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Peer-to-peer (P2P) overlay networks have been proposed to solve routing problems of big distributed infrastructures, even for Internet scale. But the research community has been questioning the security of these networks for years. Most prior work in security services was focused on trust and reputation systems, anonymity, and secure routing. However, the proper management of identities in overlays is an important prerequisite to provide most of these security services. In this paper, we propose a protocol to control the access to a P2P overlay and to assign identities in a secure way; all this preserving the anonymity of users. This protocol involves two trusted third parties (TTPs), thanks to which it is possible to preserve the users' anonymity within the network without losing traceability. Users are authenticated by a TTP using real-world digital certificates, they select their network identifier jointly with the other TTP, and finally, the two TTPs issue the internal certificate to them. The protocol also provides revocability and protection against Sybil attacks, Eclipse attacks, whitewashers, and so on. A detailed protocol description is presented, and a performance and security analysis of the protocol is also provided. Copyright (c) 2014 John Wiley & Sons, Ltd., Peer Reviewed, Postprint (published version)

The introduction of sustainability skills into higher education curricula is a natural effect of the increasing importance of sustainability in our daily lives. Topics like green computing, sustainable design or environmental engineering have become part of the knowledge required by today’s engineers. Furthermore, we strongly believe that the introduction of this skill will eventually enable future engineers to develop sustainable products, services and projects. The Final Year Project is the last academic stage facing students and a step towards their future professional engineering projects. As such, it constitutes a rehearsal for their professional future and an ideal opportunity for reflecting on whether their Final Year Project is sustainable or not, and to what extent. It also provides a good tool for reviewing the lessons learned about sustainability during the degree course and for applying them in a holistic and integrated way. In this paper, we present a guide that allows both students and advisors to think carefully about the sustainability of engineering projects, in particular the Final Year Project., Postprint (author’s final draft)

Desde hace ños, la seguridad en las redes P2P estructuradas está siendo cuestionada, y por ello se han propuesto muchos trabajos con el objetivo de proporcionar enrutamiento seguro, sistemas de reputaci´on, control de acceso, confidencialidad de los datos, etc. Sin embargo, el proceso de asignaci´on de identidades se ha dejado casi totalmente olvidado. Estas redes est´an dise ˜nadas para que cada usuario tenga un identificador ´unico (nodeID), pero la mayor´ia de los sistemas existentes permiten que los usuarios puedan obtener un conjunto de ellos, e incluso seleccionarlos. Ambas actuaciones provocan problemas importantes de seguridad, ya que gracias a ello los usuarios pueden alterar el adecuado funcionamiento de la red. En este trabajo proponemos un protocolo de asignaci´on de nodeIDs basado en la emisi´on de certificados impl´icitos. Nuestro prop´osito es proporcionar servicios de seguridad que permitan luchar contra la mayor´ia de las amenazas que sufren estas redes, con especial atenci´on a la asignaci´on de identidades. Este protocolo se basa en el uso de certificados y la generaci´on conjunta de nodeIDs por parte la Autoridad de Certificaci´on (CA) y el nuevo usuario., Peer Reviewed, Postprint (published version)

This letter presents a new and simple model for a TCP flow experiencing random packet losses due to both transmission errors and congestion events. From this model, we will derive a straightforward expression of a unified loss probability (ULP). This ULP gives the opportunity to reuse classical analytic models to analyze the performance of TCP and to size the buffer to optimize the wireless link utilization. Extensive simulations using TCP Reno in ns-2 demonstrate that our model is valid not only for the extreme cases where either transmission errors or congestion losses dominate but also in the situations where both types of losses are significant., Peer Reviewed, Postprint (published version)

In this paper, we review the workflow of the channel and physical layer modules in VANET simulations and briefly survey some of the most used propagation and packet error models in network simulators for this kind of networks. Through numerical simulations, we compare these propagation models working with a realistic and straightforward packet error model. Our results indicate that there is not a most conservative propagation model and that conservative models vary with the channel capacity. Furthermore, different propagation models specifically designed for VANETs obtain very close results in channels with intermediate capacities., Peer Reviewed, Postprint (published version)

One of the critical security issues of Vehicular Ad Hoc Networks (VANETs) is the revocation of misbehaving vehicles. While essential, revocation checking can leak potentially sensitive information. Road Side Units (RSUs) receiving the certificate status queries could infer the identity of the vehicles posing the query. An important loss of privacy results from the RSUs ability to tie the checking vehicle with the query's target. We propose a Privacy Preserving Revocation mechanism (PPREM) based on a universal one-way accumulator. PPREM provides explicit, concise, authenticated and unforgeable information about the revocation status of each certificate while preserving the users' privacy., Peer Reviewed, Postprint (published version)

