3 ms·

> “The London Underground gates don't work nearly as quick with Google Pay or any of my other contactless cards - what gives?” They used to (and still do?) wor

> “The London Underground gates don't work nearly as quick with Google Pay or any of my other contactless cards - what gives?” They used to (and still do?) work faster with the Oyster fare card. Virtually instant. But paying with EMV cards/devices does add a noticable transaction latency before the gate opens. A few hundred milliseconds, I’d say.

lxgr 1y ago
That's because for EMV, they need to run asymmetric card authentication algorithms, which unfortunately exclusively use RSA. That's just not very fast to do on the type of microcontroller common in these cards. EMV also uses faster symmetric cryptography between the card and the issuer, but the latter is not in the picture in a transit gate transaction – too much latency – so asymmetric cryptography it is. That's also the reason some (mostly older, mostly international) cards don't work with TfL: Some don't even have the coprocessor (or just additional processing power these days, I suspect) required for RSA to cut cost, and only work for online payment transactions.
- jddj 1y ago
  Is nfc via google/apple wallet faster then?
  - lxgr 1y ago
    Apple Pay uses hardware secure elements, so the same limitations largely apply. Google Pay emulates the card on the application processor, so theoretically it could be faster, but I wouldn't be surprised if anything won in terms of more performant RSA cryptography is lost to higher command processing latency between the NFC interface and application processor. It would be interesting for somebody to do a latency comparison between Apple Pay, Google Pay, and a physical card!
    - ValentineC 1y ago
      Doesn't Google Pay offload some of the processing to their cloud? Google Pay only allows a certain number of offline transactions (around three or so, I think) before I'm required to turn off airplane mode and authenticate with their servers. I believe this also allows it to work with more phones, and get around security and possibly also regulatory requirements, since there's less need for a really secure secure enclave on NFC devices.
      - lxgr 1y ago
        > Doesn't Google Pay offload some of the processing to their cloud? Not on a per transaction basis, or you couldn't make any offline payments (i.e. with your phone being offline; the terminal can usually not be offline for Apple or Google Pay in the way that it can for cards). Latency would probably also be too high/variable.

lxgr 1y ago

That's because for EMV, they need to run asymmetric card authentication algorithms, which unfortunately exclusively use RSA. That's just not very fast to do on the type of microcontroller common in these cards. EMV also uses faster symmetric cryptography between the card and the issuer, but the latter is not in the picture in a transit gate transaction – too much latency – so asymmetric cryptography it is. That's also the reason some (mostly older, mostly international) cards don't work with TfL: Some don't even have the coprocessor (or just additional processing power these days, I suspect) required for RSA to cut cost, and only work for online payment transactions.

jddj 1y ago
Is nfc via google/apple wallet faster then?
- lxgr 1y ago
  Apple Pay uses hardware secure elements, so the same limitations largely apply. Google Pay emulates the card on the application processor, so theoretically it could be faster, but I wouldn't be surprised if anything won in terms of more performant RSA cryptography is lost to higher command processing latency between the NFC interface and application processor. It would be interesting for somebody to do a latency comparison between Apple Pay, Google Pay, and a physical card!
  - ValentineC 1y ago
    Doesn't Google Pay offload some of the processing to their cloud? Google Pay only allows a certain number of offline transactions (around three or so, I think) before I'm required to turn off airplane mode and authenticate with their servers. I believe this also allows it to work with more phones, and get around security and possibly also regulatory requirements, since there's less need for a really secure secure enclave on NFC devices.
    - lxgr 1y ago
      > Doesn't Google Pay offload some of the processing to their cloud? Not on a per transaction basis, or you couldn't make any offline payments (i.e. with your phone being offline; the terminal can usually not be offline for Apple or Google Pay in the way that it can for cards). Latency would probably also be too high/variable.