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Stable Matching with Evolving Preferences [article]

Varun Kanade, Nikos Leonardos, Frédéric Magniez
2016 arXiv   pre-print
We consider the problem of stable matching with dynamic preference lists. At each time step, the preference list of some player may change by swapping random adjacent members. The goal of a central agency (algorithm) is to maintain an approximately stable matching (in terms of number of blocking pairs) at all times. The changes in the preference lists are not reported to the algorithm, but must instead be probed explicitly by the algorithm. We design an algorithm that in expectation and with
more » ... h probability maintains a matching that has at most O((log (n))^2) blocking pairs.
arXiv:1509.01988v2 fatcat:rgks7bhxa5hdncdegakwc3a25a

Hellinger volume and number-on-the-forehead communication complexity [article]

Troy Lee, Nikos Leonardos, Michael Saks, Fengming Wang
2014 arXiv   pre-print
Information-theoretic methods have proven to be a very powerful tool in communication complexity, in particular giving an elegant proof of the linear lower bound for the two-party disjointness function, and tight lower bounds on disjointness in the multi-party number-in-the-hand (NIH) model. In this paper, we study the applicability of information theoretic methods to the multi-party number-on-the-forehead model (NOF), where determining the complexity of disjointness remains an important open
more » ... oblem. There are two basic parts to the NIH disjointness lower bound: a direct sum theorem and a lower bound on the one-bit AND function using a beautiful connection between Hellinger distance and protocols revealed by Bar-Yossef, Jayram, Kumar and Sivakumar [BYJKS04]. Inspired by this connection, we introduce the notion of Hellinger volume. We show that it lower bounds the information cost of multi-party NOF protocols and provide a small toolbox that allows one to manipulate several Hellinger volume terms and lower bound a Hellinger volume when the distributions involved satisfy certain conditions. In doing so, we prove a new upper bound on the difference between the arithmetic mean and the geometric mean in terms of relative entropy. We then apply these new tools to obtain a lower bound on the informational complexity of the AND_k function in the NOF setting. Finally, we discuss the difficulties of proving a direct sum theorem for information cost in the NOF model.
arXiv:1407.5425v1 fatcat:f4i3uefklbew5k7jklu35cez4e

The Bitcoin Backbone Protocol: Analysis and Applications [chapter]

Juan Garay, Aggelos Kiayias, Nikos Leonardos
2015 Lecture Notes in Computer Science  
Bitcoin is the first and most popular decentralized cryptocurrency to date. In this work, we extract and analyze the core of the Bitcoin protocol, which we term the Bitcoin backbone, and prove two of its fundamental properties which we call common prefix and chain quality in the static setting where the number of players remains fixed. Our proofs hinge on appropriate and novel assumptions on the "hashing power" of the adversary relative to network synchronicity; we show our results to be tight
more » ... nder high synchronization. Next, we propose and analyze applications that can be built "on top" of the backbone protocol, specifically focusing on Byzantine agreement (BA) and on the notion of a public transaction ledger. Regarding BA, we observe that Nakamoto's suggestion falls short of solving it, and present a simple alternative which works assuming that the adversary's hashing power is bounded by 1/3. The public transaction ledger captures the essence of Bitcoin's operation as a cryptocurrency, in the sense that it guarantees the liveness and persistence of committed transactions. Based on this notion we describe and analyze the Bitcoin system as well as a more elaborate BA protocol, proving them secure assuming high network synchronicity and that the adversary's hashing power is strictly less than 1/2, while the adversarial bound needed for security decreases as the network desynchronizes.
doi:10.1007/978-3-662-46803-6_10 fatcat:o4dhxeta4rbylgyjhr5lwi6v3a

Flexible Sensors—From Materials to Applications

Júlio C. Costa, Filippo Spina, Pasindu Lugoda, Leonardo Garcia-Garcia, Daniel Roggen, Niko Münzenrieder
2019 Technologies  
Flexible sensors have the potential to be seamlessly applied to soft and irregularly shaped surfaces such as the human skin or textile fabrics. This benefits conformability dependant applications including smart tattoos, artificial skins and soft robotics. Consequently, materials and structures for innovative flexible sensors, as well as their integration into systems, continue to be in the spotlight of research. This review outlines the current state of flexible sensor technologies and the
more » ... ct of material developments on this field. Special attention is given to strain, temperature, chemical, light and electropotential sensors, as well as their respective applications.
doi:10.3390/technologies7020035 fatcat:p5znlbmebnefbj7jta3tk7sfwu

Oceanic Games: Centralization Risks and Incentives in Blockchain Mining [article]

