5 ms·

Author here for all your NFC chip questions :-)

by kens 2y ago

cypherpunks01 2y ago
How difficult is it to clone MIFARE Ultralight EV1 chips? You mention the UID is signed, can you simply copy this signature? Do you just need to buy one of the magical chips of the same design, that allow uid/serials to be written? What is the actual mechanism behind the DESFire and other secure NFC chips that prevents cloning?
- kens 2y ago
  I haven't really looked into the security aspects. I think that you could clone one of the Ultralight chips, but it wouldn't gain you anything because the security is in the backend. It's a lot like a printed concert ticket or boarding pass. You could print as many as you want, but the ticket is still good for just one admission. The DESFire and other secure chips contain a cryptographic key that you can't access. Without the key, you can't make a clone of the chip. The cryptography provides authentication and encryption that you don't get with the cheap Ultralight chip. I think this is all market segmentation; they don't put more security into the Ultralight chip because they don't want to cannibalize their higher-end sales.
  - lxgr 2y ago
    Ultralight supports password authentication, and you can diversify the password from the serial number, meaning that until that password is revealed by a legitimate reader as part of a validation transaction (at which time the ticket is invalidated anyway), you can't clone it. Ultralight C does support actual cryptographic authentication.
    - kens 2y ago
      I don't think the password authentication helps against cloning. You could start a transaction and stop after you get the password. Then you could clone the card. (The system could invalidate the ticket as soon as they get the UID, but that would be a reliability nightmare since a failure during the read would invalidate someone's ticket.)
      - lxgr 2y ago
        You could do that, but it still greatly raises the cost for an attacker, since they need to hang around a ticket validator for every ticket they want to clone, as opposed to e.g. a QR code ticket, which can be trivially copied by a simple screenshot. Also, many of these transit systems are eventually consistent (they're usually offline-capable for resilience, but usually manage to send all validation transactions to a backoffice system within at most a day, and often minutes). This allows detecting duplicate usage fairly quickly. In systems where you need to tap out as well as tap in to leave the turnstile, that's where ticket inspectors might take a sudden interest in you if you tap out with a cloned ticket. In the end, as with most security systems, the goal is not to make fraud absolutely impossible, but to make it economically non-viable.
        szundi 2y ago
        You have to hang out to get the id of a simple 125khz tag too - this is what cloning means
  - amluto 2y ago
    In general, the card emulation devices (e.g. the chips in phones) try to avoid letting any arbitrary UID be set. This makes cloning these cards more difficult than it would otherwise be. It’s not terribly difficult to find devices (USB-connected things and battery-less cards) that do allow arbitrary UIDs to be set, though.
    - josephcsible 2y ago
      > the card emulation devices (e.g. the chips in phones) try to avoid letting any arbitrary UID be set. I can't think of a much worse way to do security. That feels like trying to flood the market with lockpicks that don't work instead of making a more pick-resistant lock.
      - amluto 2y ago
        I imagine this is because the locks contain chips from NXP (PN532 chips), the name-brand MIFARE chips are made by NXP, and the lock picks (also PN532 chips!) are made by NXP.
jeffbee 2y ago
Is this the same system used by Boston MBTA? I was surprised to see single-use tap cards when I visited there for the first time yesterday. I wondered why the ticket isn't reloadable.
- cypherpunks01 2y ago
  Most people who live in Boston use the reloadable CharlieCard (https://www.mbta.com/fares/charliecard https://www.mbta.com/fares/charliecard) - these report as Mifare Classic 1k, which is a similar chip There are single-use fares as well, the "CharlieTicket" that you might've encountered. More CharlieCard NFC info: https://medium.com/@bobbyrsec/operation-charlie-hacking-the-mbta-charliecard-from-2008-to-present-24ea9f0aaa38 https://medium.com/@bobbyrsec/operation-charlie-hacking-the-... https://media.defcon.org/DEF%20CON%2031/DEF%20CON%2031%20presentations/Matthew%20Harris%20Zachary%20Bertocchi%20Scott%20Campbell%20Noah%20Gibson%20-%20Boston%20Infinite%20Money%20Glitch%20Hacking%20Transit%20Cards%20Without%20Ending%20Up%20In%20Handcuffs.pdf https://media.defcon.org/DEF%20CON%2031/DEF%20CON%2031%20pre...
  - jeffbee 2y ago
    Yeah I figured but you can't buy a charliecard online to load into your smartphone wallet, and I only needed it the once, and since it took more than an hour to get to Cambridge due to some combination of circus acts I used Blue bikes for the remainder of the day.
    - cypherpunks01 2y ago
      Ah yes, it's not quite there, but almost. Contactless payment directly at the turnstile is coming to Boston MBTA this year, I believe. Like how NYC works now, where you can just use your credit card for entry.
      - chgs 2y ago
        This is the London system we’ve had for a decade, it was licensed to other areas a few years ago. I found myself in Paris having to cross the other day and forgot how terrible the old way of buying tickets was, amazed that it’s still the norm in so many cities
