Privacy in NFC payments: mobile wallets and prying eyes


Continuing the exploration of privacy in NFC payments, we next focus on the code that runs on the smart phone and what those applications can learn about user transactions. Unlike an inert plastic smart-card, a smart phone is a general purpose computer connected to a network. Given that mobile devices have been implicated in many privacy intrusions, it is also natural to ask whether applications can snoop on purchase history. Due to the variation in mobile wallet architecture, there is no clear-cut answer to this but we can explore the design space for privacy.

First it is worth distinguishing between the wallet application itself and third-party apps. For example if a bank provided a mobile application for NFC payments, the privacy question is largely moot– the bank already has visibility into payments by virtue of authorizing them. (Unless of course the application allows the bank to learn additional information such as line-level item data, above and beyond what the bank is entitled to observing as card issuer.) On the other hand, if the wallet provider is distinct from the issuing bank as in the case of Apple Pay, it is fair game to ask what the mobile application sees. There is an even greater concern with third-party applications unrelated to the wallet provider or financial institution trying to snoop on purchase history.

The wallet application is typically involved in the purchase transaction for user-experience reasons. This holds true even when the actual payment credentials live on a dedicated secure element chip, with all NFC communication takes place between the point-of-sale and that secure element, as is the case when card-emulation is enabled. Recall that the secure element itself has no user-interface and can not communicate with the user to walk through various steps of the flow, such as entering a PIN, displaying a dollar amount for confirmation or signaling that a transaction has completed. Since NFC communications from the POS would be normally routed directly to the SE, this typically involves some other mechanism for notifying the host operating system. For example on Android NFC controller generates notifications for specific events, such as detecting the presence of an external NFC field or observing a command to SELECT a particular application on the secure element. Such notifications are delivered to relevant applications such as Google Wallet in the case of Android. If these notifications were more broadly available, third-party applications could also observe a transaction taking place. Typically this is prevented by having the notifications only delivered to privileged applications.

At a  minimum then the wallet application knows that a transaction was attempted. In fact the payment applets on the secure element will typically have even more information that can be queried by the host, including purchase amount and status of last transaction. This is all part of the specification for SE applets mandated by the payment network for interoperability, such as MasterCard Mobile PayPass. What the host application does not learn is also a direct consequence of what the protocol communicates to the card from the point-of-sale terminal:

  1. Identity of the merchant. Typically merchant is not authenticated in the transaction.
  2. Line-level item information; in other words, the contents of the shopping cart.

The first one is not too difficult to infer. A mobile device is equipped with a number of radios including GPS, wireless and Bluetooth. Approximate GPS location, the identity of a nearby wireless network or the presence of an iBeacon can all be used to identify a particular merchant as the setting for a shopping excursion. (This is easier in the current situation when few merchants support NFC payments, making it easier to pinpoint the only possibility given an approximate area.) In principle then the wallet application supervising NFC transactions can have at least as much visibility into transaction history as the issuing bank. Apple could in principle compile the same information on Apple Pay users that our banker friend was worried about Google getting as the issuer-of-record for virtual cards. That is not to say that Apple is collecting that information currently. iOS security guide emphasizes that “Payment transactions are between the user, the merchant and the bank,” no doubt to allay any concerns of Apple becoming yet another middleman. But it is technically possible for a mobile wallet application– and by extension its provider– to help itself to that information.

The second one however is not that easy to work around. Unless the core EMV payment protocol is somehow augmented with out-of-band data provided by the cash register, there is no way for a mobile device to divine this information using its own sensors. For all the worrying about keeping data away from Google, there is however one other leading company that does have visibility into this highly coveted consumer purchase history: Square. By providing the complete point-of-sale terminal software, Square controls the check out experience. That includes the way items are rung-up. Square in that sense is the ultimate intermediary. It acts as “merchant” as far as the payment network is concerned while also tapping into a rich data stream of line-level item information from every purchase.

CP

 

2 thoughts on “Privacy in NFC payments: mobile wallets and prying eyes

  1. As always another post from RandomOracle that is enlightening. Re: Square and the pursuit of shopping cart details, this is also true of getpoynt.com that is going for all shopping Cart details on the POS in case merchants didn’t use Square POS because CX had EMV for example.

    Can you give us one day a breakdown as to why line item details are the holy grail?

  2. Venmo goes after the peer-to-peer payment market rather than retail, allowing users to easily send payments to their friends. Once you have downloaded the app and signed up, the simple “Pay/Request” interface allows you to either pay or request payment from anyone with the app. This helps to simplify group payments for, say, restaurant bills and birthday presents, removing the need to onerously exchange bank details. The optional newsfeed, built in to the app, adds a social element to the app and provides you with notifications when people have paid.

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