Nikos Leonardos, Stefanos Leonardos, Georgios Piliouras
2019 arXiv   pre-print
Acknowledgements Stefanos Leonardos would like to thank Costis Melolidakis for drawing his attention to the concept of Oceanic Games.  ...  Stefanos Leonardos and Georgios Piliouras acknowledge that this work was supported in part by the National Research Foundation (NRF), Prime Minister's Office, Singapore, under its National Cybersecurity  ... 
arXiv:1904.02368v3 fatcat:zbui45a6jrdjtkpee4ybfeoiui

The Bitcoin Backbone Protocol with Chains of Variable Difficulty [chapter]

Juan Garay, Aggelos Kiayias, Nikos Leonardos
2017 Lecture Notes in Computer Science  
Bitcoin's innovative and distributedly maintained blockchain data structure hinges on the adequate degree of difficulty of so-called "proofs of work," which miners have to produce in order for transactions to be inserted. Importantly, these proofs of work have to be hard enough so that miners have an opportunity to unify their views in the presence of an adversary who interferes but has bounded computational power, but easy enough to be solvable regularly and enable the miners to make progress.
more » ... As such, as the miners' population evolves over time, so should the difficulty of these proofs. Bitcoin provides this adjustment mechanism, with empirical evidence of a constant block generation rate against such population changes. In this paper we provide the first (to our knowledge) formal analysis of Bitcoin's target (re)calculation function in the cryptographic setting, i.e., against all possible adversaries aiming to subvert the protocol's properties. We extend the q-bounded synchronous model of the Bitcoin backbone protocol [Eurocrypt 2015], which posed the basic properties of Bitcoin's underlying blockchain data structure and shows how a robust public transaction ledger can be built on top of them, to environments that may introduce or suspend parties in each round. We provide a set of necessary conditions with respect to the way the population evolves under which the "Bitcoin backbone with chains of variable difficulty" provides a robust transaction ledger in the presence of an actively malicious adversary controlling a fraction of the miners strictly below 50% at each instant of the execution. Our work introduces new analysis techniques and tools to the area of blockchain systems that may prove useful in analyzing other blockchain protocols.
doi:10.1007/978-3-319-63688-7_10 fatcat:erpkke25fjfhllazp4shxu37fm

Lower Bounds on the Randomized Communication Complexity of Read-Once Functions

Nikos Leonardos, Michael Saks
2010 Computational Complexity  
We prove lower bounds on the randomized two-party communication complexity of functions that arise from read-once boolean formulae. A read-once boolean formula is a formula in propositional logic with the property that every variable appears exactly once. Such a formula can be represented by a tree, where the leaves correspond to variables, and the internal nodes are labeled by binary connectives. Under certain assumptions, this representation is unique. Thus, one can define the depth of a
more » ... la as the depth of the tree that represents it. The complexity of the evaluation of general read-once formulae has attracted interest mainly in the decision tree model. In the communication complexity model many interesting results deal with specific read-once formulae, such as DISJOINTNESS and TRIBES. In this paper we use information theory methods to prove lower bounds that hold for any read-once formula. Our lower bounds are of the form n(f )/c d(f ) , where n(f ) is the number of variables and d(f ) is the depth of the formula, and they are optimal up to the constant in the base of the denominator.
doi:10.1007/s00037-010-0292-2 fatcat:hiy5kdsiibbldhp73h3owfzgu4

An Improved Lower Bound for the Randomized Decision Tree Complexity of Recursive Majority, [chapter]

Nikos Leonardos
2013 Lecture Notes in Computer Science  
We prove that the randomized decision tree complexity of the recursive majority-of-three is Ω(2.55 d ), where d is the depth of the recursion. The proof is by a bottom up induction, which is same in spirit as the one in the proof of Saks and Wigderson in their 1986 paper on the complexity of evaluating game trees. Previous work includes an Ω (7/3) d lower bound, published in 2003 by Jayram, Kumar, and Sivakumar. Their proof used a top down induction and tools from information theory. In 2011,
more » ... gniez, Nayak, Santha, and Xiao, improved the lower bound to Ω (5/2) d and the upper bound to O(2.64946 d ).
doi:10.1007/978-3-642-39206-1_59 fatcat:sjvfccrhjncx5ogtvetuzuv75a

Stable Matching with Evolving Preferences

Varun Kanade, Nikos Leonardos, Frédéric Magniez, Marc Herbstritt
2016 International Workshop on Approximation Algorithms for Combinatorial Optimization  
Leonardos, and F.  ...  Leonardos, and F. Magniez 36:9 for sufficiently large C. We have P T k=1 X k > 2C log n ≤ P T > Cn log n + P T k=1 X k > 2C log n ∧ T ≤ Cn log n ≤ P T > Cn log n + P m k=1 X k > 2C log n .  ... 
doi:10.4230/lipics.approx-random.2016.36 dblp:conf/approx/KanadeLM16 fatcat:lw2nltfnu5aedcpqgehpifsdh4

