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Most cities around the world require drivers to pay for the time they occupy a parking spot. In this way, drivers are encouraged to shorten parking time so that other drivers are given a reasonable chance of finding parking. The traditional way, based on moving to a pay station and placing the issued parking ticket on the dashboard of the car, presents several drawbacks like having to predict in advance the duration of parking or the need to move to the car in case the parking time has to be extended. Over the last few years, several applications permitting to pay through the mobile phone have appeared. Such applications manage detailed information about parking operations so that accurate profiles of parking habits of car owners can be created. In this paper we propose a system to pay for parking by phone which preserves the privacy of drivers in the sense that the information managed by the system is proven not to help an attacker with full access to it to do better that she would do by patrolling the city for collecting information about parked cars. ; The authors acknowledge partial support by the Spanish Government under project MTM2013–46949–P and by the Government of Catalonia under grants 2014SGR–1666 and 2016FI–B1 00155.

Social networks can be a very successful tool to engage users to discuss relevant topics for society. However, there are also some dangers that are associated with them, such as the emergence of polarization in online discussions. Recently, there has been a growing interest to try to understand this phenomenon, as some consider that this can be harmful concerning the building of a healthy society in which citizens get used to polite discussions and even listening to opinions that may be different from theirs. In this work, we face the problem of defining a precise measure that can quantify in a meaningful way the level of polarization present in an online discussion. We focus on the Reddit social network, given that its primary focus is to foster discussions, in contrast to other social networks that have some other uses. Our measure is based on two different characteristics of an online discussion: the existence of a balanced bipartition of the users of the discussion, where one partition contains mainly users in agreement (regarding the topic of the discussion) and the other users in disagreement, and the degree of negativity of the sentiment of the interactions between these two groups of users. We discuss how different characteristics of the discussions affect the value of our polarization measure, and we finally perform an empirical evaluation over different sets of Reddit discussions about diverse classes of topics. Our results seem to indicate that our measure can capture differences in the polarization level of different discussions, which can be further understood when analyzing the values of the different factors used to define the measure. ; This research was funded by Spanish Project PID2019-111544GB-C22 (MINECO/FEDER), by the European Union's Horizon 2020 Research and Innovation Program under Grant Agreements 723596, 768824, 764025 and 814945, and by 2017 SGR 1537.

Online debating forums are important social media for people to voice their opinions and engage in debates with each other. Measuring user relevance on these forums can be useful to identify different user profiles or behaviors in online debates, for example, users that tend to participate at the beginning of a debate and whose comments trigger participation, or users that post relevant comments but are not replied too much. To help users to distinguish such different user profiles, we propose graded measures based on users' influence, the controversy that they generate throughout the debates, their contribution to the polarization of the debates, and their social acceptance, that we extract by analyzing the debates in which the users participate. Our approach is based on an argumentation-based analysis that represents a debate as a valued argumentation framework, in which comments of a debate are arguments, the attack relation between arguments models disagreement between comments, and values for arguments represent the overall support of users for comments. Finally, we test our measures with a sample of users from Reddit debates, identifying four main groups of users, from users with almost no impact on the debate to very active ones with decisive comments for the outcome of the debate. ; This work was partially funded by Spanish Project TIN2015-71799-C2-2-P (MINECO/FEDER), by the European Union's Horizon 2020 Research and Innovation Program under Grant Agreements 723596, 768824, 764025 and 814945, and by 2017 SGR 1537. The authors would like to thank anonymous reviewers for providing helpful comments.

Comunicació presentada a la XV Reunión Española sobre Criptología y Seguridad de la Información, celebada a Granada del 3 al 5 d'octubre de 2018. ; Fine-grained electricity consumption information has been proven to allow to infer people's habits from their consumption patterns. Hence, smart metering systems need a secure way to regularly transmit the electricity consumption to the supplier in such a way that the privacy of customers is preserved. Several proposals can be found in the literature. Some of them, classified as aggregative solutions, organize the meters into groups and employ homomorphic cryptography in such a way that the supplier only obtains the aggregated consumption values of the meters in each group. In this paper we propose a technique which allows to remove the presence of a trusted dealer in one of the most efficient current proposals. Furthermore, we increase the overall performance of the resulting system by employing elliptic curve cryptography. The resulting system only requires a single message sent from each smart meter to its substation, without needing the meters to communicate among them. With the use of elliptic curve cryptography, the only relevant computation that the smart meters need to perform is a single elliptic curve point multiplication by a scalar. The system has been proven to be feasible and practical even for large neigborhoods including several thousands of meters. ; Research of the authors was supported in part by the European Regional Development Fund of the European Union in the scope of the "Programa Operatiu FEDER de Catalunya 2014–2020" (project COMRDI16-1-0060), and by the Spanish Ministry of Science, Innovation and Universities (project MTM2017-83271-R). The Cryptography & Graphs Research Group of Universitat de Lleida has been recognized as a Consolidated Research Group by Generalitat de Catalunya (2017 SGR 1158).