Recommendation systems are information-filtering systems that tailor information to users on the basis of knowledge about their preferences. The ability of these systems to profile users is what enables such intelligent functionality, but at the same time, it is the source of serious privacy concerns. In this paper we investigate a privacy-enhancing technology that aims at hindering an attacker in its efforts to accurately profile users based on the items they rate. Our approach capitalizes on the combination of two perturbative mechanisms—the forgery and the suppression of ratings. While this technique enhances user privacy to a certain extent, it inevitably comes at the cost of a loss in data utility, namely a degradation of the recommendation’s accuracy. In short, it poses a trade-off between privacy and utility. The theoretical analysis of such trade-off is the object of this work. We measure privacy as the Kullback-Leibler divergence between the user’s and the population’s item distributions, and quantify utility as the proportion of ratings users consent to forge and eliminate. Equipped with these quantitative measures, we find a closed-form solution to the problem of optimal forgery and suppression of ratings, an optimization problem that includes, as a particular case, the maximization of the entropy of the perturbed profile. We characterize the optimal trade-off surface among privacy, forgery rate and suppression rate,and experimentally evaluate how our approach could contribute to privacy protection in a real-world recommendation system., Peer Reviewed, Postprint (published version)

Personalized information systems are information-filtering systems that endeavor to tailor information-exchange functionality to the specific interests of their users. The ability of these systems to profile users is, on the one hand, what enables such intelligent functionality, but on the other, the source of innumerable privacy risks. In this paper, we justify and interpret KL divergence as a criterion for quantifying the privacy of user profiles. Our criterion, which emerged from previous work in the domain of information retrieval, is here thoroughly examined by adopting the beautiful perspective of the method of types and large deviation theory, and under the assumption of two distinct adversary models. In particular, we first elaborate on the intimate connection between Jaynes' celebrated method of entropy maximization and the use of entropies and divergences as measures of privacy; and secondly, we interpret our privacy metric as false positives and negatives in a binary hypothesis testing. (C) 2013 Elsevier B.V. All rights reserved., Peer Reviewed, Postprint (published version)

El estudio que se presenta a continuación aporta las reflexiones necesarias para empezar a trabajar en la consecución de un sistema de información que permita recopilar los datos estadísticos necesarios para extraer el conocimiento sobre la evolución del sistema universitario español. Demuestra la viabilidad legal, académica y técnica del sistema sin dudas ni lagunas. Aporta las especificaciones necesarias para su implementación. Y, finalmente, pone en manos de la voluntad la necesidad de su creación., Peer Reviewed, Postprint (published version)

In recent years, the general interest in routing for vehicular ad hoc networks (VANETs) has increased notably. Many proposals have been presented to improve the behavior of the routing decisions in these very changeable networks. In this paper, we propose a new routing protocol for VANETs that uses four different metrics. which are the distance to destination, the vehicles’ density, the vehicles’ trajectory and the available bandwidth, making use of the information retrieved by the sensors of the vehicle, in order to make forwarding decisions, minimizing packet losses and packet delay. Through simulation, we compare our proposal to other protocols, such as AODV (Ad hoc On-Demand Distance Vector), GPSR (Greedy Perimeter Stateless Routing), I-GPSR (Improvement GPSR) and to our previous proposal, GBSR-B (Greedy Buffer Stateless Routing Building-aware). Besides, we present a performance evaluation of the individual importance of each metric to make forwarding decisions. Experimental results show that our proposed forwarding decision outperforms existing solutions in terms of packet delivery., Peer Reviewed, Postprint (published version)

Collaborative tagging is one of the most popular services available online, and it allows end user to loosely classify either online or offline resources based on their feedback, expressed in the form of free-text labels (i.e., tags). Although tags may not be per se sensitive information, the wide use of collaborative tagging services increases the risk of cross referencing, thereby seriously compromising user privacy. In this paper, we make a first contribution toward the development of a privacy-preserving collaborative tagging service, by showing how a specific privacy-enhancing technology, namely tag suppression, can be used to protect end-user privacy. Moreover, we analyze how our approach can affect the effectiveness of a policy-based collaborative tagging system that supports enhanced web access functionalities, like content filtering and discovery, based on preferences specified by end users., Peer Reviewed, Postprint (published version)

This article presents a cross-layer packet scheduler to provide end-to-end QoS guarantees for Digital Video Broadcasting-Second Generation (DVB-S2) broadband satellite systems. The scheduler design is based on a cross-layer mechanism defined between the physical and the network layer. It includes an algorithm to guarantee the required QoS specifications established in the service level agreement. The algorithm calculation depends basically on two parameters: the available bandwidth present in a DVB-S2 satellite link and the QoS requirements of each traffic class defined by the satellite operator. The cross-layer scheduler's operation is demonstrated using the NS-2 simulator tool. The results show that the proposed mechanism maximizes the bandwidth utilization while enforcing the priority level of each service class when an extreme reduction of bandwidth caused by rain events is experienced. Copyright (C) 2012 John Wiley & Sons, Ltd., Peer Reviewed, Postprint (published version)

Many proposals have studied the optimization of the EDCA mechanism used by the IEEE 802.11p MAC layer to provide QoS. In this work, we focus on the queuing process done by the different Access Categories (ACs). We propose the use of a single common buffer with a prioritized buffer allocation algorithm inspired on the MAX-MIN principle for a fair sharing of a common resource. We test the proposed algorithm with a fixed and a dynamic guaranteed queue size for each AC. Results obtained from simulations in a multihop VANET scenario show that our approach outperforms traditional queuing used in MAC 802.11p., Peer Reviewed, Postprint (published version)

