Techniques for Implementing Derived Credentials on Mobile Devices

Update (April 3, 2013). There is a more recent blog post with important new information on the topic of derived credentials.

Update (September 25, 2012). We made a presentation on this topic at the Cryptographic Key Management Workshop that was held on September 10-11 at NIST.

We live in the Age of Mobile, and US Federal agencies, like all enterprises, want their employees to use smart phones and tablets. But they face a serious obstacle: how to authenticate users on mobile devices securely.

As I noted in the previous post, ordinary passwords are even less secure on mobile devices than on desktops and laptops, and one-time passwords provide only limited security because they can be intercepted or observed and they remain valid for several minutes. Authentication of federal employees requires the much stronger cryptographic and biometric security provided by Personal Identity Verification (PIV) smartcards in civilian agencies and Common Access Cards (CAC) in the Department of Defense.

It is difficult to use a smartcard to authenticate a user who is accessing an application on a mobile device. A contactless card could communicate with the device via Near Field Communication (NFC), but some mobile devices, including the iPhone, are not equipped with NFC today. A card reader could communicate with the mobile device via Bluetooth or WiFi, but that requires the user to carry three pieces of equipment: the card, the phone and the card reader.

NIST is working on a better authentication solution: derived credentials, which would provide the same security strength as PIV credentials but would be stored in the mobile device rather than in a separate smartcard. The Electronic Authentication Guideline defines a derived credential as a credential issued based on proof of possession and control of a token associated with a previously issued credential, so as not to duplicate the identity proofing process.

Derived credentials are a very good idea, but they present several challenges. One challenge is the cost of verifying a client certificate chain, in terms of bandwidth, latency and battery life. Another challenge is the lack of tamper resistant storage for credentials and biometric data in mobile devices. Yet another challenge is the complexity of cryptographic and biometric technology, which most app developers are not familiar with.

I believe that these challenges can be addressed using three techniques used in the mobile authentication methods that we described in the white paper

which I summarized in the previous post. We have written another white paper,

that describes each technique separately.

The first technique eliminates the costs associated with verifying client certificate chains by using public key cryptography without certificates. The device demonstrates knowledge of a private key, and the application verifies that the hash of the associated public key matches a field of a device record stored in an enterprise directory. The device record, in turn, contains a reference to a user record, identifying the user as the owner of the device.

The second technique obviates the need for tamper-resistant storage. Tamper-resistant storage is usually needed when a PIN and/or a biometric sample is used to enable the use of a key pair, so that an attacker cannot extract the key pair and use it without providing the PIN and/or the biometric, or extract the biometric template, or mount an offline attack against the PIN. We avoid the need for tamper resistance by regerenating the key pair from the PIN or from a biometric key derived from a biometric sample and an auxiliary string. An attacker who tampers with the device gains no advantage because the only way to know if a regenerated key is the correct one is by using it for online authentication.

The third technique shields app developers from the complexities of cryptography and biometrics by encapsulating the cryptographic and biometric computations in a Prover Black Box, which can be provided as a separate native app on the mobile device, and a Verifier Black Box, which can implemented as a server appliance. The application, which may interact with the user via a browser or via a native front-end, outsources authentication to the black boxes using interapp communication facilities available at least in iOS and Android.

The white paper has figures and more details.

Leave a Reply

Your email address will not be published. Required fields are marked *