Filters








18 Hits in 1.2 sec

Joining Local Knowledge to Communicate Reliably (Extended Abstract) [article]

Aris Pagourtzis, Giorgos Panagiotakos, Dimitris Sakavalas
2017 arXiv   pre-print
A fundamental primitive in distributed computing is Reliable Message Transmission (RMT), which refers to the task of correctly sending a message from a party (or player) to another, in a network where some intermediate relays might be controlled by an adversary. We address the problem under the realistic assumption that the topological knowledge of players is restricted to a certain subgraph and specifically study the role of local information exchange in the feasibility of RMT. We employ the
more » ... neral Adversary model of Hirt and Maurer and the recently introduced Partial Knowledge Model which subsume all known models for the adversary and local knowledge respectively. Tight feasibility conditions, naturally involving the network topology, the adversary and the local knowledge of players, are presented.
arXiv:1711.01725v1 fatcat:gdwoxxjz65dhzfd2gxy46bexpy

Reliable broadcast with respect to topology knowledge

Aris Pagourtzis, Giorgos Panagiotakos, Dimitris Sakavalas
2016 Distributed computing  
We study the Reliable Broadcast problem in incomplete networks against a Byzantine adversary. We examine the problem under the locally bounded adversary model of Koo (2004) and the general adversary model of Hirt and Maurer (1997) and explore the tradeoff between the level of topology knowledge and the solvability of the problem. We refine the local pair-cut technique of Pelc and Peleg (2005) in order to obtain impossibility results for every level of topology knowledge and any type of
more » ... n distribution. On the positive side we devise protocols that match the obtained bounds and thus, exactly characterize the classes of graphs in which Reliable Broadcast is possible. Among others, we show that Koo's Certified Propagation Algorithm (CPA) is unique against locally bounded adversaries in ad hoc networks, that is, it can tolerate as many local corruptions as any other non-faulty algorithm; this settles an open question posed by Pelc and Peleg. We also provide an adaptation of CPA against general adversaries and show its uniqueness in this case too. To the best of our knowledge this is the first optimal algorithm for Reliable Broadcast in generic topology ad hoc networks against general adversaries.
doi:10.1007/s00446-016-0279-6 fatcat:zcgrj45w7bfyboovk75h23ensm

Reliable Broadcast with Respect to Topology Knowledge [chapter]

Aris Pagourtzis, Giorgos Panagiotakos, Dimitris Sakavalas
2014 Lecture Notes in Computer Science  
We study the Reliable Broadcast problem in incomplete networks against a Byzantine adversary. We examine the problem under the locally bounded adversary model of Koo (2004) and the general adversary model of Hirt and Maurer (1997) and explore the tradeoff between the level of topology knowledge and the solvability of the problem. We refine the local pair-cut technique of Pelc and Peleg (2005) in order to obtain impossibility results for every level of topology knowledge and any type of
more » ... n distribution. On the positive side we devise protocols that match the obtained bounds and thus, exactly characterize the classes of graphs in which Reliable Broadcast is possible. Among others, we show that Koo's Certified Propagation Algorithm (CPA) is unique against locally bounded adversaries in ad hoc networks, that is, it can tolerate as many local corruptions as any other non-faulty algorithm; this settles an open question posed by Pelc and Peleg. We also provide an adaptation of CPA against general adversaries and show its uniqueness in this case too. To the best of our knowledge this is the first optimal algorithm for Reliable Broadcast in generic topology ad hoc networks against general adversaries.
doi:10.1007/978-3-662-45174-8_8 fatcat:mu43bnhnxnf23lhqfskbf6qcgu

Reliable Communication via Semilattice Properties of Partial Knowledge [chapter]