Traces related to our identity are left every day while we browse the Internet. Being the user’s information a very valued asset for most of the companies, user activities on Internet are permanently monitored, and the information obtained from this process is used by big advertising companies. Accurate user profiles are built based on web searches, tags, tweets and even clicks issued by users. Collecting and processing this huge amount of personal data represents a serious risk for the user’s privacy but most of people are not aware of such risk. We describe in this paper a way to measure the effectiveness of a query obfuscation method. Since privacy level of user profiles is only estimated theoretically in some previous work, we firstly create a privacy risk measuring tool in a Firefox add-on which we named PrivMeter. Besides warning the user about his privacy levels, this tool is especially useful to evaluate the obfuscation mechanism offered by another very well-known add-on called TrackMeNot. We find that, for identification attacks, TrackMeNot importantly improved the user’s privacy. However, against more sophisticated attacks, such as classification attacks, the obfuscation mechanism was not successful enough., Peer Reviewed, Postprint (published version)

Vehicular Ad Hoc Networks (VANETs) require mechanisms to authenticate messages, identify valid vehicles, and remove misbehaving vehicles. A Public Key Infrastructure (PKI) can be utilized to provide these functionalities using digital certificates. However, if a vehicle is no longer trusted, its certificates have to be immediately revoked and this status information has to be made available to other vehicles as soon as possible. The goal of this chapter is to introduce and describe in detail a certificate revocation mechanism based on the Merkle Hash Tree (MHT), which allows to efficiently distribute certificate revocation information in VANETs. For this, an extended-CRL is created by embedding a hash tree in each standard certificate revocation list (CRL). A node possessing an extended-CRL can respond to certificate status requests without having to send the complete CRL. Instead, the node can send a short response (less than 1 KB) that fits in a single UDP message. This means that any node possessing an extended-CRL, including Road Side Units (RSUs) or intermediate vehicles, can produce short certificate-status responses that can be easily authenticated. The main procedures involved in the proposed mechanism are described in detail. General security issues related to the mechanism are treated as well., Peer Reviewed, Postprint (published version)

A key aspect in the design of smart cities is, undoubtedly, a plan for the efficient management of utilities, enabled by technologies such as those entailing smart metering of the residential consumption of electricity, water or gas. While one cannot object to the appealing advantages of smart metering, the privacy risks posed by the submission of frequent, data-rich measurements cannot simply remain overlooked. The objective of this paper is to provide a general perspective on the contrasting issues of privacy and efficient utility management, by surveying the main requirements and tools, and by establishing exploitable connections. Copyright (c) 2013 John Wiley & Sons, Ltd., Peer Reviewed, Postprint (published version)

Buffer overflow is an important phenomenon in data networks that has much bearing on the overall network performance. Such overflow critically depends on the amount of storage space allotted to the transmission channels. To properly dimension this buffering capacity a detailed knowledge of some set of probabilities is needed. In real practice, however, that information is seldom available, and only a few average values are at the analyst disposal. In this paper, the use of a solution to this quandary based on maximum entropy is proposed. On the other hand, when wireless devices are taken into account, the transmission over a shared medium imposes additional restrictions. This paper also presents an extension of the maximum entropy approach for this kind of devices. The main purpose is that wireless nodes become able to dynamically self-configure their buffer sizes to achieve more efficient memory utilization while keeping bounded the packet loss probability. Simulation results using different network settings and traffic load conditions demonstrate meaningful improvement in memory utilization efficiency. This could potentially benefit devices of different wireless network technologies like mesh routers with multiple interfaces, or memory constraint sensor nodes. Additionally, when the available memory resources are not a problem, the buffer memory reduction also contributes to prevent the high latency and network performance degradation due to overbuffering. And it also facilitates the design and manufacturing of devices with faster memory technologies and future all-optical routers., Peer Reviewed, Postprint (published version)

This article proposes VSPLIT, a new architecture based on TCP cross-layering and splitting techniques for optimizing the transport layer performance in vehicular networks for Internet-based Vehicle-to-Infrastructure (V2I) communications. Our architecture mainly pretends to enhance the performance of TCP handovers in 802.11 networks. VSPLIT includes a cross-layer TCP protocol, called VSPLIT-TCP, that adapts the congestion control during the handover, learning the new characteristics of the network after the handover using the mechanisms provided by the IEEE 802.21 Media Independent Handover (MIH) services. VSPLIT has been implemented and tested in the NS-3 simulator. We include the some of the most interesting performance evaluation results, which show a good performance of our proposal for the intended scenario., Peer Reviewed, Postprint (published version)

This paper presents an analysis of several Transmission Control Protocol (TCP) variants working over a digital video broadcasting-second generation (DVB-S2) satellite link with the support of the Differentiated Services (DiffServ) architecture to provide quality of service (QoS). This analysis is carried out using the NS-2 simulator tool. Three TCP variants are considered: SACK TCP, Hybla TCP, and CUBIC TCP. These TCP variants are taken as a starting point because they have proven to be the most suitable variants to deal with long delays present in satellite links. The DVB-S2 link also introduces the challenge of dealing with variable bandwidth, whereas the DiffServ architecture introduces the challenge of dealing with different priorities. In this paper, we propose a DiffServ model that includes a modified queuing mechanism to enhance the goodput of the assured forwarding traffic class. This modified DiffServ model is simulated and tested, considering the interaction of the selected TCP variants. In addition, we present evaluation metrics, significant simulations results, and conclusions about the performance of these TCP variants evaluated over the proposed scenario. As a general conclusion, we show that CUBIC TCP is the TCP variant that shows the best performance in terms of goodput, latency, and friendliness., Peer Reviewed, Postprint (published version)