modeless 2y ago
I am interested in the plastic layer with conductive traces for the antenna. How are these made? Do you know of a source that talks about the production process for them?
- kens 2y ago
  I don't know personally about the antenna manufacturing, but one web page talks about printable conductive silver ink for producing RFID antennas. https://www.sunchemical.com/product/printed-antenna/ https://www.sunchemical.com/product/printed-antenna/
politelemon 2y ago
Could someone just use an nfc enabled phone to get it to act as a ticket?
- lxgr 2y ago
  For MIFARE Ultralight, yes – it's essentially just a bearer token with no encryption/authentication. I believe there's a password mechanism, though, which might just be good enough for single-use tickets. That password can be derived/diversified from the card's serial number, making such a scheme still significantly better than e.g. simple QR codes. MIFARE Ultralight C and larger/more expensive chips allow challenge-response authentication, making them pratically uncloneable. These are usually used for reloadable and monthly passes.
- numpad0 2y ago
  Layer 3 and down yes, 4 and up depends
ak217 2y ago
Thanks for the write-up. How does this fit into the broader NFC ecosystem? What do other big metro systems like Omny, Clipper, Smartrip etc use? Apple and Google seem to implement some NFC protocols in their devices but in a much more programmable way, how does that work? Is the protocol used in credit cards related at all? And how do these relate to Felica, the system used everywhere in Japan (which was in the news for a while because the factory where they made the chips burned down and they had a chip shortage - giving Apple an opening to move into the market with iPhone NFC)?
- kens 2y ago
  That seems like a question for @lxgr :-) As far as I can tell, the NFC ecosystem is a mess of competing, incompatible protocols from different companies, as well as incompatibilities for historical reasons. For example, Clipper uses MIFARE DESFire, which is the more secure sibling of the Ultralight chip that I examined. Washington's SmarTrip cards use MIFARE Plux X. New York City's OMNY, on the other hand, is apparently built on top of the Mastercard payment network using EMV. Montreal's rechargeable OPUS card (not the disposable one I examined) uses the completely different Calypso standard. FeliCa was developed in Japan along a different path and has a different standard (NFC-F vs NFC-A) with different modulation, protocol, and data rates. The NFC chips used in phones try to be compatible with as much as possible. These NFC systems all use the same 13.56 MHz frequency, so the radio hardware is compatible across them.
  - Tijdreiziger 2y ago
    How and why did you learn about this topic? :)
  - lxgr 2y ago
    On it, but I couldn't have said it better :) To expand on Felica vs. ISO a bit: Theoretically Felica is a different stack from ISO 14443, but it's close enough that it almost got specified as a variant of ISO 14443 as well (C; MIFARE and most other systems use A). NFC does specify Felica as one possible official tag type (then called NFC-F, as opposed to NFC-A and NFC-B), so practically, most mobile devices can just also read it. For anybody wanting to experiment a bit, I can highly recommend getting any Android device and installing NFC tag reader by NXP; it'll show you what technology exactly a given card uses, and in some cases can show you other interesting information as well. There's also an app that lets you read the current balance of various transit cards.
teruakohatu 2y ago
That was a god read, thank you. Do you have any insight into the economics of this in general compared to other disposable solutions. Are manufacturing old school magnetic stripe tickets, or just optical scanning/barcodes a lot cheaper? I imagine magnetic stripes have a higher failure to read rate at the turnstile causing issues, while both them and optical scanning requires the ticket to be inserted into the machine, adding complexity and moving parts.
- kens 2y ago
  I couldn't find a nice price breakdown. I'd expect the magnetic stripe tickets to be cheaper to manufacture, but since the NFC tickets cost pennies, there isn't a lot of money to save. I agree with you that magnetic stripes would have a much higher maintenance cost due to the mechanical aspect and the read/write head. Optical scanning seems less likely to work the first time, based on my experience with airplane checkins. NFC is probably the best from an ecosystem perspective since it can work with credit cards and phones as well. NFC readers are probably the cheapest since they are produced in large volumes for credit card point of sale.
  - amluto 2y ago
    I’ve occasionally gotten to watch transit workers open up and service the magnetic stripe card readers in the BART. Those things are complicated. It may well cost less to outright replace a contactless reader module on a fare gate than to service a magnetic stripe ticket machine once. Even an Adafruit PN532 board is only $40.
    - teruakohatu 2y ago
      And if a machine jams not only do you need staff to spend hours to repair it, but you need to pay staff, or contractors, to do it.
    - edub 2y ago
      I've not worked with the Adafruit PN532, but for an extra $10 you can get a Pepper C1 USB from Eccel which is very easy to work with. It is a stand-alone device, so you don't have to connect it to anything but power. Has WiFi & BT built in and has a built-in web server to configure it with, you can have it make calls via REST, MQTT, WebSocket.