Aris Pagourtzis, Giorgos Panagiotakos, Dimitris Sakavalas
2017 Lecture Notes in Computer Science  
A fundamental communication primitive in distributed computing is Reliable Message Transmission (RMT), which refers to the task of correctly sending a message from a party to another, despite the presence of Byzantine corruptions. In this work we address the problem in the general adversary model of Hirt and Maurer [5] , which subsumes earlier models such as the global or local threshold adversaries. Regarding the topology knowledge, we employ the recently introduced Partial Knowledge Model
more » ... , which encompasses both the full knowledge and the ad hoc model; the latter assumes knowledge of the local neighborhood only. Our main contribution is a tight condition for achieving RMT in the partial knowledge model under a general adversary. A key algorithmic tool that we define and use is the joint view operation which imposes a semilattice structure on the partial knowledge possessed by different parties. In this context, we prove that the worst possible adversary structure, conforming with the initial knowledge of a set of parties, can be expressed as the supremum of the parties' knowledge under the semilattice partial order. The new operation allows for the definition of an appropriate network separator notion that yields a necessary condition for achieving RMT. In order to show the sufficiency of the condition, we propose the RMT Partial Knowledge Algorithm (RMT-PKA), an algorithm that also employs the joint view operation to solve RMT whenever the condition is met. This implies that RMT-PKA achieves reliable message transmission in every instance where this is possible, therefore it is a unique algorithm [13] . To the best of our knowledge, this is the first unique protocol for RMT against general adversaries in the partial knowledge model. Due to the generality of the model, our results provide, for any level of topology knowledge and any adversary structure, an exact characterization of instances where RMT is possible and an algorithm to achieve RMT on such instances.
doi:10.1007/978-3-662-55751-8_29 fatcat:7kow2q525re5fc7dtq27i7vpoa

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

Determinants of Processing Speed Trajectories among Middle Aged or Older Adults, and Their Association with Chronic Illnesses: The English Longitudinal Study of Aging

Viktor Gkotzamanis, Giorgos Koliopanos, Albert Sanchez-Niubo, Beatriz Olaya, Francisco Félix Caballero, José Luis Ayuso-Mateos, Somnath Chatterji, Josep Maria Haro, Demosthenes Panagiotakos
2021 Life  
The aim of this study was to identify latent groups of similar trajectories in processing speed through aging, as well as factors that are associated with these trajectories. In the context of the Ageing Trajectories of Health: Longitudinal Opportunities and Synergies (ATHLOS) project, data from the English Longitudinal Study of Aging (ELSA) (n = 12099) were analyzed. Latent groups of similar trajectories in the processing scores as well as their predictors and covariates were investigated,
more » ... g group-based trajectory models (GBTM). The coefficient estimates for potential group predictors correspond to parameters of multinomial logit functions that are integrated in the model. Potential predictors included sex, level of education, marital status, level of household wealth, level of physical activity, and history of smoking, while time-varying covariates included incidence of cardiovascular disease (CVD), diabetes mellitus, depressive symptoms, and sleep disturbances. Four trajectories were identified and named after their baseline scores and shapes: High (4.4%), Middle/Stable (31.5%), Low/Stable (44.5%), and Low Decline (19.6%). Female sex, higher levels of education, mild level of physical activity, having been married, and higher level of wealth were associated with a higher probability of belonging to any of the higher groups compared to the Low/Decline that was set as reference, while presence of CVD, diabetes mellitus, and depressive symptoms were associated with lower processing speed scores within most trajectories. All the aforementioned factors might be valid targets for interventions to reduce the burden of age-related cognitive impairment.
doi:10.3390/life11040357 pmid:33919625 fatcat:6ioswwhzrfhyrn3irhw5g6qdeq

Arterial hypertension assessed "out-of-office" in a contemporary cohort of rheumatoid arthritis patients free of cardiovascular disease is characterized by high prevalence, low awareness, poor control and increased vascular damage-associated "white coat" phenomenon

Athanase D Protogerou, Demosthenis B Panagiotakos, Evangelia Zampeli, Antonis A Argyris, Katerina Arida, Giorgos D Konstantonis, Christos Pitsavos, George D Kitas, Petros P Sfikakis
2013 Arthritis Research & Therapy  
doi:10.1186/ar4324 pmid:24286134 pmcid:PMC3978881 fatcat:tdr22ozotfe3pml4nzriqaa4yq