Wireless vehicular communications are a key technology to provide drivers with novel services such as collision avoidance, safety warnings and real-time traffic information. Other services are high-speed toll collection, infotainment and wireless ubiquitous connectivity. It is clear that vehicular communications will be a cornerstone of the future transport systems which will significantly change our daily lives. Vehicular ad hoc networks (VANETs) have rapidly emerged and raised novel research challenges such as the design of network protocols adapted to the specific features of VANETs, e.g. the high speed of vehicles. Also, the design of realistic simulation frameworks is an important goal to speed up the development of VANETs. This paper presents the design and evaluation of GBSR-B, a routing protocol based on GPSR that seeks to improve the performance of VANETs over urban scenarios. Our proposal includes a novel algorithm to select the optimal next-hop forwarding node. This paper also presents an alternative to the perimeter mode used in GPSR. We compare our proposal GBSR-B to GPSR and AODV using the network simulator NCTUns 6.0, showing better results in terms of packet losses., Peer Reviewed, Postprint (published version)

Given the all-pervasiveness of Systems thinking -which consists of thinking about things as systemsas a way of reasoning, in this work we will describe its application to make an interpretation of how to conceive and design a final year CDIO capstone course. Both the student teamwork structure as well as the complex engineering system itself addressed in the project are described in terms of entities, links, form and function, thereby pointing out their formal and functional interaction. The ultimate goal of the Systems thinking perspective is, given the necessary ingredients, to try maximizing the chances of the emergence of a fruitful capstone course, namely a culminating project that yields a set of students qualified to CDIO complex engineering systems., Peer Reviewed, Postprint (published version)

Recommender systems select the most interesting products for costumers based on their interests. The move of a recommender system to a cloud faces many challenges from the perspective of the protection of the participants. Little work has been done regarding secure recommender systems or how to cope with the legal liability of the cloud provider and any virtual machine inside the cloud. We propose DocCloud, a recommender system that focused on the protection of all participants against legal attacks. We present the architecture of DocCloud and analyze the security mechanisms that the system includes. Specifically, we study the properties of plausible deniability and anonymity of the recommenders and intermediate nodes. This way, nodes can recommend products to the customers while deny any knowledge about the product they are recommending or their participation in the recommendation process., Peer Reviewed, Postprint (published version)

In this paper, a new mechanism for topology control in wireless mesh networks is proposed. We evaluate the application to this problem of the centrality metrics developed by social network analysts. Our target network is a wireless mesh network created by user hand-held devices. For this kind of networks, we aim to construct a connected dominating set that includes the most central nodes. Many advantages result from selecting just a subset of stations for routing tasks: reduction of collisions, protocol overhead, interference and energy consumption, better network organization and scalability. The resulting performance using the three most common centrality measures (degree, closeness and betweenness) is evaluated. As we are working with dynamic and decentralized networks, Peer Reviewed, Postprint (published version)

Road safety has become a main issue for governments and car manufacturers in the last twenty years. The concept of Intelligent Transportation Systems (ITS) is used when talking about communication technologies between vehicles and infrastructure to improve, among others, road safety. In this paper, we propose a warning service to prevent accidents by alerting drivers about accidents and dangerous road conditions. This service includes the definition of a new broadcast dissemination mechanism. A VANET highway scenario is simulated to evaluate how the use of safety schemes decreases the driver’s reaction time when an unexpected situation occurs., Peer Reviewed, Postprint (published version)

El dimensionado del tamaño de los buffers de almacenamiento de paquetes en los equipos de red es un aspecto fundamental debido a sus consecuencias directas sobre la calidad de servicio observada por los usuarios finales. Dicho tamaño debe ser el más pequeño posible que cumpla con unos determinados requisitos respecto a algún parámetro de prestaciones de la red (caudal, tiempo extremo a extremo, probabilidad de pérdida de paquetes, etc.). En este artículo se propone un mecanismo de dimensionado que permite mantener la probabilidad de pérdida por debajo de un umbral determinado y que se adapta dinámicamente a las variaciones de carga en la red. El mecanismo, basado en una aproximación de máxima entropía, permite además el autodimensionado de buffers por parte de cada equipo, sin necesidad de la intervención de equipos especiales, y puede utilizarse también cuando se desea diferenciar diferentes flujos de tráfico y pedir para ellos distintas prestaciones., Peer Reviewed, Postprint (published version)

