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22:17 [Pub][ePrint] Identity-Based Key-Encapsulation Mechanism from Multilinear Maps, by Hao Wang and Lei Wu Zhihua Zheng

  We construct an Identity-Based Key Encapsulation Mechanism (IB-KEM) in a generic \"leveled\" multilinear map setting and make it translated to the GGH framework, which defined an \"approximate\" version of a multilinear group family from ideal lattices. Then, we prove their security in the selective-ID model.

22:17 [Pub][ePrint] Fair Two-Party Computations via the BitCoin Deposits, by Marcin Andrychowicz and Stefan Dziembowski and Daniel Malinowski and Łukasz Mazurek


22:17 [Pub][ePrint] An improved compression technique for signatures based on learning with errors, by Shi Bai and Steven D. Galbraith

  We present a new approach to the compression technique of Lyubashevsky et al for lattice-based signatures based on learning with errors (LWE).

Our ideas seem to be particularly suitable for signature schemes whose security, in the random oracle model, is based on standard worst-case computational assumptions. Our signatures are shorter than any previous proposal for provably-secure signatures based on standard lattice problems: at the 128-bit level we improve signature size from (more than) 16500 bits to around 9000 to 12000 bits.

22:17 [Pub][ePrint] Lattice Decoding Attacks on Binary LWE, by Shi Bai and Steven D. Galbraith

  We consider the binary-LWE problem, which is the learning with errors problem when the entries of the secret vector are chosen from $\\{ 0, 1\\}$ or $\\{ -1, 0, 1 \\}$ (and the error vector is sampled from a discrete Gaussian distribution). Our main result is an algorithm for binary-LWE that first translates the problem to the inhomogeneous short integer solution (ISIS) problem, and then solves the closest vector problem using a re-scaling of the lattice. We also discuss modulus switching as an approach to the problem. Our conclusions are that binary-LWE is easier than general LWE. We give experimental results that will be of help when choosing parameters for binary-LWE to achieve certain security levels.

22:17 [Pub][ePrint] (Efficient) Universally Composable Oblivious Transfer Using a Minimal Number of Stateless Tokens, by Seung Geol Choi and Jonathan Katz and Dominique Schröder and Arkady Yerukhimovich and Hong Sheng Z

  We continue the line of work initiated by Katz (Eurocrypt 2007) on using tamper-proof hardware for universally composable secure computation. As our main result, we show an efficient oblivious-transfer (OT) protocol in which two parties each create and exchange a single, stateless token and can then run an unbounded number of OTs. Our result yields what we believe is the most practical and efficient known approach for oblivious transfer based on tamper-proof tokens, and implies that the parties can perform (repeated) secure computation of arbitrary functions without exchanging additional tokens.

Motivated by this result, we investigate the minimal number of stateless tokens needed for universally composable OT/secure computation. We prove that our protocol is optimal in this regard for constructions making black-box use of the tokens (in a sense we define). We also show that nonblack-box techniques can be used to obtain a construction using only a single stateless token.

22:17 [Pub][ePrint] Trust Views for the Web PKI, by Johannes Braun, Florian Volk, Johannes Buchmann and Max Mühlhäuser

  The steadily growing number of certication authorities (CAs)

assigned to the Web Public Key Infrastructure (Web PKI) and trusted

by current browsers imposes severe security issues. Apart from being

impossible for relying entities to assess whom they actually trust, the

current binary trust model implemented with the Web PKI makes each

CA a single point of failure. In this paper, we present the concept of

trust views to manage variable trust levels for exactly those CAs actually

required by a relying entity. This reduces the set of trusted CAs

and minimizes the risk to rely on malicious certicates issued due to CA

failures or compromises.

