Target breach, credit card security and NFC (part I)

This holiday season has not been kind to retailers. First Target experienced one of the worst data breaches in recent memory, with the damage toll continuing to rise and the scope threatening to expand. From 40 million initial estimates and only in-store purchases, the retailer added another 70 million online shoppers, also including email, phone numbers and email addresses in the compromised data. Then the upscale retailer Neimann Marcus jumped into the fray, announcing that it too had experienced an intrusion resulting in the loss of credit card data. Not to be outdone, the crafts supplier Michaels announced that it had detected a successful attack resulting in loss of credit cards.

The great chip & PIN diversion

In a subtle attempt to shift blame back to the credit-card networks, the Target CEO went on the record to praise the virtues of chip & PIN cards. Kim Zetter of Wired quickly pointed out a slightly inconvenient fact: Target rejected a program back in 2004 to upgrade point-of-sale terminals for accepting chip & PIN technology. It turns out there is even more recent evidence of how little Target cared about supporting new payment technologies: very few of their stores accept NFC payments. Contrast this with Walgreens, CVS and Whole Foods which accept contactless payments at most locations. This state of affairs casts some doubt on Target’s avowed commitment to chip & PIN; NFC is a bridge technology to full chip & PIN.  NFC-enabled debit/credit cards as well as their mobile incarnations such as Google Wallet implement simplified versions (“profiles” more accurately) of the same EMV protocols.

Would NFC have protected Target customers?

Answering this question requires a closer look at the protocol. At first blush it seems that one can achieve much better security with contactless payments, at least against the specific risk Target customers faced: compromised point-of-sale terminals. A plastic card is a passive, inert object encoding static data. By contrast NFC payments involve smart cards with embedded chips– in other words miniature computers– or even full-scale mobile devices such as phones. Being programmable environments, they can support elaborate payment protocols leveraging strong cryptography. The question is how far that promise is realized in current deployments.

Protocols on paper and in the field

EMV defines the umbrella standard for contactless payments, while each particular payment network has slight tweaks and a proprietary brand for their variant: Mastercard PayPass, Visa payWave, American Express ExpressPay and Discover Zip. These protocols are not exactly interchangeable, but they are designed for coexistence: it is possible to have a single card/phone contain both Mastercard and Visa payment instruments, with the POS selecting one based on some combination of user and merchant preference. For our purposes, the critical detail is that commonly fielded NFC systems in the US use a “mild” version of the full chip & PIN protocol, walking a fine line between remaining compatible with existing infrastructure and providing additional security features.

It’s all magnetic-stripes

Specifically these systems implement the magnetic-stripe profile of EMV protocols. They do not use the heavy-weight cryptography in full chip & PIN, such as static-data authentication or even more robust dynamic-data authentication using unique RSA keys. Instead they emulate an old-school magnetic stripe at the logical level. Emphasis on logical; not to be confused with programmable/dynamic stripe technology such as Coin, which does feature a physical incarnation of a magnetic stripe driven by chips embedded into the card. By contrast an NFC payment does not involve any object resembling a stripe being magnetically read. Physical characteristics of the communication between NFC reader and card look nothing like the act of swiping a plastic card: the induction field used for powering the circuitry embedded in the card, the specific frequency for transmission over the air (13.56Mhz) and data encoding.

Instead of low-level hardware characteristics, it is the data-format associated with magnetic-stripes being simulated. At the end of the exchange between card and reader, the reader constructs a result that looks similar to what a plain magnetic stripe reader might output after processing an old-school plastic card. For example there are two tracks of data, the first one contains  credit card number and expiration date, while the second track has a field reserved for the card-holder name, all of this specified by ISO/IEC 7813.



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