Towards A Global Record Of Stocks And Fisheries

Yannis Tzitzikas, Yannis Marketakis, Nikos Minadakis, Michalis Mountantonakis, Leonardo Candela, Francesco Mangiacrapa, Pasquale Pagano, Costantino Perciante, Donatella Castelli, Marc Taconet, Aureliano Gentile, Giulia Gorelli
2017 Zenodo  
The collation of information for the monitoring of fish stocks and fisheries is a difficult and time-consuming task, as the information is scattered across different databases and is modelled using different formats and semantics. Our purpose is to offer a unified view of the existing stocks and fisheries information harvested from three different database sources (FIRMS, RAM and FishSource), by relying on innovative data integration and manipulation facilities. In this paper, we describe the
more » ... tivities carried out to realize the Global Record of Stocks and Fisheries (GRSF) which aims at offering an integrated and enriched view on data about fish stocks and fisheries from the database sources. More specifically we describe the model, the workflow and the software components for producing GRSF records and make them easily available to the users.
doi:10.5281/zenodo.1118191 fatcat:qg7kldsui5az5hdhwhy45d2kau

Optimal Rate Private Information Retrieval from Homomorphic Encryption

Aggelos Kiayias, Nikos Leonardos, Helger Lipmaa, Kateryna Pavlyk, Qiang Tang
2015 Proceedings on Privacy Enhancing Technologies  
This research was performed while Leonardos and Tang were at the University of Athens. Kiayias, Leonardos and Tang were supported by ERC project CODAMODA.  ... 
doi:10.1515/popets-2015-0016 dblp:journals/popets/KiayiasLLPT15 fatcat:g7uy66menrbdbpemzot2vu75bq

Bootstrapping the Blockchain, with Applications to Consensus and Fast PKI Setup [chapter]

Juan A. Garay, Aggelos Kiayias, Nikos Leonardos, Giorgos Panagiotakos
2018 Lecture Notes in Computer Science  
The Bitcoin backbone protocol (Eurocrypt 2015) extracts basic properties of Bitcoin's underlying blockchain data structure, such as "common prefix" and "chain quality," and shows how fundamental applications including consensus and a robust public transaction ledger can be built on top of them. The underlying assumptions are "proofs of work" (POWs), adversarial hashing power strictly less than 1/2 and no adversarial pre-computation-or, alternatively, the existence of an unpredictable "genesis"
more » ... lock. In this paper we first show how to remove the latter assumption, presenting a "bootstrapped" Bitcoin-like blockchain protocol relying on POWs that builds genesis blocks "from scratch" in the presence of adversarial pre-computation. Importantly, the round complexity of the genesis block generation process is independent of the number of participants. Next, we consider applications of our construction, including a PKI generation protocol and a consensus protocol without trusted setup assuming an honest majority (in terms of computational power). Previous results in the same setting (unauthenticated parties, no trusted setup, POWs) required a round complexity linear in the number of participants. Related work. Nakamoto [32] proposed Bitcoin, the first decentralized currency system based on POWs while relaxing the anonymity property of a digital currency to mere pseudonymity. This work was followed by a multitude of other related proposals including Litecoin, Primecoin [29] , and Zerocash [4], and further analysis improvements (e.g., [17, 18] ), to mention a few. As mentioned above, we work in a model that generalizes the model put forth by Garay et al. [21] , who abstracted out and formalized the core of the Bitcoin protocol-the Bitcoin backbone. As presented in [21] , however, the protocol considers as valid any chain that extends the empty chain, which is not going to work in our model. Indeed, if the adversary is allowed polynomial-time pre-computation, he can prepare a very long, private chain; then, by revealing blocks of this chain at the rate that honest players compute new blocks, he can break security. As also mentioned above, to overcome this problem one can assume that at the time honest parties start the computation, they have access to a fresh common random string (a "genesis" block). Then, if we consider as valid only the chains that extend this block, all results proved in [21] follow, since the probability that the adversary can use blocks mined before honest players "woke up" is negligible in the security parameter. In this paper we show how to establish such genesis block directly, and in a number of rounds essentially independent of the number of participants. To our knowledge, the idea of using POWs to distributedly agree on something (specifically, a PKI) in an unauthenticated setting with no trusted setup was first put forth by Aspnes et al. [2] , who suggested to use them as an identityassignment tool as a way to combat Sybil attacks [14] , and in such a way that the number of identities assigned to the honest and adversarial parties can be made proportional to their aggregate computational power, respectively. For example, by assuming that the adversary's computational power is less than 50%, one of the algorithms in [2] results in a number of adversarial identities less than half of that obtained by the honest parties. By running this procedure in a pre-processing stage, it is then suggested in [2] that a standard authenticated broadcast protocol (specifically, the one by Dolev and Strong [13]) could be run. Such protocols, however, would require that the PKI be consistent, details of which are not laid out in [2] . They are in [1], where Andrychowicz and Dziembowski address the more general goal of secure computation in this setting based on POWs, as mentioned earlier; the POWs are used to build a "graded" PKI, where keys have "ranks." The graded PKI is an instance of a "graded agreement," or "partial consistency" problem [12, 19, 20] , where honest parties do not disagree "by much," according to some metric. In [19] , Fitzi calls this the b-set-neighboring problem ("proxcast" in [12]), with b the number of possible "grades," and shows how to achieve global consistency by running the b-set-neighboring protocol multiple times. In [1], the
doi:10.1007/978-3-319-76581-5_16 fatcat:iipo5643ojdefohdovodx4fwvi

