers,
Roger
____________
Please reply to Newsgroup. Whilst email address is valid, it is seldom
checked.

On 20/05/2013 22:31, Andy Burns wrote:
newshound wrote:

Surely *someone* has access to an X-ray machine so that we can map how
to disable them without shafting the chip!

By shining a bright LED torch though various MiFARE type cards I can
easily see the aerial wires running round the cards, but on a Barclays
contactless card I can't see anything other than the chip itself ...


Good thinking. Works on two of my site passes but not the third. Can't
spot an obvious aeriel loop on any of my Visa cards, only one of them is
marked contactless enabled. Perhaps it is behind the ferrite stripe or
the tamper proof signature strip? One of the two holograms is "blotchy",
the other is completely opaque.

"Roger Mills" wrote in message
...
On 20/05/2013 20:59, Andy Cap wrote:
On 05/20/2013 08:10 PM, Roger Mills wrote:


Advantage is they can probably only grab £20 at a time.

But they can probably do it multiple times before being asked for a PIN
- and rack up quite a bill in the process.

I believe it's £50/day max and they promise to refund any fraudulent
transactions.

So how do you prove that *you* didn't make the transaction?

It's called "trust" LOL. Some years back someone was pulling from my debit
card, 2 or 3K ish. I didn't know until the statement arrived. Phoned the
bank, told them which payments weren't made by me and they simply refunded.
I maybe had to sign something, can't remember.

Roger Mills wrote:

On 20/05/2013 20:59, Andy Cap wrote:

I believe it's £50/day max and they promise to refund any fraudulent
transactions.

So how do you prove that *you* didn't make the transaction?

I think the boot's on the other foot

http://www.legislation.gov.uk/uksi/2.../contents/made

god knows where in that lot though ...

On Mon, 20 May 2013 20:14:37 +0100, Harry Bloomfield wrote:

Roger Mills explained on 20/05/2013 :
This article http://www.bbc.co.uk/news/business-22545804 appears to
vindicate what I have always said about contactless cards.

Not only are they vulnerable to fraud by villains with scanners who
don't need to get close enough to arouse suspicion, but it now seems
that they're also vulnerable to *accidental* use!

I'm very glad that I ditched my CapitalOne card when they insisted [1]
on replacing it with a contactless card.

More and more financial institutions are now using these things, making
it very difficult to avoid unless you do away with credit and debit
cards altogether. I've unfortunately still ended up with one or two,
but they never leave the house and are used only for on-line
transactions.

The sooner the whole idea is re-thought, the better!

[1] Using the very dubious tactic of pretending that my previous card
had been "compromised" (which I'm damn sure it hadn't!) in order to
provide a pretext for needing to issue a new card long before the old
one had expired.

A sheet of cooking foil, in with the notes of your wallet, would
probably be enough to defeat it.

http://www.lessemf.com/fabric.html



--
Use the BIG mirror service in the UK: http://www.mirrorservice.org
My posts (including this one) are my copyright and if @diy_forums on
Twitter wish to tweet them they can pay me £30 a post
*lightning surge protection* - a w_tom conductor

Roger Mills wrote:


But they can probably do it multiple times before being asked for a PIN

4

- and rack up quite a bill in the process.

Therefore £80



Not insignificant!

I think the reverification policy is actually an issue for the bank,
based on the perceived level of risk. They, or the card, might well
trigger a verification if there were a smaller number of transactions
all very close to the limit, or on a random basis.

In part the amount at risk is chosen to similar to the amount of cash
that might be lost to a pick pocket.

Bob Eager wrote:
On Mon, 20 May 2013 20:14:37 +0100, Harry Bloomfield wrote:


A sheet of cooking foil, in with the notes of your wallet, would
probably be enough to defeat it.

http://www.lessemf.com/fabric.html

A lot of those materials are not specified at the frequencies used for
the cards, which are in the short waves. Most of the materials seem to
be designed for microwave use.

On 20/05/2013 17:44, John Rumm wrote:
....
I seem to recall that my bank wrote to me saying they were going to
issue these things, and to object if you did not want one. I did, and
later they sent one anyway. I wrote and pointed out I had requested not
to have one, and they said basically "tough, its policy now!"


My bank wrote to me to ask if I wanted them and also offered me a
sticker to put on my mobile phone that would work as a contactless card.
I said no to both and my personal cards from that bank are not
contactless, although my business cards from the same bank are.

Colin Bignell

wrote:
Andy Wade wrote:
On 20/05/2013 18:00, Martin Brown wrote:

Probably needs to be a sandwich of mu-metal inside copper inside silver
to get maximum screening in minimum thickness.

Looking at BS ISO/IEC 14443-2 reveals that the operating frequency for
these cards is 13.56 MHz - one of the ISM 'free-radiation' freqs.

Hence low-frequency screening techniques such as mu-metal aren't
particularly relevant. Wrapping the card in aluminium foil ought to
work fine. Ideally all the seams should make good contact, but you're
not looking for a particularly high screening factor and more or less
any sort of overall wrap ought to do the trick.