seizethecheese 2y ago
I’ve been curious about the orientation of these devices. For examples, if I want to track an item’s presence in a box, would I have to coat the entire item in these chips to get one to be in the right orientation?
- kens 2y ago
  I think it depends on the type of antenna. A linearly polarized RFID antenna is sensitive to the tag's orientation, but a circularly polarized antenna is less sensitive to orientation. Systems can also use more than one RFID antenna to get better coverage.
maaarghk 2y ago
Not an NFC chip question, but what kind of microscope do you need to get silicon photos of a chip so tiny?
- kens 2y ago
  The trick is to use a metallurgical microscope, which shines the light down through the lens. A regular microscope illuminates from below, which works fine for cells, but not for opaque chips. Specifically, I use an AmScope ME300TZB-2L-10M microscope, which my friends consider an entry-level microscope, but it works for my needs.
throwadobe 2y ago
How much cheaper and smaller can they get before we see them used in parcel and freight?
- poslathian 2y ago
  Several such pilots for this have been in flight the last couple years.
- account42 2y ago
  You already see RFID tags used in retail so I would not be surprised if that isn't already the case somewhere in the shipping industry as well. I imagine those can use even simpler chips that are completely read-only over the air and only have a UID programmed.
rendaw 2y ago
Kind of a software question, but why isn't nfc with asymmetric keys a thing? It seems like at best this is a custom javacard app on select expensive cards ($4 per card if you buy 1000 on aliexpress) or $70 for a yubikey otherwise. Is getting the signature time fast enough just impossible with current hardware/tramission power restrictions?
jFriedensreich 2y ago
as always a delight to read ken! im curious about speculation how to do the bonding and mounting of these chips at scale. at this size even the general handling and cutting of wafers are hard to imagine for me. how did the connections to the antenna look like and was there an indication of different glue / adhesive layers apart from the coatings you described?
caf 2y ago
I'm curious about how the unique ID is programmed into each chip. Presumably all the chips on the wafer come out identical - at which point in the process are they individually selected and given a unique personality? Is it done with direct electrical contact that is then fused off, or using the near field link?
- grishka 2y ago
  Maybe some kind of custom NFC command that writes the UID into the EEPROM and sets a flag that it's now read only?
  - account42 2y ago
    It's probably done through normal write commands if there is any explicit lock bit at all (it could doesn't just check if any of the UID bits are already non-zero and then reject the write). You can actually make other parts of the memory read-only too by setting bits at a specified address [0] (which then cannot be unset again). [0] https://www.nxp.com/docs/en/data-sheet/MF0ICU1.pdf#G4008599 https://www.nxp.com/docs/en/data-sheet/MF0ICU1.pdf#G4008599
- kens 2y ago
  Since they need to probe each die to test it on the wafer, they set the UID at the same time. According to the datasheet, "These bytes are programmed and write protected in the production test."
qaisjp 2y ago
I really enjoyed this read, thanks. In the footnotes you said: > One complication is that the counters have an "anti-tearing" feature for additional security Two questions: 1. Why is it a "complication"? Is it just that it makes the counters more complicated, or is there something frustrating about the counters? 2. I would love to learn more about how the anti-tearing feature works!
- kens 2y ago
  The problem is that if the user tears the card away from the reader in the middle of an update, that card can end up with corrupted data. This makes implementing the increment-only counters more complicated. For instance, the straightforward approach might hold 00 FF in two bytes. If you increment the counter by updating the low-order byte first, but the card gets torn away before you update the high-order byte, you end up with 00 00, and the counter has gone backward. A simple way of preventing tearing is to have two copies of each counter; if there is tearing, then the two values will be different. Looking at an NXP patent [1], they use a much more complicated approach, using a level of indirection. They write the new value to a different memory page and then update a pointer to the new page. There are various progress bits recorded along the way so they can roll back as needed. [1]: https://patents.google.com/patent/EP3226141A1 https://patents.google.com/patent/EP3226141A1 Here's an article describing an attack on the anti-tearing feature: https://blog.quarkslab.com/rfid-monotonic-counter-anti-tearing-defeated.html https://blog.quarkslab.com/rfid-monotonic-counter-anti-teari...
crote 2y ago
How do they make the chips so incredibly thin? Surely they're not using 75µm/120µm wafers throughout the entire production process - that's literally the thickness of a human hair! Can a 200/300mm wafer of that thickness even support itself, let alone all the stresses in the production process?
- abainbridge 2y ago
  https://en.wikipedia.org/wiki/Wafer_backgrinding https://en.wikipedia.org/wiki/Wafer_backgrinding