Leontio Lyceum ALbuminuria (3L Study) epidemiological study: aims, design and preliminary findings

Costas Tsioufis, Dimitris Tsiachris, Kyriakos Dimitriadis, Costas Thomopoulos, Dimitris Syrseloudis, Eirini Andrikou, Dimitris Chatzis, Efstathios Taxiarchou, Maria Selima, Anastasia Mazaraki, Giorgos Chararis, Panagiotis Tolis (+8 others)
Hellenic Journal of Cardiology  
The significance of microalbuminuria (MA) in paediatric essential hypertension has yet to be established. The Leontio Lyceum ALbuminuria Study (3L Study) was designed to determine the prevalence of MA among Greek schoolchildren and to evaluate these rates in relation to the children's anthropometric and lifestyle characteristics, and dietary habits. During April 2009, 498 students from the Leontio Lyceum, aged 12-17 years (7th-12th grade), were asked to participate in the 3L Study. For each
more » ... d a questionnaire was completed that was developed for the purposes of the study to retrieve information on socio-demographic and lifestyle characteristics, as well as dietary habits (through a semi-quantitative Food Frequency Questionnaire), and physical activity status. Overweight and obesity were defined using the international body mass index cut-off points established for children and young people. Office blood pressure (BP) was measured on two different occasions and those students who had BP >95th percentile for gender, age and height on both occasions were considered as hypertensives. Microalbuminuria was determined as albumin to creatinine ratio >or=22 mg/g in boys and >or=31 mg/g in girls in a morning spot urine sample using a quantitative assay (DCA 2000). The prevalence of MA was found to be 12.9% and that of childhood hypertension 5.2%. The prevalence of overweight status was 25.8% and 5.8% of the students were classified as obese. Low physical activity was reported by 7% of boys and girls, while 46.5% of the students reported participation in vigorous physical activities during a normal week. Based on the KIDMED score of each student, only 6% of them were classified as high adherers to a Mediterranean diet and 41.9% were classified as having very low diet quality. In this paper we present the aims, design and preliminary results of an epidemiological study on MA, hypertension, increased body size and lifestyle characteristics among Greek schoolchildren.
pmid:19942561 fatcat:gezf3suhr5d7xne5bp46ru3mru

Acute effects of second-hand smoke on complete blood count

Petros C. Dinas, Giorgos S. Metsios, Athanasios Z. Jamurtas, Manolis N. Tzatzarakis, A. Wallace Hayes, Yiannis Koutedakis, Aristidis M. Tsatsakis, Andreas D. Flouris
2013 International Journal of Environmental Health Research  
Indeed, some authors reported that WBC is increased in individuals commonly exposed to SHS (Ronchetti et al. 1990; Panagiotakos et al. 2004) , while others report no such changes (Husgafvel-Pursiainen  ...  Some epidemiological studies have assessed the effects of chronic exposure to SHS on CBC, indicating that white blood cell (WBC) count is the parameter most often affected (Ronchetti et al. 1990; Panagiotakos  ... 
doi:10.1080/09603123.2013.782603 pmid:23544435 fatcat:25jmhip42navjkmvdmfrmwfaiq

The mediterranean dietary pattern and breast cancer risk in Greek-Cypriot women: a case-control study