At 13.56 Mhz the wavelength is something around 20 metres so any sort of
mesh with holes much smaller than that (hardly difficult!) would screen
it very effectively.

Aluminium wallets are sposed to do the trick.
http://www.ebay.com.au/sch/i.html?_nkw=aluminium+wallet

In uk.d-i-y Roger Mills wrote:
On 20/05/2013 18:53, Theo Markettos wrote:
In uk.d-i-y Andy wrote:
There's several Youtube videos on disabling them but I suspect they're
using US cards pre-Chip and Pin and it looks like you'd damage that area
as well.

A carefully placed drill hole should do the trick.

If anyone asks, you drilled it to keep cards on your keychain like those
Tesco Clubcard thingies.

Theo

Do you have a template for where it needs to be drilled? [I assume that
it's not as simple as drilling out the contactless icon].

I've not tried this, but:
http://linuxcentre.net/disabling-contactless-cards

The loop antenna goes around the edge, so any cut sufficiently deep into the
three edges furthest from the chip should do it. However that's not
mechanically stable (ie is liable to turn into a larger tear).

X-rays:
http://imageshack.us/a/img89/4470/cardswb.jpg

Based on the sample I have here, I think the top two cards may be from TCT
(look at the tiny print on the top right of the back of your card) - I think
that's Thames Card Technology (who provide the cards to the bank, but aren't
themselves the manufacturer). What's curious about these is the lack of
visible antenna on the X-ray. However it may be a polymer antenna not
metal.

Assuming the polymer antenna is in the same place as the copper on other
cards, drilling about 5mm in from the long edge of the card should do the
trick.

Theo

In message , David Woolley
writes
Bob Eager wrote:
On Mon, 20 May 2013 20:14:37 +0100, Harry Bloomfield wrote:


A sheet of cooking foil, in with the notes of your wallet, would
probably be enough to defeat it.
http://www.lessemf.com/fabric.html

A lot of those materials are not specified at the frequencies used for
the cards, which are in the short waves. Most of the materials seem to
be designed for microwave use.
Something I read earlier today suggested keeping two (or more) cards
close together as they interfere and cancel out any accidental
transaction. This certainly works for my security passes - the correct
card has to be removed from the stack in the card holder to work. A M&S
PoS terminal was tested by I can't remember whom and found to have a
maximum NFC range of 5cm.
--
Nick (=----)

In article ,
Nick wrote:

Something I read earlier today suggested keeping two (or more) cards
close together as they interfere and cancel out any accidental
transaction. This certainly works for my security passes - the correct
card has to be removed from the stack in the card holder to work. A M&S
PoS terminal was tested by I can't remember whom and found to have a
maximum NFC range of 5cm.


I wouldn't rely on that. I regularly travel in London with both my Oyster
card and my word ID card. Both are Mifare classics - if I have them both
in my wallet about 75% of the time the oyster readers get it right, 25% of
the time I get an error. Removing my id card cures it. Likewise the other
way around - word security readers barf around 25% of the time if my
oyster card is in my wallet.

So while it does appear to confuse it sometimes, it's far from a reliable
method! (might be different for non-mifare classic cards, but I'd not
rely on it).

Darren

Roger Mills wrote:
This article http://www.bbc.co.uk/news/business-22545804 appears to
vindicate what I have always said about contactless cards.

Not only are they vulnerable to fraud by villains with scanners who
don't need to get close enough to arouse suspicion, but it now seems
that they're also vulnerable to *accidental* use!

I'm very glad that I ditched my CapitalOne card when they insisted [1]
on replacing it with a contactless card.

More and more financial institutions are now using these things, making
it very difficult to avoid unless you do away with credit and debit
cards altogether. I've unfortunately still ended up with one or two, but
they never leave the house and are used only for on-line transactions.

The sooner the whole idea is re-thought, the better!

[1] Using the very dubious tactic of pretending that my previous card
had been "compromised" (which I'm damn sure it hadn't!) in order to
provide a pretext for needing to issue a new card long before the old
one had expired.
If you want to use the contactless facility every now and then, store it
in your wallet with an oyster card on top. The mifare system cannot
differentiate between two co located cards.

Otherwise just disable it by cutting a part or all the way through with
a Stanley knife as shown here on my barclaycard
http://i115.photobucket.com/albums/n...s/IMG_1790.jpg

This has no effect on the chip and pin function or the mag stripe swipe.

D.M.Chapman wrote:


I wouldn't rely on that. I regularly travel in London with both my Oyster
card and my word ID card. Both are Mifare classics - if I have them both
in my wallet about 75% of the time the oyster readers get it right, 25% of
the time I get an error. Removing my id card cures it. Likewise the other
way around - word security readers barf around 25% of the time if my
oyster card is in my wallet.

That's handled by the discovery protocol, which should try to establish
the serial number of the card and address further requests to other
cards. In theory it is supposed to be able to detect multiple numbers
and abort the transaction, but it sounds like it might not be as good as
it might be.

I believe the discovery protocol applies to all the ISO standard cards,
on that frequency, not just NXP's Mifare ones.