Author here for all your NFC chip questions :-)

by kens 2y ago

Author here for all your NFC chip questions :-)

How difficult is it to clone MIFARE Ultralight EV1 chips? You mention the UID is signed, can you simply copy this signature? Do you just need to buy one of the magical chips of the same design, that allow uid/serials to be written? What is the actual mechanism behind the DESFire and other secure NFC chips that prevents cloning?

kens 2y ago
I haven't really looked into the security aspects. I think that you could clone one of the Ultralight chips, but it wouldn't gain you anything because the security is in the backend. It's a lot like a printed concert ticket or boarding pass. You could print as many as you want, but the ticket is still good for just one admission. The DESFire and other secure chips contain a cryptographic key that you can't access. Without the key, you can't make a clone of the chip. The cryptography provides authentication and encryption that you don't get with the cheap Ultralight chip. I think this is all market segmentation; they don't put more security into the Ultralight chip because they don't want to cannibalize their higher-end sales.
- lxgr 2y ago
  Ultralight supports password authentication, and you can diversify the password from the serial number, meaning that until that password is revealed by a legitimate reader as part of a validation transaction (at which time the ticket is invalidated anyway), you can't clone it. Ultralight C does support actual cryptographic authentication.
  - kens 2y ago
    I don't think the password authentication helps against cloning. You could start a transaction and stop after you get the password. Then you could clone the card. (The system could invalidate the ticket as soon as they get the UID, but that would be a reliability nightmare since a failure during the read would invalidate someone's ticket.)
    - lxgr 2y ago
      You could do that, but it still greatly raises the cost for an attacker, since they need to hang around a ticket validator for every ticket they want to clone, as opposed to e.g. a QR code ticket, which can be trivially copied by a simple screenshot. Also, many of these transit systems are eventually consistent (they're usually offline-capable for resilience, but usually manage to send all validation transactions to a backoffice system within at most a day, and often minutes). This allows detecting duplicate usage fairly quickly. In systems where you need to tap out as well as tap in to leave the turnstile, that's where ticket inspectors might take a sudden interest in you if you tap out with a cloned ticket. In the end, as with most security systems, the goal is not to make fraud absolutely impossible, but to make it economically non-viable.
      - szundi 2y ago
        You have to hang out to get the id of a simple 125khz tag too - this is what cloning means
- amluto 2y ago
  In general, the card emulation devices (e.g. the chips in phones) try to avoid letting any arbitrary UID be set. This makes cloning these cards more difficult than it would otherwise be. It’s not terribly difficult to find devices (USB-connected things and battery-less cards) that do allow arbitrary UIDs to be set, though.
  - josephcsible 2y ago
    > the card emulation devices (e.g. the chips in phones) try to avoid letting any arbitrary UID be set. I can't think of a much worse way to do security. That feels like trying to flood the market with lockpicks that don't work instead of making a more pick-resistant lock.
    - amluto 2y ago
      I imagine this is because the locks contain chips from NXP (PN532 chips), the name-brand MIFARE chips are made by NXP, and the lock picks (also PN532 chips!) are made by NXP.

