*04:17* [Pub][ePrint]
How to Use Bitcoin to Design Fair Protocols, by Iddo Bentov and Ranjit Kumaresan
We study a model of fairness in secure computation in which an adversarial party that aborts on receiving output is forced to pay a mutually predefined monetary penalty. We then show how the Bitcoin network can be used to achieve the above notion of fairness in the two-party as well as the multiparty setting (with a dishonest majority). In particular, we propose new ideal functionalities and protocols for fair secure computation and fair lottery in this model. One of our main contributions is the definition of an ideal primitive, which we call $\\mathcal{F}_{\\mathrm{CR}}^\\star$ ($\\mathrm{CR}$ stands for ``claim-or-refund\'\'), that formalizes and abstracts the exact properties we require from the Bitcoin network to achieve our goals. Naturally, this abstraction allows us to design fair protocols in a hybrid model in which parties have access to the $\\mathcal{F}_{\\mathrm{CR}}^\\star$ functionality, and is otherwise independent of the Bitcoin ecosystem.

We also show an efficient realization of $\\mathcal{F}_{\\mathrm{CR}}^\\star$ that requires only two Bitcoin transactions to be made on the network.

Our constructions also enjoy high efficiency. In a multiparty setting, our protocols only require a constant number of calls to $\\mathcal{F}_{\\mathrm{CR}}^\\star$ per party on top of a standard multiparty secure computation protocol. Our fair multiparty lottery protocol improves over previous solutions which required a quadratic number of Bitcoin transactions.

*04:17* [Pub][ePrint]
Efficient Revocable Identity-Based Encryption via Subset Difference Methods, by Kwangsu Lee and Dong Hoon Lee and Jong Hwan Park
Providing an efficient revocation mechanism for identity-based encryption (IBE) is very important since a user\'s credential (or private key) can be expired or revealed. Revocable IBE (RIBE) is an extension of IBE that provides an efficient revocation mechanism. Previous RIBE schemes essentially use the complete subtree (CS) scheme for key revocation. In this paper, we present a new technique for RIBE that uses the efficient subset difference (SD) scheme or the layered subset difference (LSD) scheme instead of using the CS scheme to improve the size of update keys.Following our new technique, we first propose an efficient RIBE scheme in prime-order bilinear groups by combining the IBE scheme of Boneh and Boyen and the SD scheme and prove its selective security under the standard assumption. Our RIBE scheme is the first RIBE scheme in bilinear groups that has $O(r)$ number of group elements in update keys. Next, we also propose another RIBE scheme in composite-order bilinear groups and prove its full security under static assumptions. Our RIBE schemes also can be integrated with the LSD scheme to reduce the size of private keys.

*04:17* [Pub][ePrint]
Kummer strikes back: new DH speed records, by Daniel J. Bernstein and Chitchanok Chuengsatiansup and Tanja Lange and Peter Schwabe
This paper introduces high-security constant-time variable-base-point Diffie--Hellman software using just 274593 Cortex-A8 cycles, 91460 Sandy Bridge cycles, 90896 Ivy Bridge cycles, or 72220 Haswell cycles. The only higher speed appearing in the literature for any of these platforms is a claim of 60000 Haswell cycles for unpublished software performing arithmetic on a binary elliptic curve.The new speeds rely on a synergy between (1) state-of-the-art formulas for genus-2 hyperelliptic curves and (2) a modern trend towards vectorization in CPUs. The paper introduces several new techniques for efficient vectorization of Kummer-surface computations.

*04:17* [Pub][ePrint]
Anonymous Two-Factor Authentication: Certain Goals Are Beyond Attainment, by Ding Wang, Ping Wang, and Debiao He
Despite a decade of intensive research, it still remains a challenge to design a practical dynamic id-based two-factor authentication scheme, for the designers are confronted with an impressive list of security requirements (e.g., resistance to mart card loss attack) and desirable attributes (e.g., local and secure password update). Dozens of solutions have been proposed, yet most of them are shortly found either unable to satisfy some security requirements or short of important features. To overcome this unsatisfactory situation, researchers often work around it in hopes of a new solution (but no one has succeeded so far), while paying little attention to the question: Whether or not there are inherent limitations (conflicts) that prevents us from designing an ideal scheme that satisfies all of these goals?In this work, we attempt to provide an answer to this question. We revisit two latest (and reprentative) proposals, i.e. Xie\'s scheme and Li\'s scheme, and explore some inherent conflicts and unavoidable trade-offs in designing such schemes. Our results highly indicate that, under the current widely accepted adversary model, certain goals are beyond attainment. To the best of knowledge, the present study makes the first step towards understanding the underlying evaluation metric for dynamic id-based two-factor authentication, which we believe will facilitate better design of two-factor protocols that offer acceptable trade-offs between usability, security and privacy.

