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80
An improved LPN algorithm
 In Roberto De Prisco and Moti Yung, editors, SCN, volume 4116 of Lecture
"... Abstract. HB + is a sharedkey authentication protocol, proposed by Juels and Weis at Crypto 2005, using prior work of Hopper and Blum. Its very low computational cost makes it attractive for lowcost devices such as radiofrequency identification(RFID) tags. Juels and Weis gave a security proof, re ..."
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Abstract. HB + is a sharedkey authentication protocol, proposed by Juels and Weis at Crypto 2005, using prior work of Hopper and Blum. Its very low computational cost makes it attractive for lowcost devices such as radiofrequency identification(RFID) tags. Juels and Weis gave a security proof, relying on the hardness of the “learning parity with noise” (LPN) problem. Here, we improve the previous best known algorithm proposed by Blum, Kalai, and Wasserman for solving the LPN problem. This new algorithm yields an attack for HB + in the detectionbased model with work factor 2 52. 1
Robust gossiping with an application to consensus
 Journal of Computer and System Sciences
"... We study deterministic gossiping in synchronous systems with dynamic crash failures. Each processor is initialized with an input value called rumor. In the standard gossip problem, the goal of every processor is to learn all the rumors. When processors may crash, then this goal needs to be revised, ..."
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We study deterministic gossiping in synchronous systems with dynamic crash failures. Each processor is initialized with an input value called rumor. In the standard gossip problem, the goal of every processor is to learn all the rumors. When processors may crash, then this goal needs to be revised, since it is possible, at a point in an execution, that certain rumors are known only to processors that have already crashed. We define gossiping to be completed, for a system with crashes, when every processor knows either the rumor of processor v or that v has already crashed, for any processor v. We design gossiping algorithms that are efficient with respect to both time and communication. Let t < n be the number of failures, where n is the number of processors. If n − t = Ω(n/polylog n), then one of our algorithms completes gossiping in O(log 2 t) time and with O(n polylog n) messages. We develop an algorithm that performs gossiping with O(n 1.77) messages and in O(log 2 n) time, in any execution in which at least one processor remains nonfaulty. We show a tradeoff between time and communication in gossiping algorithms: if the number of messages is at most O(n polylog n), then the time has to be at least Ω ( log n. By way of application, we show that if n − t = Ω(n), then log(n log n)−log t consensus can be solved in O(t) time and with O(n log 2 t) messages.
Efficient FullySimulatable Oblivious Transfer
 the Journal of Cryptology
, 2007
"... Oblivious transfer, first introduced by Rabin, is one of the basic building blocks of cryptographic protocols. In an oblivious transfer (or more exactly, in its 1outof2 variant), one party known as the sender has a pair of messages and the other party known as the receiver obtains one of them. So ..."
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Oblivious transfer, first introduced by Rabin, is one of the basic building blocks of cryptographic protocols. In an oblivious transfer (or more exactly, in its 1outof2 variant), one party known as the sender has a pair of messages and the other party known as the receiver obtains one of them. Somewhat paradoxically, the receiver obtains exactly one of the messages (and learns nothing of the other), and the sender does not know which of the messages the receiver obtained. Due to its importance as a building block for secure protocols, the efficiency of oblivious transfer protocols has been extensively studied. However, to date, there are almost no known oblivious transfer protocols that are secure in the presence of malicious adversaries under the real/ideal model simulation paradigm (without using general zeroknowledge proofs). Thus, efficient protocols that reach this level of security are of great interest. In this paper we present efficient oblivious transfer protocols that are secure according to the ideal/real model simulation paradigm. We achieve constructions under the DDH, Nth residuosity and quadratic residuosity assumptions, as well as under the assumption that homomorphic encryption exists. 1
Group Testing with Probabilistic Tests: Theory, Design and Application
"... Identification of defective members of large populations has been widely studied in the statistics community under the name of group testing. It involves grouping subsets of items into different pools and detecting defective members based on the set of test results obtained for each pool. In a class ..."
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Identification of defective members of large populations has been widely studied in the statistics community under the name of group testing. It involves grouping subsets of items into different pools and detecting defective members based on the set of test results obtained for each pool. In a classical noiseless group testing setup, it is assumed that the sampling procedure is fully known to the reconstruction algorithm, in the sense that the existence of a defective member in a pool results in the test outcome of that pool to be positive. However, this may not be always a valid assumption in some cases of interest. In particular, we consider the case where the defective items in a pool can become independently inactive with a certain probability. Hence, one may obtain a negative test result in a pool despite containing some defective items. As a result, any sampling and reconstruction method should be able to cope with two different types of uncertainty, i.e., the unknown set of defective items and the partially unknown, probabilistic testing procedure. In this work, motivated by the application of detecting infected people in viral epidemics, we design nonadaptive sampling procedures that allow successful identification of the defective items through a set of probabilistic tests. Our design requires only a small number of tests to single out the defective items.
Exploring the design space of social networkbased Sybil defense
 In Proceedings of the 4th International Conference on Communication Systems and Network (COMSNETS’12
, 2012
"... Abstract—Recently, there has been significant research interest in leveraging social networks to defend against Sybil attacks. While much of this work may appear similar at first glance, existing social networkbased Sybil defense schemes can be divided into two categories: Sybil detection and Sybil ..."