jeffbee 2y ago

Is this the same system used by Boston MBTA? I was surprised to see single-use tap cards when I visited there for the first time yesterday. I wondered why the ticket isn't reloadable.

cypherpunks01 2y ago
Most people who live in Boston use the reloadable CharlieCard (https://www.mbta.com/fares/charliecard https://www.mbta.com/fares/charliecard) - these report as Mifare Classic 1k, which is a similar chip There are single-use fares as well, the "CharlieTicket" that you might've encountered. More CharlieCard NFC info: https://medium.com/@bobbyrsec/operation-charlie-hacking-the-mbta-charliecard-from-2008-to-present-24ea9f0aaa38 https://medium.com/@bobbyrsec/operation-charlie-hacking-the-... https://media.defcon.org/DEF%20CON%2031/DEF%20CON%2031%20presentations/Matthew%20Harris%20Zachary%20Bertocchi%20Scott%20Campbell%20Noah%20Gibson%20-%20Boston%20Infinite%20Money%20Glitch%20Hacking%20Transit%20Cards%20Without%20Ending%20Up%20In%20Handcuffs.pdf https://media.defcon.org/DEF%20CON%2031/DEF%20CON%2031%20pre...
- jeffbee 2y ago
  Yeah I figured but you can't buy a charliecard online to load into your smartphone wallet, and I only needed it the once, and since it took more than an hour to get to Cambridge due to some combination of circus acts I used Blue bikes for the remainder of the day.
  - cypherpunks01 2y ago
    Ah yes, it's not quite there, but almost. Contactless payment directly at the turnstile is coming to Boston MBTA this year, I believe. Like how NYC works now, where you can just use your credit card for entry.
    - chgs 2y ago
      This is the London system we’ve had for a decade, it was licensed to other areas a few years ago. I found myself in Paris having to cross the other day and forgot how terrible the old way of buying tickets was, amazed that it’s still the norm in so many cities

modeless 2y ago

I am interested in the plastic layer with conductive traces for the antenna. How are these made? Do you know of a source that talks about the production process for them?

kens 2y ago
I don't know personally about the antenna manufacturing, but one web page talks about printable conductive silver ink for producing RFID antennas. https://www.sunchemical.com/product/printed-antenna/ https://www.sunchemical.com/product/printed-antenna/

politelemon 2y ago

Could someone just use an nfc enabled phone to get it to act as a ticket?

lxgr 2y ago
For MIFARE Ultralight, yes – it's essentially just a bearer token with no encryption/authentication. I believe there's a password mechanism, though, which might just be good enough for single-use tickets. That password can be derived/diversified from the card's serial number, making such a scheme still significantly better than e.g. simple QR codes. MIFARE Ultralight C and larger/more expensive chips allow challenge-response authentication, making them pratically uncloneable. These are usually used for reloadable and monthly passes.
numpad0 2y ago
Layer 3 and down yes, 4 and up depends

ak217 2y ago

Thanks for the write-up. How does this fit into the broader NFC ecosystem? What do other big metro systems like Omny, Clipper, Smartrip etc use? Apple and Google seem to implement some NFC protocols in their devices but in a much more programmable way, how does that work? Is the protocol used in credit cards related at all? And how do these relate to Felica, the system used everywhere in Japan (which was in the news for a while because the factory where they made the chips burned down and they had a chip shortage - giving Apple an opening to move into the market with iPhone NFC)?

