Despite two decades of intensive research, it remains a challenge to design a practical anonymous two-factor authentication scheme, for the designers are confronted with an impressive list of security requirements (e.g., resistance to smart card loss attack) and desirable attributes (e.g., local password update). Numerous solutions have been proposed, yet most of them are shortly found either unable to satisfy some critical security requirements or short of a few important features. To overcome this unsatisfactory situation, researchers often work around it in hopes of a new proposal (but no one has succeeded so far), while paying little attention to the fundamental question: whether or not there are inherent limitations that prevent us from designing an “ideal” scheme that satisfies all the desirable goals? In this work, we aim to provide a definite answer to this question. We first revisit two foremost proposals, i.e. Tsai et al.’s scheme and Li’s scheme, revealing some subtleties and challenges in designing such schemes. Then, we systematically explore the inherent conflicts and unavoidable trade-offs among the design criteria. Our results indicate that, under the current widely accepted adversarial model, certain goals are beyond attainment. This also suggests a negative answer to the open problem left by Huang et al. in 2014. To the best of knowledge, the present study makes the first step towards understanding the underlying evaluation metric for anonymous two-factor authentication, which we believe will facilitate better design of anonymous two-factor protocols that offer acceptable trade-offs among usability, security and privacy.

Anonymous Two-Factor Authentication in Distributed Systems: Certain Goals Are Beyond Attainment

As the most prevailing two-factor authentication mechanism, smart-card-based password authentication has been a subject of intensive research in the past two decades, and hundreds of this type of schemes have wave upon wave been proposed. In most of these studies, there is no comprehensive and systematical metric available for schemes to be assessed objectively, and the authors present new schemes with assertions of the superior aspects over previous ones, while overlooking dimensions on which their schemes fare poorly. Unsurprisingly, most of them are far from satisfactory—either are found short of important security goals or lack of critical properties, especially being stuck with the security-usability tension. To overcome this issue, in this work we first explicitly define a security model that can accurately capture the practical capabilities of an adversary and then suggest a broad set of twelve properties framed as a systematic methodology for comparative evaluation, allowing schemes to be rated across a common spectrum. As our main contribution, a new scheme is advanced to resolve the various issues arising from user corruption and server compromise, and it is formally proved secure under the harshest adversary model so far. In particular, by integrating “honeywords”, traditionally the purview of system security, with a “fuzzy-verifier”, our scheme hits “two birds”: it not only eliminates the long-standing security-usability conflict that is considered intractable in the literature, but also achieves security guarantees beyond the conventional optimal security bound.

Two Birds with One Stone: Two-Factor Authentication with Security Beyond Conventional Bound

SUMMARY

Understanding security failures of cryptographic protocols is the key to both patching existing protocols and designing future schemes. In this paper, we analyze two recent proposals in the area of password-based remote user authentication using smart cards. First, we point out that the scheme of Chen et al. cannot achieve all the claimed security goals and report its following flaws: (i) it is vulnerable to offline password guessing attack under their nontamper resistance assumption of the smart cards; and (ii) it fails to provide forward secrecy. Then, we analyze an efficient dynamic ID-based scheme without public-key operations introduced by Wen and Li in 2012. This proposal attempts to overcome many of the well-known security and efficiency shortcomings of previous schemes and supports more functionalities than its counterparts. Nevertheless, Wen–Li's protocol is vulnerable to offline password guessing attack and denial of service attack, and fails to provide forward secrecy and to preserve user anonymity. Furthermore, with the security analysis of these two schemes and our previous protocol design experience, we put forward three general principles that are vital for designing secure smart-card-based password authentication schemes: (i) public-key techniques are indispensable to resist against offline password guessing attack and to preserve user anonymity under the nontamper resistance assumption of the smart card; (ii) there is an unavoidable trade-off when fulfilling the goals of local password update and resistance to smart card loss attack; and (iii) at least two exponentiation (respectively elliptic curve point multiplication) operations conducted on the server side are necessary for achieving forward secrecy. The cryptanalysis results discourage any practical use of the two investigated schemes and are important for security engineers to make their choices correctly, whereas the proposed three principles are valuable to protocol designers for advancing more robust schemes. Copyright © 2012 John Wiley & Sons, Ltd.

Yi-Xiang Hung

Papers

Two robust remote user authentication protocols using smart cards