PIV card and mobile devices: NFC as missing link

An article in the Government Computing News titled Are mobile devices already making PIV cards obsolete? draws attention to the incompatibility of US government PIV standard with newfangled mobile devices. The author asks whether the shift to smart-phones and tablets is threatening to render the ID card program obsolete barely after it has gained momentum. With the prevalence of NFC in mobile devices (mostly owing to Android ecosystem, although Nokia, Blackberry predate it) this perceived incompatibility may be increasingly an artifact of  design decisions in the PIV specification rather than any intrinsic limitation of smartcards. After all PIV cards are dual-interface: they have both old-school metal contacts for insertion into traditional card reader, as well as NFC antennas for communicating wirelessly. Since NFC-capable phones can act as card reader, one might expect that using the contactless interface will solve the problem– modulo NFC adoption, which is helped by the fact that the Pentagon favors Android for military applications. But it turns out that deliberate design decisions in PIV protocols frustrate that  expectation.

Traditional contact-based card readers were historically used in conjunction with desktop machines, where having a separate gadget with dangling USB cable going back to the PC was less of a problem because the setup was stationary. Overtime came readers integrated into existing hardware such as keyboards, to better blend in with the existing peripherals.  Laptops posed a slight s carrying yet another gadget quickly becomes a usability problem, even when the gadget in question can be quite tiny. In response manufacturers designed  readers intended for the PC Card and later ExpressCard slot directly such that it can be left permanently fixed in place. (Strangely the move from PC Card to ExpressCard standard made the ergonomics worse. Now part of the reader must just out of the narrower slot in order to match ID-1 card dimensions, instead of being flush against the laptop edge in previous designs.)

Mobile devices however continue to pose a challenge due to the paucity of options. It is not that there are no card readers available– they are just very awkward looking. The generic availability of USB in Android makes it possible to reuse existing USB card readers, as ACR has done. Alternatively some manufacturers designed custom readers for phones, since they are no longer required to follow the USB CCID standard prevalent on Windows. There are a couple of products marketed as mobile CAC readers taking that route on iOS, Blackberry and other mobile operating systems. These gadgets are expensive, almost comparable to the cost of the phone and unwieldy. They combine the problems of one-more-widget-to-carry-around (or forget/lose) when not in use, with the problem of poor ergonomics when needed. Some of them functions as sleeve for the phone– a design that ironically would not fly on Android because it would interfere with NFC, with the card being recognized as NFC tag while also being activated on contact interface. Perhaps the least intrusive design is the baiMobile 3000MP which acts as a sleeve for the card and links up to the phone via Bluetooth.

What about NFC? Considering all card functionality can be accessed equally well over NFC, such kluges to get contact readers playing well with mobile device are no longer necessary for the latest crop of phones. In effect the devices are shipping with built-in contactless readers at no extra cost.

There is a catch. While it is true that communicating to the card works equally well from either interface, it does not follow that applications will respond identically. In fact card environments permit applications to determine what interface they have been invoked from and behave differently. In the extreme case, that could mean declining all requests from one interface. There are good security arguments for such discriminatory behavior. Case in point: payment applications running in a secure element inside a mobile device have reason to be suspicious of access from contact interface. That is where host applications and malware lurk. Contactless access from an NFC reader is the proper path for a legitimate point-of-sale terminal, and the payment application can check this during a transaction.

PIV also mandates similar restrictions, except in the other direction. The standard has a significant bias in favor of the contact interface, forbidding most operations over NFC. A look at the PIV data model in NIST SP 800-73 section 1 shows how bad the situation is.  Appendix A lists up to four active X509 certificates and associated key pairs, identified by their purpose: card authentication, PIV authentication, signature and key management. Of these four, only the card authentication certificate can be used over NFC. Worse, that key does not provide two-factor authentication because there is no PIN entry required. It is primarily intended for low-security physical access scenarios. Employees tap their badge against a reader to open doors. (Even in that scenario, FIPS defines “restricted” and “exclusionary” areas where PIN entry and use of a different card key is required, which is only possible by inserting the card into a contact reader.)

The upshot is PIV cards can be accessed from an NFC-enabled mobile device, but they can not be used for any purpose other than physical access. Other applications such as Kerberos authentication with PKINIT, document signing or encrypted email call for using keys that are disallowed for contactless mode. These restrictions are not without good justification: NFC provides no encryption at the transport layer. This is unlike Bluetooth for example, where the pairing process also negotiates keys for protecting future traffic. If PIV messages between card and phone were carried over the air instead of direct contact, it would create new privacy problems. Most notably the user PIN sent to the card, as well as any decrypted data returned from the card would be susceptible to eavesdropping within NFC range. Future protocol improvements can overcome these limitations, but that will not help already deployed cards.


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