Nikos Leonardos

Nikos Leonardos
National and Kapodistrian University of Athens | uoa · Sector of Theoretical Computer Science

Doctor of Philosophy

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21
Publications
4,413
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1,285
Citations

Publications

Publications (21)
Preprint
Full-text available
We investigate the randomized decision tree complexity of a specific class of read-once threshold functions. A read-once threshold formula can be defined by a rooted tree, every internal node of which is labeled by a threshold function $T_k^n$ (with output 1 only when at least $k$ out of $n$ input bits are 1) and each leaf by a distinct variable. S...
Preprint
Full-text available
Resource-based consensus is the backbone of permissionless distributed ledger systems. The security of such protocols relies fundamentally on the level of resources actively engaged in the system. The variety of different resources (and related proof protocols, some times referred to as PoX in the literature) raises the fundamental question whether...
Chapter
We consider (n, f)-evacuation on a circle, an evacuation problem of a hidden exit on the perimeter of a unit radius circle for n>1 robots, f of which are faulty. All the robots start at the center of the circle and move with maximum speed 1. Robots must first find the exit and then move there to evacuate in minimum time. The problem is considered c...
Chapter
We consider (n, f)-search on a circle, a search problem of a hidden exit on a circle of unit radius for robots, f of which are faulty. All the robots start at the centre of the circle and can move anywhere with maximum speed 1. During the search, robots may communicate wirelessly. All messages transmitted by all robots are tagged with the robots’ u...
Conference Paper
To participate in the distributed consensus of permissionless blockchains, prospective nodes-or miners-provide proof of designated, costly resources. However, in contrast to the intended decentralization, current data on blockchain mining unveils increased concentration of these resources in a few major entities, typically mining pools. To study st...
Preprint
To participate in the distributed consensus of permissionless blockchains, prospective nodes -- or miners -- provide proof of designated, costly resources. However, in contrast to the intended decentralization, current data on blockchain mining unveils increased concentration of these resources in a few major entities, typically mining pools. To st...
Conference Paper
Full-text available
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...
Article
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 li...
Conference Paper
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...
Article
Full-text available
We consider the problem of minimizing the communication in single-database private information retrieval protocols in the case where the length of the data to be transmitted is large. We present first rate-optimal protocols for 1-out-of-n computationallyprivate information retrieval (CPIR), oblivious transfer (OT), and strong conditional oblivious...
Conference Paper
Full-text available
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 h...
Conference Paper
Full-text available
Asymmetric fingerprinting schemes — introduced by Pfitzmann and Schunter in Eurocrypt 1996 — enable the transmission of a file stored in a server to a set of users so that each user obtains a variation of the file. The security considerations of these schemes are as follows: if any (appropriately bounded) subset of users collude to produce a “pirat...
Article
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...
Conference Paper
We prove that the randomized decision tree complexity of the recursive majority-of-three is Ω(2.55d ), 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 \(\Omega\...
Article
In this paper, the berth allocation problem with stochastic vessel handling times is formulated as a bi-objective problem. To solve the resulting problem, an evolutionary algorithm-based heuristic and a simulation-based Pareto front pruning algorithm is proposed. Computational examples show that the proposed approach provides solutions superior to...
Article
In this chapter we propose a game theoretic modeling framework based on the concept of Pareto optimality to schedule incoming trucks to the inbound doors of a freight distribution facility. We also attempt to address the issue of travel time variability from the truck's origin to the distribution facility. The problem is formulated as a bi-objectiv...
Article
Full-text available
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...
Conference Paper
Full-text available
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...

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