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Abstract—Recently, there has been significant research interest in leveraging social networks to defend against Sybil attacks. While much of this work may appear similar at first glance, existing social networkbased Sybil defense schemes can be divided into two categories: Sybil detection and Sybil tolerance. These two categories of systems both leverage global properties of the underlying social graph, but they rely on different assumptions and provide different guarantees: Sybil detection schemes are applicationindependent and rely only on the graph structure to identify Sybil identities, while Sybil tolerance schemes rely on applicationspecific information and leverage the graph structure and transaction history to bound the leverage an attacker can gain from using multiple identities. In this paper, we take a closer look at the design goals, models, assumptions, guarantees, and limitations of both categories of social networkbased Sybil defense systems. I.
Sybildefender: Defend against sybil attacks in large social networks
 In IEEE INFOCOM
, 2012
"... Abstract—Distributed systems without trusted identities are particularly vulnerable to sybil attacks, where an adversary creates multiple bogus identities to compromise the running of the system. This paper presents SybilDefender, a sybil defense mechanism that leverages the network topologies to de ..."
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Abstract—Distributed systems without trusted identities are particularly vulnerable to sybil attacks, where an adversary creates multiple bogus identities to compromise the running of the system. This paper presents SybilDefender, a sybil defense mechanism that leverages the network topologies to defend against sybil attacks in social networks. Based on performing a limited number of random walks within the social graphs, SybilDefender is efficient and scalable to large social networks. Our experiments on two 3,000,000 node realworld social topologies show that SybilDefender outperforms the state of the art by one to two orders of magnitude in both accuracy and running time. SybilDefender can effectively identify the sybil nodes and detect the sybil community around a sybil node, even when the number of sybil nodes introduced by each attack edge is close to the theoretically detectable lower bound. Besides, we propose two approaches to limiting the number of attack edges in online social networks. The survey results of our Facebook application show that the assumption made by previous work that all the relationships in social networks are trusted does not apply to online social networks, and it is feasible to limit the number of attack edges in online social networks by relationship rating. I.
Towards Reliable Broadcasting using ACKs
"... Abstract — We propose a mechanism for reliable broadcasting in wireless networks, that consists of two components: a method for bandwidth efficient acknowledgment collection, and a coding scheme that uses acknowledgments. Our approach combines ideas from network coding and distributed space time cod ..."
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Abstract — We propose a mechanism for reliable broadcasting in wireless networks, that consists of two components: a method for bandwidth efficient acknowledgment collection, and a coding scheme that uses acknowledgments. Our approach combines ideas from network coding and distributed space time coding. I.
Canal: Scaling Social NetworkBased Sybil Tolerance Schemes
"... There has been a flurry of research on leveraging social networks to defend against multiple identity, or Sybil, attacks. A series of recent works does not try to explicitly identify Sybil identities and, instead, bounds the impact that Sybil identities can have. We call these approaches Sybil toler ..."
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There has been a flurry of research on leveraging social networks to defend against multiple identity, or Sybil, attacks. A series of recent works does not try to explicitly identify Sybil identities and, instead, bounds the impact that Sybil identities can have. We call these approaches Sybil tolerance; they have shown to be effective in applications including reputation systems, spam protection, online auctions, and content rating systems. All of these approaches use a social network as a credit network, rendering multiple identities ineffective to an attacker without a commensurate increase in social links to honest users (which are assumed to be hard to obtain). Unfortunately, a hurdle to practical adoption is that Sybil tolerance relies on computationally expensive network analysis, thereby limiting widespread deployment.
On spanners of geometric graphs
 In Proc. of the 10th Scandinavian Workshop on Algorithm Theory (SWAT), volume 4059 of LNCS
, 2006
"... Given a connected geometric graph G, we consider the problem of constructing a tspanner of G having the minimum number of edges. We prove that for every t with 1 < t < 1 log n, there exists a con4 nected geometric graph G with n vertices, such that every tspanner of G contains Ω(n1+1/t) edges. Th ..."
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Given a connected geometric graph G, we consider the problem of constructing a tspanner of G having the minimum number of edges. We prove that for every t with 1 < t < 1 log n, there exists a con4 nected geometric graph G with n vertices, such that every tspanner of G contains Ω(n1+1/t) edges. This bound almost matches the known upper bound, which states that every connected weighted graph with n vertices contains a tspanner with O(tn1+2/(t+1) ) edges. We also prove that the problem of deciding whether a given geometric graph contains a tspanner with at most K edges is NPhard. Previously, this NPhardness result was only known for nongeometric graphs. 1
Local Algorithms for Finding Interesting Individuals in Large Networks
"... Abstract: We initiate the study of local, sublinear time algorithms for finding vertices with extreme topological properties — such as high degree or clustering coefficient — in large social or other networks. We introduce a new model, called the Jump and Crawl model, in which algorithms are permitt ..."
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Abstract: We initiate the study of local, sublinear time algorithms for finding vertices with extreme topological properties — such as high degree or clustering coefficient — in large social or other networks. We introduce a new model, called the Jump and Crawl model, in which algorithms are permitted only two graph operations. The Jump operation returns a randomly chosen vertex, and is meant to model the ability to discover “new ” vertices via keyword search in the Web, shared hobbies or interests in social networks such as Facebook, and other mechanisms that may return vertices that are distant from all those currently known. The Crawl operation permits an algorithm to explore the neighbors of any currently known vertex, and has clear analogous in many modern networks. We give both upper and lower bounds in the Jump and Crawl model for the problems of finding vertices of high degree and high clustering coefficient. We consider both arbitrary graphs, and specializations in which some common assumptions are made on the global topology (such as power law degree distributions or generation via preferential attachment). We also examine local algorithms for some related vertex or graph properties, and discuss areas for future investigation.