19:17 [Pub][ePrint] Is Bitcoin a Decentralized Currency?, by Arthur Gervais and Ghassan Karame and Srdjan Capkun and Vedran Capkun

  Bitcoin has achieved large-scale acceptance and popularity by promising its users a low-cost, anonymous, and completely decentralized exchange of transactions. However, recent incidents and observations are revealing the true limits of decentralization in the Bitcoin system. In this article, we show that the vital operations and decisions that Bitcoin is currently undertaking are not decentralized. More specifically, we show that a limited set of entities currently control

the services, decision making, mining, and the incident resolution processes in Bitcoin. We also show that third-party entities can unilaterally decide to \"devalue\" any specific set of Bitcoin addresses pertaining to any entity participating in the system. Finally, we explore possible avenues to enhance the decentralization in the Bitcoin system.


  At Eurocrypt\'12, Pandey and Rouselakis~\\cite{PR12} proposed the notion of property preserving symmetric encryption ({\\PPE}).

They defined several security notions for {\\PPE} and studied their relationship. They also proposed a concrete scheme which preserves

the orthogonality of encrypted vectors. The proposed construction is claimed to achieve the strongest security notion

of property preserving encryption, called {\\LoR} security. In this work, we take a critical look at the three security theorems in the context of {\\PPE} from~\\cite{PR12}. In particular, we show

that the Pandey-Rouselakis construction does not even satisfy the weakest notion of security for {\\PPE}. We also note that

their separation results between different notions of security for {\\PPE} stand vacuous in the absence of any concrete example. We fill up this gap in the separation results of~\\cite{PR12} by suggesting an example construction of


19:17 [Pub][ePrint] Provable Security Proofs and their Interpretation in the Real World, by Vikram Singh

  This paper analyses provable security proofs, using the EDL signature scheme as its case study, and interprets their benefits and drawbacks when applied to the real world.

Provable security has been an area of contention. Some, such as Koblitz and Menezes, give little credit to the potential extra security provided and argue that it is a distracting goal. However, others believe that an algorithm with a security proof is superior to one without it, and are prepared to accept the impact to performance that their use might involve. Goldreich has been notable for his defence of the security proof, and for his opposition to the view of Koblitz and Menezes.

This paper is designed to help the reader make their own decisions on security proofs. We achieve this by giving an introduction to the typical security model used, then give a description of the EDL signature scheme and its tight reduction to the CDH problem in the Random Oracle Model, then analyse the proof\'s assumptions, meaning, validity and overhead for real world security.

19:17 [Pub][ePrint] Practical Dynamic Searchable Encryption with Small Leakage, by Emil Stefanov and Charalampos Papamanthou and Elaine Shi

  Dynamic Searchable Symmetric Encryption (DSSE) enables a client to encrypt his document collection in a way that it is still searchable and efficiently updatable. However, all DSSE constructions that have been presented in the literature so far come with several problems: Either they leak a significant amount of information (e.g., hashes of the keywords contained in the updated document) or are inefficient in terms of space or search/update time (e.g., linear in the number of documents).

In this paper we revisit the DSSE problem. We propose the first DSSE scheme that achieves the best of both worlds, i.e., both small leakage and efficiency. In particular, our DSSE scheme leaks significantly less information than any other previous DSSE construction and supports both updates and searches in sublinear time in the worst case, maintaining at the same time a data structure of only linear size. We finally provide an implementation of our construction, showing its practical efficiency.

19:17 [Pub][ePrint] Verifier-Based Password-Authenticated Key Exchange: New Models and Constructions, by Fabrice Benhamouda and David Pointcheval

  While password-authenticated key exchange (or PAKE) protocols have

been deeply studied, a server corruption remains the main threat, with

many concrete cases nowadays. Verifier-based PAKE (or VPAKE)

protocols, initially called Augmented-PAKE, have been proposed to

limit the impact of any leakage. However, no satisfactory security

model has ever been proposed to quantify the actual security of a

protocol in the standard model. The unique model proposed so far is an

ideal functionality in the universal composability (UC) framework, but

is only meaningful in idealized models.

In this paper, we first enhance the Bellare-Pointcheval-Rogaway

game-based model for PAKE to VPAKE protocols, and then propose the

first game-based security model for both PAKE and VPAKE protocols that

additionally handles related passwords. It also allows a VPAKE

protocol to be secure in the standard model. We then propose several

VPAKE candidates which involve smooth projective hash functions and

multi-linear maps.