In this paper we propose an architecture to provide Quality of Service (QoS) guarantees for DVB-S2 broadband satellite systems. The proposed architecture provides low complexity on its implementation and can seamlessly inter-operate with terrestrial IP networks. The solution is designed in compliance with the ETSI-BSM-QoS framework and provides a detailed design at the Satellite Independent- Satellite Dependent (SI-SD) layers in order to provide QoS guarantees by means of traffic priorities. Particularly, at the SI layers several mechanism are defined to deal with QoS differentiation based on the DiffServ framework. Conversely, at the SD layers the application of different DVB-S2 channel adaptations are assumed. The proposed architecture is evaluated using the NS-2 simulator. The key results show that the implementation of this architecture, enables to keep control of the satellite system load while guaranteeing the QoS levels for the high priority traffic classes even though bandwidth variations due to rain events are experienced., Peer Reviewed, Postprint (published version)

Mobile ad hoc networks (MANETs) are infrastructureless networks formed by wireless mobile devices. Recently, the demand over multimedia services such as video streaming has increased specially since the number of mobile end users is growing as well. MPEG-2 VBR is one of the most fitting video coding techniques for MANETs which improves the distribution of video streams specially when it is used with a proper multipath routing scheme. In this article, we aimed to design a routing scheme to dynamically select the forwarding paths using a game-theoretic approach over a multipath routing protocol. Our proposal seeks to describe an equation of the probability p of sending video frames through the best available path. p depends on network parameters that vary throughout time. The aim is that the most important video frames (I+P) will be sent through the best path with a certain probability p and will be sent through the second best path with a probability 1-p. To achieve that, we carried out simulations done with fixed values of p and after that we applied a lineal regression method to obtain the coefficients of the equation for p. Simulations have been done to show the benefits of our proposal where interfering traffic and mobility of the nodes are present., Peer Reviewed, Postprint (published version)

Personal Health Systems (PHS) allow to move the point of care from hospitals to the patient's home. Moreover, a PHS usually handles much more information and provides more appropriate diagnostic and personalized treatments to individuals. In this paper, we present the objectives, structure and expected innovations of the TAMESIS project. TAMESIS aims to contribute in advancing the state of the art of some of the technologies needed for the development of Personal Health Systems. Specifically, we propose techniques for preventing denial of service, sensor node malfunctioning and traffic injection. In addition, we explore a protocol that makes use of mobile agents for the exchange of medical records between networked databases. As a novel aspect, the protocol will not only support bilateral agreements, already existing in the literature, but multilateral agreements. In reference to clinical data, is critical to preserve the privacy and intimacy of patients. Thus, the data, at the time of being collected as for when it is exchanged, should be processed so as to avoid leakage of information that is not strictly necessary for the parties to fulfill their task correctly. A privacy metric suitable for PHS, and the incorporation of mechanisms needed for privacy and intimacy are also key objectives of this project. Finally, usability aspects of all the system interfaces will be considered., Peer Reviewed, Postprint (published version)

In 2010, Sood-Sarje-Singh proposed two dynamic ID-based remote user authentication schemes. The first scheme is a security improvement of Liao et al.’s scheme and the second scheme is a security improvement of Wang et al.’s scheme. In both cases, the authors claimed that their schemes can resist many attacks. However, we find that both schemes have security flaws. In addition, their schemes require a verification table and time-synchronization, making the schemes unfeasible and unsecured for electronic services. In order to remedy the security flaws of Sood et al.’s schemes, we propose a robust scheme which resists the well-known attacks and achieves all the desirable security goals., Peer Reviewed, Postprint (published version)

This paper proposes an adaptation of the collision probability used in the measure of the available bandwidth designed for Mobile Ad hoc Networks (MANETs) and which is described in ABE [12]. Instead, we propose a new ABE+ that includes a new function to estimate the probability of losses. This new function has been specially designed for Vehicular Ad hoc Networks, to be suited to the high mobility and variable density in vehicular environments. In this analysis we do not only consider the packet size, but also other metrics, such as, density and speed of the nodes. We include the ABE+ algorithm in the forwarding decisions of the GBSR-B protocol [14], which is an improvement of the well-known GPSR protocol. Finally through simulations, we compare the performance of our new ABE+ compared to the original ABE. These results show that ABE+ coupled with GBSR-B achieves a good trade-o in terms of packet losses and packet end-to-end delay., Peer Reviewed, Postprint (published version)

Recently, Chen-Hsiang-Shih proposed a new dynamic ID-based remote user authentication scheme. The authors claimed that their scheme was more secure than previous works. However, this paper demonstrates that theirscheme is still unsecured against different kinds of attacks. In order to enhance the security of the scheme proposed by Chen-Hsiang-Shih, a new scheme is proposed. The scheme achieves the following security goals: without verification table, each user chooses and changes the password freely, each user keeps the password secret, mutual authentication, the scheme establishes a session key after successful authentication, and the scheme maintains the user's anonymity. Security analysis and comparison demonstrate that the proposed scheme is more secure than Das-Saxena-Gulati's scheme, Wang et al.'s scheme and Chen-Hsiang-Shih., Peer Reviewed, Postprint (published version)

Secure frameproof code is the name given to a separating code when studied in relation to fingerprinting schemes. Separating codes are combinatorial objects that have found to be useful in many areas such as technical diagnosis and the protection of distribution rights. A relaxed definition of the properties of separation and frameproofness, in the sense of only requiring the properties to hold with high probability, shows that for the relaxed definitions these notions are different. In this paper we address the construction of almost secure frameproof codes based on small-bias probability spaces., Peer Reviewed, Postprint (published version)