kens 2y ago
That seems like a question for @lxgr :-) As far as I can tell, the NFC ecosystem is a mess of competing, incompatible protocols from different companies, as well as incompatibilities for historical reasons. For example, Clipper uses MIFARE DESFire, which is the more secure sibling of the Ultralight chip that I examined. Washington's SmarTrip cards use MIFARE Plux X. New York City's OMNY, on the other hand, is apparently built on top of the Mastercard payment network using EMV. Montreal's rechargeable OPUS card (not the disposable one I examined) uses the completely different Calypso standard. FeliCa was developed in Japan along a different path and has a different standard (NFC-F vs NFC-A) with different modulation, protocol, and data rates. The NFC chips used in phones try to be compatible with as much as possible. These NFC systems all use the same 13.56 MHz frequency, so the radio hardware is compatible across them.
- Tijdreiziger 2y ago
  How and why did you learn about this topic? :)
- lxgr 2y ago
  On it, but I couldn't have said it better :) To expand on Felica vs. ISO a bit: Theoretically Felica is a different stack from ISO 14443, but it's close enough that it almost got specified as a variant of ISO 14443 as well (C; MIFARE and most other systems use A). NFC does specify Felica as one possible official tag type (then called NFC-F, as opposed to NFC-A and NFC-B), so practically, most mobile devices can just also read it. For anybody wanting to experiment a bit, I can highly recommend getting any Android device and installing NFC tag reader by NXP; it'll show you what technology exactly a given card uses, and in some cases can show you other interesting information as well. There's also an app that lets you read the current balance of various transit cards.

teruakohatu 2y ago

That was a god read, thank you. Do you have any insight into the economics of this in general compared to other disposable solutions. Are manufacturing old school magnetic stripe tickets, or just optical scanning/barcodes a lot cheaper? I imagine magnetic stripes have a higher failure to read rate at the turnstile causing issues, while both them and optical scanning requires the ticket to be inserted into the machine, adding complexity and moving parts.

kens 2y ago
I couldn't find a nice price breakdown. I'd expect the magnetic stripe tickets to be cheaper to manufacture, but since the NFC tickets cost pennies, there isn't a lot of money to save. I agree with you that magnetic stripes would have a much higher maintenance cost due to the mechanical aspect and the read/write head. Optical scanning seems less likely to work the first time, based on my experience with airplane checkins. NFC is probably the best from an ecosystem perspective since it can work with credit cards and phones as well. NFC readers are probably the cheapest since they are produced in large volumes for credit card point of sale.
- amluto 2y ago
  I’ve occasionally gotten to watch transit workers open up and service the magnetic stripe card readers in the BART. Those things are complicated. It may well cost less to outright replace a contactless reader module on a fare gate than to service a magnetic stripe ticket machine once. Even an Adafruit PN532 board is only $40.
  - teruakohatu 2y ago
    And if a machine jams not only do you need staff to spend hours to repair it, but you need to pay staff, or contractors, to do it.
  - edub 2y ago
    I've not worked with the Adafruit PN532, but for an extra $10 you can get a Pepper C1 USB from Eccel which is very easy to work with. It is a stand-alone device, so you don't have to connect it to anything but power. Has WiFi & BT built in and has a built-in web server to configure it with, you can have it make calls via REST, MQTT, WebSocket.

seizethecheese 2y ago

I’ve been curious about the orientation of these devices. For examples, if I want to track an item’s presence in a box, would I have to coat the entire item in these chips to get one to be in the right orientation?

kens 2y ago
I think it depends on the type of antenna. A linearly polarized RFID antenna is sensitive to the tag's orientation, but a circularly polarized antenna is less sensitive to orientation. Systems can also use more than one RFID antenna to get better coverage.