Towards A Global Record Of Stocks And Fisheries

Yannis Tzitzikas, Yannis Marketakis, Nikos Minadakis, Michalis Mountantonakis, Leonardo Candela, Francesco Mangiacrapa, Pasquale Pagano, Costantino Perciante, Donatella Castelli, Marc Taconet, Aureliano Gentile, Giulia Gorelli
2017 Zenodo  
The collation of information for the monitoring of fish stocks and fisheries is a difficult and time-consuming task, as the information is scattered across different databases and is modelled using different formats and semantics. Our purpose is to offer a unified view of the existing stocks and fisheries information harvested from three different database sources (FIRMS, RAM and FishSource), by relying on innovative data integration and manipulation facilities. In this paper, we describe the
more » ... tivities carried out to realize the Global Record of Stocks and Fisheries (GRSF) which aims at offering an integrated and enriched view on data about fish stocks and fisheries from the database sources. More specifically we describe the model, the workflow and the software components for producing GRSF records and make them easily available to the users.
doi:10.5281/zenodo.1099798 fatcat:o6sdkmln4bf7tly6q366cfjini

Improved periodic data retrieval in asynchronous rings with a faulty host

Evangelos Bampas, Nikos Leonardos, Euripides Markou, Aris Pagourtzis, Matoula Petrolia
2015 Theoretical Computer Science  
The exploration problem has been extensively studied in unsafe networks containing malicious hosts of a highly harmful nature, called black holes, which completely destroy mobile agents that visit them. In a recent work, Královič and Miklík [SIROCCO 2010, LNCS 6058, pp. 157-167] considered various types of malicious host behavior in the context of the Periodic Data Retrieval problem in asynchronous ring networks with exactly one malicious host. In this problem, a team of initially co-located
more » ... nts must report data from all safe nodes of the network to the homebase, infinitely often. The malicious host can choose whether to kill visiting agents or allow them to pass through (gray hole). In another variation of the model, the malicious host can, in addition, alter its whiteboard contents in order to deceive visiting agents. The goal is to design a protocol for Periodic Data Retrieval using as few agents as possible. In this paper, we present the first nontrivial lower bounds on the number of agents for Periodic Data Retrieval in asynchronous ring networks. Specifically, we show that at least 4 agents are needed when the malicious ⋆ Partial support by the ANR project DISPLEXITY (ANR-11-BS02-014). ⋆⋆ host is a gray hole, and at least 5 agents are needed when the malicious host whiteboard is unreliable. This improves the previous lower bound of 3 in both cases and answers an open question posed in the aforementioned paper. On the positive side, we propose an optimal protocol for Periodic Data Retrieval in asynchronous rings with a gray hole, which solves the problem with only 4 agents. This improves the previous upper bound of 9 agents and settles the question of the optimal number of agents in the gray-hole case. Finally, we propose a protocol with 7 agents when the whiteboard of the malicious host is unreliable, significantly improving the previously known upper bound of 27 agents. Along the way, we set forth a detailed framework for studying networks with malicious hosts of varying capabilities.
doi:10.1016/j.tcs.2015.09.019 fatcat:6hwbtigaknat7k3p4dywrojemq

The nutritional value of algae grown under different culture conditions for Mytilus edulis L. larvae

Nikos Leonardos, Ian A.N Lucas
2000 Aquaculture  
Factors influencing algal Ž . biochemical composition have been described by Leonardos 1998 .  ...  Environmental Ž conditions have been shown to influence algal biochemical composition Leonardos and . Lucas, 2000a .  ... 
doi:10.1016/s0044-8486(99)00269-0 fatcat:ucuqxm2ounhgpmmo4tydtbyxoq
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