The security of the Peer-to-Peer (P2P) overlays networks has been questioned for years. Many works have been proposed to provide secure routing, anonymity, reputation systems, confidentiality, etc. However, the identity assignment has been less considered. These networks are designed so that each user has a unique identifier (nodeID), but the most of identity assignment systems allow malicious users to obtain a set of nodeIDs or even select certain identifiers. Thus, these users can disrupt the proper operation of a P2P overlay. In this paper, we propose a nodeID assignment protocol based on the issue of implicit certificates. Our purpose is to provide security services to struggle against the most of security threats in these networks with special attention to the identity assignment. This approach is based on the use of certificates and the joint generation of nodeIDs between a Certification Authority (CA) and the user. In addition, the use of implicit certificates presents certain advantages over the use of traditional certificates (explicit certificates)., Peer Reviewed, Postprint (published version)

Machine-to-machine networks are spreading over every sector of our society due to their self-organisation capabilities. In these networks, thousands of devices are left unattended for years of operation without the possibility of human intervention. In this sense, every step forward into avoiding early exhaustion of the network nodes is of paramount importance. We have introduced a novel authentication scheme that is able to discard non-intended and/or non-legitimate packets just after the reception of the physical preamble. This proposal was shown to yield enormous energy saving with regard to both node exhaustion attacks and normal network operation. In this paper, we extend that work with a novel synchronisation protocol that addresses previous desynchronisation issues. Besides, we analyse and propose the more appropriate deployment parameters that maximise the overall energy savings. We also detail the necessary key generation and key updating processes required to manage the in use keying material. Moreover, we show how to fit the proposed mechanism into the IEEE 802.15.4e amendment to the IEEE 802.15.4-2006 standard, as many companies have decide to go for this technology for the development of machine-to-machine networks, Peer Reviewed, Postprint (published version)

Muchas overlays P2P requieren ciertos servicios de seguridad para poder ser utilizadas como aplicaciones comerciales, y una simple Infraestructura de Clave P´ublica (PKI) podr´ia solucionar el problema. Sin embargo, estas infraestructuras tienen que estar vinculadas a un sistema de revocaci´on, como por ejemplo las Listas de Certificados Revocados (CRLs). Un sistema con una estructura cliente/servidor, donde una Autoridad de Certificaci´on (CA) juega el papel de servidor central. Por lo tanto, propenso a sufrir problemas en grandes redes por el hecho de tener un ´unico punto de fallo. Y si adem´as tenemos en cuenta que los usuarios de muchas redes P2P pueden cambiar de identidad, o incluso disponer de m´as de una, los problemas crecen. El tama˜no de las CRLs crecer´a exponencialmente con el n´umero de usuarios y deberemos actualizarlas con mucha frecuencia para garantizar la frescura de la informaci´on que contienen. Nosotros proponemos un nuevo sistema de revocaci´on distribuido para la red KAD. La distribuci´on de las CRLs se lleva a cabo utilizando la propia overlay, y para no comprometer la capacidad de almacenamiento de los nodos, las CRLs son divididas en segmentos. Este mecanismo mejora la accesibilidad de la informaci´on de revocaci´on, incrementa la disponibilidad de los segmentos, y garantiza la frescura de la informaci´on mediante la emisi´on de los segmentos de forma independiente., Peer Reviewed, Postprint (published version)

Vehicular ad hoc networks (VANETs) are emerging as a novel paradigm for safety services, supporting real-time applications (e.g., video-streaming, Internet browsing, online gaming, etc.). However, maintaining ubiquitous connectivity remains a challenge due to both high vehicle speed, and non-homogeneous nature of the network access infrastructure. Getting access to the network infrastructure must be controlled and only authorized users should be able to use it. However, the authentication process incurs in a not-negligible delay which can result in packet losses and other issues during handoffs. Hence, a fast and secure handoff scheme is essential. Although some solutions have been given in IEEE 802.11i and 802.11r standards, the handoff latency is still above 50 ms. Other protocols such as CAPWAP and HOKEY include support for fast handoff but have not been evaluated in a vehicular network. In this article, we analyze the security properties and performance of current proposals. Finally, simulations are conducted to date the effectiveness of the handoffs schemes., Peer Reviewed, Postprint (published version)

Vehicular ad hoc networks (VANETs) have emerged to leverage the power of modern communication technologies, applied to both vehicles and infrastructure. Allowing drivers to report traffic accidents and violations through the VANET may lead to substantial improvements in road safety. However, being able to do so anonymously in order to avoid personal and professional repercussions will undoubtedly translate into user acceptance. The main goal of this work is to propose a new collaborative protocol for enforcing anonymity in multi-hop VANETs, closely inspired by the well-known Crowds protocol. In a nutshell, our anonymous-reporting protocol depends on a forwarding probability that determines whether the next forwarding step in message routing is random, for better anonymity, or in accordance with the routing protocol on which our approach builds, for better quality of service (QoS). Different from Crowds, our protocol is specifically conceived for multi-hop lossy wireless networks. Simulations for residential and downtown areas support and quantify the usefulness of our collaborative strategy for better anonymity, when users are willing to pay an eminently reasonable price in QoS., Peer Reviewed, Postprint (published version)

