This is the first of a series of posts discussing the paper A Comprehensive Approach to Cryptographic and Biometric Authentication from a Mobile Perspective
In the next few posts I will be reporting on research that we have been doing over the last six months related to cryptographic and biometric authentication, focused on mobile devices. I have held off from writing while we were doing the research but now I have a lot to say, so stay tuned.
By the way, in the last six months we have also moved from San Diego to San Jose. I used to work in Silicon Valley, so it’s nice to be back here and renew old friendships. If you are interested in cryptographic and/or biometric authentication and you are based in Silicon Valley or passing by, let me know; I would be happy to meet for coffee and chat.
The starting point of the this latest research was the work we presented at the NIST Cryptographic Key Management workshop last September (Key Management Challenges of Derived Credentials and Techniques for Addressing Them) and at the Internet Identity Workshop last October (New Authentication Method for Mobile Devices), and wrote up in the paper Strong and Convenient Multi-Factor Authentication on Mobile Devices.
In that early work we devised a mobile authentication architecture where the user authenticates with an uncertified key pair, and a method for regenerating an RSA key pair from a PIN and/or a biometric key. The architecture facilitates implementation by encapsulating the complexities of cryptography and biometrics in a Prover Black Box located in the device and Verifier Black Box located in the cloud, while the key pair regeneration method protects the credential against an adversary who captures the user’s mobile device, by preventing an offline attack against the PIN and/or the biometric key. The architecture was primarily intended for mobile devices but could be adapted for use in traditional PCs by means of browser extensions.
The early work left three questions open:
- Can the key pair regeneration method be adapted to cryptosystems other than RSA? This question is practically important because RSA can be used for encryption, and is therefore subject to export controls. The export restrictions have been relaxed a lot since the nineties, but they are so complex that consultation with a lawyer may be required to figure out whether and to what extent they are applicable to a particular product.
- Can the mobile authentication architecture accomodate credentials other than uncertified key pairs, including public key certificates and privacy-enhancing credentials such as U-Prove tokens and Idemix anonymous credentials? Uncertified key pairs are ideal for returning-user authentication, but they cannot be used to provide evidence that the user is entitled to attributes asserted by authoritative third parties.
- Does the architecture support single sign-on (SSO)? SSO is an essential usability feature when multiple frequently used applications require multifactor authentication.
I am happy to report that we have found good answers to all three questions. First, we have found efficient regeneration methods for DSA and ECDSA key pairs; since DSA and ECDSA can only be used for digital signature, they are not subject to export restrictions. Second, we have found a way of extending the architecture to accomodate a variety of credentials, including public key certificates and privacy-enhancing credentials, without giving up on the strong security properties of the original architecture. Third, we found have found two different ways of providing SSO, one of them well suited for web-wide consumer SSO, the other for enterprise SSO; and both applicable to a mix of web-based apps and apps with native front-ends.
An unanticipated result of the research was the discovery of a defense against an adversary who has succeeded in spoofing a TLS server certificate. Spoofing a certificate is difficult, but not unheard of. The defense, which relies on a form of mutual cryptographic authentication, prevents a man-in-the-middle attack and helps the user detect that a server controlled by the adversary is masquerading as a legitimate server using the spoofed certificate.
We have written all this up in a technical whitepaper,
The paper is quite long, because we thought it was important to describe everything in one place, showing how it all fits together. It would be difficult to discuss the entire paper at once, but in the next few posts I will go one by one over some of the topics in the paper; hopefully that will make it easier to discuss each topic. Watch for the next post in a few days.