*04:17* [Pub][ePrint]
Isolated Execution on Many-core Architectures, by Ramya Jayaram Masti and Devendra Rai and Claudio Marforio and Srdjan Capkun
We explore how many-core platforms can be used to enhance the security of future systems and to support important security properties such as runtime isolation using a small Trusted Computing Base (TCB). We focus on the Intel Single-chip Cloud Computer (SCC) to show that such properties can be implemented in current systems. We design a system called \\archname{} which offers strong security properties while maintaining high performance and flexibility enabled by a small centralized security kernel. We further implement and evaluate the feasibility of our design. Currently, our prototype security kernel is able to execute applications in isolation and accommodate dynamic resource requests from them. We show that, with minor modifications, many-core architectures can offer some unique security properties, not supported by existing single- and multi-core architectures, such as application context awareness. Context awareness, a new security property that we define and explore in this work, allows each application to discover, without any interaction with the security kernel, which other parts of the system are allowed to interact with it and access its resources. We also discuss how an application can use context awareness to defend itself from an unlikely, yet potentially compromised security kernel.

*04:17* [Pub][ePrint]
Efficient, Oblivious Data Structures for MPC, by Marcel Keller and Peter Scholl
We present oblivious implementations of several data structures for secure multiparty computation (MPC) such as arrays, dictionaries, and priority queues. The resulting oblivious data structures have only polylogarithmic overhead compared with their classical counterparts. To achieve this, we give secure multiparty protocols for the ORAM of Shi et al. (Asiacrypt `11) and the Path ORAM scheme of van Dijk et al. (CCS `13), and we compare the resulting implementations. We subsequently use our oblivious priority queue for secure computation of Dijkstra\'s shortest path algorithm on general graphs, where the graph structure is secret. To the best of our knowledge, this is the first implementation of a non-trivial graph algorithm in multiparty computation with polylogarithmic overhead.We implemented and benchmarked all of our protocols using the SPDZ protocol of Damgaard et al. (Crypto `12), which works in the preprocessing model and ensures active security against an adversary corrupting all but one players. For two parties, the access time for an oblivious array of size 1 million is under 250 ms.

*04:17* [Pub][ePrint]
Short Signatures from Diffie-Hellman, Revisited: Sublinear Public Key, CMA Security, and Tighter Reduction, by Jae Hong Seo
Designing practical signature scheme based on the standard assumption such as the Computational Diffie-Hellman (CDH) assumption is important both from a practical and a theoretical point of view. Currently, there are only three standard model CDH-based signature schemes with short signatures due to Waters (EUROCRYPT 2005), Seo, and B\\\"ohl et al. (the merged paper in EUROCRYPT 2013). The Waters signature scheme achieves the Existentail UnForgeability against Chosen Message Attack (EUF-CMA) with nearly optimal reduction. However, this scheme suffers from large public keys. To shorten public key size, Seo and B\\\"ohl et al. proposed new approaches, respectively, but each approach has a weak point rather than the Waters signature scheme; Seo\'s approach could prove only a rather weak security, called the bounded CMA security, and B\\\"ohl et al.\'s approach inherently accompanies a loose reduction. In this paper, we aim at stepping towards practical EUF-CMA secure signatures with tighter reduction; that is, we achieve sublinear public keys with preserving the same security as the Waters signatures. To this end, we revisit the Seo signature scheme and devise an alternative and simple analysis leading the standard EUF-CMA security with tighter reduction. In particular, our security proof has a reduction loss of $O(\\lambda q)$, which is less than $O(\\sqrt{\\frac{\\lambda}{\\log}}\\lambda q)$ of the original security proof, where $\\lambda$ is the security parameter, and is almost the same as that of the Water signature scheme.