Las redes ad hoc vehiculares (VANETs) han emergido como una tecnolog´ıa clave que posibilita el despliegue diversas aplicaciones. Proporcionar seguridad a los servicios de informaci´on y entretenimiento en estos entornos requiere del uso de unidades a pie de la carretera (RSU) como puertas de acceso a los recursos solicitados. Idealmente, las RSUs deben desplegarse para proporcionar conectividad continua. Si bien esto aumenta la capacidad y cobertura, tambi´en acarrea un aumento de la interferencia que puede degradar seriamente el rendimiento de la VANET. En este trabajo se estudia el impacto de la interferencia entre RSUs y sostenemos que la versi´on actual de la norma IEEE 1609.4 no puede hacer frente a la alta densidad vehicular. Extensas simulaciones apoyan las conclusiones obtenidas en este trabajo., Peer Reviewed, Postprint (published version)

This work describes the security architecture of MOSAIC, a protocol for clinical data exchange with multilateral agreement support. The blocks of the architecture are derived from a series of common attacks that can be done to the protocol. The fair exchange problem of the protocol is analyzed introducing the management messages that the agents must exchange in order to authorize or not the use of data. Due to multilateral agreements, loops can appear in the negotiation stage of the protocol. We describe the mechanisms to manage this loops, and we propose a solution to avoid that malicious agents can take advantage when there is a loop in the negotiation stage of the protocol., Peer Reviewed, Postprint (published version)

Actualmente, la monitorización de los usuarios en Internet es permanente, y la información obtenida en este proceso es de enorme interés para grandes compañías de publicidad e incluso gobiernos. Además, la gran cantidad de datos susceptibles de recopilarse por los sistemas de información personalizados representa un grave riesgo para la privacidad del usuario en Internet. Quizá aún más crítico es que muchos usuarios no son conscientes de este riesgo, ya que éste no es tan manifiesto como en el mundo físico. En este artículo presentamos un add-on de navegador que estima el riesgo de privacidad del perfil de un usuario, quien por sus hábitos de navegación, está expuesto a mecanismos de profiling en Internet. El nivel de riesgo se muestra, de manera comprensible y accesible en la interfaz gráfica del navegador y se calcula tomando en cuenta diferentes modelos de adversario., Peer Reviewed, Postprint (published version)

Message encryption does not prevent eavesdroppers from unveiling who is communicating with whom, when, or how frequently, a privacy risk wireless networks are particularly vulnerable to. The Crowds protocol, a well-established anonymous communication system, capitalizes on user collaboration to enforce sender anonymity. This work formulates a mathematical model of a Crowd-like protocol for anonymous communication in a lossy network, establishes quantifiable metrics of anonymity and quality of service (QoS), and theoretically characterizes the trade-off between them. The anonymity metric chosen follows the principle of measuring privacy as an attacker's estimation error. By introducing losses, we extend the applicability of the protocol beyond its original proposal. We quantify the intuition that anonymity comes at the expense of both delay and end-to-end losses. Aside from introducing losses in our model, another main difference with respect to the traditional Crowds is the focus on networks with stringent QoS requirements, for best effort anonymity, and the consequent elimination of the initial forwarding step. Beyond the mathematical solution, we illustrate a systematic methodology in our analysis of the protocol. This methodology includes a series of formal steps, from the establishment of quantifiable metrics all the way to the theoretical study of the privacy QoS trade-off. Copyright © 2013 John Wiley & Sons, Ltd., Peer Reviewed, Postprint (published version)

Vehicular Ad Hoc Networks (VANETs) require some mechanism to authenticate messages, identify valid vehicles, and remove misbehaving ones. A Public Key Infrastructure (PKI) can provide this functionality using digital certificates. In PKI, key management and corresponding issuance and revocation of digital certificates is one of the key issues that have to be solved. The IEEE 1609.2 standard states that VANETs will rely on the use of certificate revocation lists (CRLs) to achieve revocation. In this paper, we analyze the problems of using CRLs in these type of networks. Moreover, we describe the Risk Aware Revocation (RAR) mechanism that improves the traditional use of CRLs. RAR takes advantage of the two distinct channel types in VANETs to increase the freshness of the revocation information. Moreover, RAR allows users to gauge the risk of operating in a VANET when using CRLs., Peer Reviewed, Postprint (published version)

Peer Reviewed, Postprint (published version)

Las redes vehiculares requieren de alg ́ un meca- nismo para autenticar los mensajes, identificar a los veh ́ ıculos leg ́ ıtimos y sacar de la red aqu ́ ellos que no presenten un comportamiento adecuado. La infrastructura de clave p ́ ublica (PKI) puede proporcionar estos requisitos mediante el uso de certificados digitales. Sin embargo, la adopci ́ on de una PKI, conlleva la necesidad de gestionar no tan s ́ olo la emisi ́ on de certificados sino tambi ́ en su revocaci ́ on. El est ́ andar IEEE 1609.2 apunta que la revocaci ́ on de certificados en redes vehiculares debe depender del uso de Listas de Certificados Revocados (CRLs). En este art ́ ıculo, analizamos los problemas derivados del uso de CRLs en este tipo de redes. Asimismo, proponemos un mecanismo para gestionar el riesgo inherente del uso de estas listas el cual mejora el uso tradicional de las CRLs. Ayud ́ andose del canal de control de este tipo de redes, nuestro mecanismo es capaz de dar a conocer la frescura de los datos de revocaci ́ on en tiempo real. Adem ́ as, este mecanismo permite a los usuarios estimar el riesgo operacional que asumen al usar las CRLs., Postprint (published version)