Christiana A Demetriou, Andreas Hadjisavvas, Maria A Loizidou, Giorgos Loucaides, Ioanna Neophytou, Sabina Sieri, Eleni Kakouri, Nicos Middleton, Paolo Vineis, Kyriacos Kyriacou
2012 BMC Cancer  
Lastly, we would like to thank dieticians Giorgos Loucaides, Rodoula Papalambrianou and Despo Michaelidou for the design of the FFQ as well as Stalo Lazarou for her advice and guidance concerning use of  ...  Mediterranean diet score by Panagiotakos The mean intake of the 11 food indices used to construct the Mediterranean Diet score by Panagiotakos among post-menopausal cases and controls is shown in Table  ...  A unit increase in the overall diet score by Panagiotakos was not associated with risk of post-menopausal BC (Table 4) .  ... 
doi:10.1186/1471-2407-12-113 pmid:22443862 pmcid:PMC3323439 fatcat:u2l46e54mrd35iw76h6eqysi5m

Christian Zervos, Cahiers d'art et le IVe Congrès international d'architecture moderne

Eleni Stavroulaki
2020 L'Âge d'or  
Cela dit, l'architecte Djelepy a parachevé ses études à Paris, entre 1918 et 1921, à l'École spéciale d'architecture, Papadakis également fut élève à l'École spéciale, entre 1924 et 1929, Giorgos Kalyvas  ...  Du point de vue grec, plusieurs architectes présentés dans l'article de Cahiers d'art, tels que Karantinos, Panagiotakos, Mitsakis, Papadakis, avaient reconnu, bien avant les années 1930, l'importance  ... 
doi:10.4000/agedor.3752 fatcat:34rd6zcpiva4tlb4n4zmuelgi4

Effects of Topology Knowledge and Relay Depth on Asynchronous Consensus [article]

Dimitris Sakavalas, Lewis Tseng, Nitin H. Vaidya
2018 arXiv   pre-print
URL: https: //doi.org/10.1007/s00446-016-0279-6, doi:10.1007/s00446-016-0279-6. 31 Aris Pagourtzis, Giorgos Panagiotakos, and Dimitris Sakavalas.  ...  Panagiotakos, and Dimitris Sakavalas.  ... 
arXiv:1803.04513v3 fatcat:2bppzfnv65awxmbvyag6a2chci

Mediterranean Lifestyle in Relation to Cognitive Health: Results from the HELIAD Study

Costas Anastasiou, Mary Yannakoulia, Meropi Kontogianni, Mary Kosmidis, Eirini Mamalaki, Efthimios Dardiotis, Giorgos Hadjigeorgiou, Paraskevi Sakka, Angeliki Tsapanou, Anastasia Lykou, Nikolaos Scarmeas
2018 Nutrients  
Adherence to the Mediterranean pattern was evaluated through the Mediterranean Dietary Score (MedDiet Score) proposed by Panagiotakos et al. [39] .  ... 
doi:10.3390/nu10101557 fatcat:k7pdast4frc2ba3yfh7ngy53la

Mediterranean diet and cognitive health: Initial results from the Hellenic Longitudinal Investigation of Ageing and Diet

Costas A. Anastasiou, Mary Yannakoulia, Mary H. Kosmidis, Efthimios Dardiotis, Giorgos M. Hadjigeorgiou, Paraskevi Sakka, Xanthi Arampatzi, Anastasia Bougea, Ioannis Labropoulos, Nikolaos Scarmeas, Catherine Feart
2017 PLoS ONE  
Adherence to the MeDi pattern was evaluated through the Mediterranean Dietary Score (MedDietScore), proposed by Panagiotakos et al. [71] .  ... 
doi:10.1371/journal.pone.0182048 pmid:28763509 pmcid:PMC5538737 fatcat:bfzi7ttqyfb6jark2szskwjih4

Proofs of Proofs of Work with Sublinear Complexity [chapter]

Aggelos Kiayias, Nikolaos Lamprou, Aikaterini-Panagiota Stouka
2016 Lecture Notes in Computer Science  
The authors wish to thank Giorgos Panagiotakos for helpful discussions as well as the anonymous referees of the 3rd Workshop on Bitcoin and Blockchain Research for their valuable comments.  ... 
doi:10.1007/978-3-662-53357-4_5 fatcat:k7ibpcsh25gaxepgzmjyou5sbm
« Previous Showing results 1 — 15 out of 18 results