maaarghk 2y ago

Not an NFC chip question, but what kind of microscope do you need to get silicon photos of a chip so tiny?

kens 2y ago
The trick is to use a metallurgical microscope, which shines the light down through the lens. A regular microscope illuminates from below, which works fine for cells, but not for opaque chips. Specifically, I use an AmScope ME300TZB-2L-10M microscope, which my friends consider an entry-level microscope, but it works for my needs.

throwadobe 2y ago

How much cheaper and smaller can they get before we see them used in parcel and freight?

poslathian 2y ago
Several such pilots for this have been in flight the last couple years.
account42 2y ago
You already see RFID tags used in retail so I would not be surprised if that isn't already the case somewhere in the shipping industry as well. I imagine those can use even simpler chips that are completely read-only over the air and only have a UID programmed.

rendaw 2y ago

Kind of a software question, but why isn't nfc with asymmetric keys a thing? It seems like at best this is a custom javacard app on select expensive cards ($4 per card if you buy 1000 on aliexpress) or $70 for a yubikey otherwise. Is getting the signature time fast enough just impossible with current hardware/tramission power restrictions?

jFriedensreich 2y ago

as always a delight to read ken! im curious about speculation how to do the bonding and mounting of these chips at scale. at this size even the general handling and cutting of wafers are hard to imagine for me. how did the connections to the antenna look like and was there an indication of different glue / adhesive layers apart from the coatings you described?

caf 2y ago

I'm curious about how the unique ID is programmed into each chip. Presumably all the chips on the wafer come out identical - at which point in the process are they individually selected and given a unique personality? Is it done with direct electrical contact that is then fused off, or using the near field link?

grishka 2y ago
Maybe some kind of custom NFC command that writes the UID into the EEPROM and sets a flag that it's now read only?
- account42 2y ago
  It's probably done through normal write commands if there is any explicit lock bit at all (it could doesn't just check if any of the UID bits are already non-zero and then reject the write). You can actually make other parts of the memory read-only too by setting bits at a specified address [0] (which then cannot be unset again). [0] https://www.nxp.com/docs/en/data-sheet/MF0ICU1.pdf#G4008599 https://www.nxp.com/docs/en/data-sheet/MF0ICU1.pdf#G4008599
kens 2y ago
Since they need to probe each die to test it on the wafer, they set the UID at the same time. According to the datasheet, "These bytes are programmed and write protected in the production test."

qaisjp 2y ago

I really enjoyed this read, thanks. In the footnotes you said: > One complication is that the counters have an "anti-tearing" feature for additional security Two questions: 1. Why is it a "complication"? Is it just that it makes the counters more complicated, or is there something frustrating about the counters? 2. I would love to learn more about how the anti-tearing feature works!

kens 2y ago
The problem is that if the user tears the card away from the reader in the middle of an update, that card can end up with corrupted data. This makes implementing the increment-only counters more complicated. For instance, the straightforward approach might hold 00 FF in two bytes. If you increment the counter by updating the low-order byte first, but the card gets torn away before you update the high-order byte, you end up with 00 00, and the counter has gone backward. A simple way of preventing tearing is to have two copies of each counter; if there is tearing, then the two values will be different. Looking at an NXP patent [1], they use a much more complicated approach, using a level of indirection. They write the new value to a different memory page and then update a pointer to the new page. There are various progress bits recorded along the way so they can roll back as needed. [1]: https://patents.google.com/patent/EP3226141A1 https://patents.google.com/patent/EP3226141A1 Here's an article describing an attack on the anti-tearing feature: https://blog.quarkslab.com/rfid-monotonic-counter-anti-tearing-defeated.html https://blog.quarkslab.com/rfid-monotonic-counter-anti-teari...

crote 2y ago

How do they make the chips so incredibly thin? Surely they're not using 75µm/120µm wafers throughout the entire production process - that's literally the thickness of a human hair! Can a 200/300mm wafer of that thickness even support itself, let alone all the stresses in the production process?

abainbridge 2y ago
https://en.wikipedia.org/wiki/Wafer_backgrinding https://en.wikipedia.org/wiki/Wafer_backgrinding