One of the hardest tasks of a Public Key Infrastructure (PKI) is to manage revocation. Different revocation mechanisms have been proposed to invalidate the credentials of compromised or misbe- having users. All these mechanisms aim to optimize the transmission of revocation data to avoid unnecessary network overhead. To that end, they establish release policies bas ed on the assumption that the revoca- tion data follows uniform or Poisson distribution. Temporal distribution of the revocation data has a significant influence on the performance and scalability of the revocation service. In this paper, we demonstrate that the temporal distribution of the daily number of revoked certificates is statistically self-similar, and that the currently assumed Poisson distribu- tion does not capture the statistical properties of the distribution. None of the commonly used revocation models takes into account this fractal behavior, though such behavior has serious implications for the design, control, and analysis of revocation protocols such as CRL or delta-CRL., Peer Reviewed, Postprint (published version)

The ability to communicate securely is needed for many network applications. Public key infrastructure (PKI) is the most extended solution to verify and confirm the identity of each party involved in any secure transaction and transfer trust over the network. One of the hardest tasks of a certification infrastructure is to manage revocation. Research on this topic has focused on the trade-offs that different revocation mechanisms offer. However, less effort has been paid to understand the benefits of improving the revocation policies. In this paper, we analyze the behavior of the oligopoly of certificate providers that issue digital certificates to clients facing identical independent risks. We found the prices in the equilibrium, and we proof that certificate providers that offer better revocation information are able to impose higher prices to their certificates without sacrificing market share in favor of the other oligarchs. In addition, we show that our model is able to explain the actual tendency of the SSL market where providers with worst QoS are suffering loses., Postprint (published version)

List decoding is a decoding strategy that provides a set of codewords at the output of the channel decoder. Since this technique corrects errors beyond the correcting bound of the code, upper layers in the application or in the communications protocol can choose the appropriate candidate codeword among the elements of the set. The Koetter-Vardy algorithm is a soft-decision decoding algorithm for Reed-Solomon codes. It is based on two sequential processes: interpolation and factorization. In most applications it is interesting to efficiently decode in real time. This paper discusses some parallelization results about the interpolation process, which is the highest time-consuming part of the Koetter-Vardy algorithm., Postprint (published version)

Cognitive radio networks (CRNs) can improve the utilization of the spectrum by making use of licensed spectrum in an opportunistic manner. With such purpose, coexistence mechanisms among CRN nodes or secondary users and legitimate users of the spectrum or primary users are defined. However, due to the particular features of CRNs, new security threats arise, such as the primary user emulation (PUE) attack, which is the most challenging among all. With the aim of detecting such kind of attacks, in this paper we propose a cooperative localization method specifically suited to CRNs which relies on TDoA measurements and Taylor-series estimations. Simulations results show the goodness of the proposed method and its suitability to typical CRN scenarios., Peer Reviewed, Postprint (published version)

—We address the problem of query profile obfuscation by means of partial query exchanges between two users, in order for their profiles of interest to appear distorted to the information provider (database, search engine, etc.). We illustrate a methodology to reach mutual privacy gain, that is, a situation where both users increase their own privacy protection through collaboration in query exchange. To this end, our approach starts with a mathematical formulation, involving the modeling of the users’ apparent profiles as probability distributions over categories of interest, and the measure of their privacy as the corresponding Shannon entropy. The question of which query categories to exchange translates into finding optimization variables representing exchange policies, for various optimization objectives based on those entropies, possibly under exchange traffic constraints., Peer Reviewed, Postprint (published version)

Road safety has become a main issue for governments and car manufacturers in the last twenty years. The development of new vehicular technologies has favoured companies, researchers and institutions to focus their efforts on improving road safety. During the last decades, the evolution of wireless technologies has allowed researchers to design communication systems where vehicles participate in the communication networks. Thus, new types of networks, such as Vehicular Ad Hoc Networks (VANETs), have been created to facilitate communication between vehicles themselves and between vehicles and infrastructure. New concepts where vehicular networks play an important role have appeared the last years, such as smart cities and living labs [1]. Smart cities include intelligent traffic management in which data from the TIC (Traffic Information Centre) infrastructures could be reachable at any point. To test the possibilities of these future cities, living labs (cities in which new designed systems can be tested in real conditions) have been created all over Europe. The goal of our framework is to transmit information about the traffic conditions to help the driver (or the vehicle itself) take adequate decisions. In this work, the development of a warning system composed of Intelligent Traffic Lights (ITLs) that provides information to drivers about traffic density and weather conditions in the streets of a city is proposed and evaluated through simulations., Peer Reviewed, Postprint (published version)