http://hdl.handle.net/2117/3531 2013-05-23T22:22:16Z http://hdl.handle.net/2117/17225 Title: Cifrado homomorfico de clave publica basado en Residuosidad Cuadratica Authors: Herranz Sotoca, Javier; Sisternes, Juan Ramón Abstract: Los esquemas de cifrado de clave p´ ublica con propiedades homom´orficas tienen muchas utilidades en aplicaciones reales. Entre los esquemas con propiedades homom´orficas aditivas existentes, hay una familia (desde el esquema de Goldwasser-Micali hasta el esquema de Paillier) cuya seguridad se basa en problemas computacionalmente dif´ıciles relacionados con el problema de factorizar un n´umero grande N. Los esquemas de esta familia tienen diferentes propiedades tanto en lo referente a la eficiencia, como al problema de teor´ıa de n´umeros concreto en el que basan su seguridad. En este art´ıculo proponemos un nuevo esquema a a˜nadir a esta familia. La hip´otesis computacional en la que se basa la seguridad de nuestro esquema es la hip´otesis de la Residuosidad Cuadr´atica m´odulo N. En t´erminos de eficiencia, por un lado nuestro esquema mejora todos los esquemas anteriores cuya seguridad se basa en la hip´otesis de la Residuosidad d-´esima m´odulo N, para d 2; por otro lado, nuestro esquema es en general menos eficiente (tiempo de descifrado) que algunos esquemas como el de Paillier, cuya seguridad se basa en otra hip´otesis (Residuosidad N-´esima m´odulo N2). Sin embargo, si los mensajes a cifrar son cortos, la eficiencia de nuestro esquema es esencialmente la misma que la del esquema de Paillier 2013-01-09T10:09:28Z http://hdl.handle.net/2117/17224 Title: Firmas digitales con verificación distribuida en el modelo de seguridad estándar Authors: Herranz Sotoca, Javier; Ruiz Rodríguez, Alexandre; Sáez Moreno, Germán Abstract: Las firmas digitales con verificaci´on distribuida protegen en cierta manera el nivel de anonimato o privacidad del firmante, ya que un subconjunto autorizado de usuarios deben colaboran para verificar la (in)validez de una firma. En trabajos anteriores se propusieron esquemas de este tipo pero que o no alcanzaban el nivel m´aximo de seguridad o bien lo hac´ıan en el modelo del or´aculo aleatorio. Proponemos aqu´ı el primer esquema de firma digital con verificaci´on distribuida que consigue seguridad m´axima, en t´erminos de infalsificabilidad y privacidad, y con seguridad demostrable en el modelo de computaci´on est´andar. 2013-01-09T10:02:20Z http://hdl.handle.net/2117/16248 Title: On method-specific record linkage for risk assessment Authors: Nin Guerrero, Jordi; Herranz Sotoca, Javier; Torra i Reventós, Vicenç Abstract: Nowadays, the need for privacy motivates the use of methods that permit us to protect a microdata file both minimizing the disclosure risk and preserving the statistical utility. Nevertheless, research is usually focused on how data utility is preserved, and much less research effort is dedicated to the study of the tools that an intruder might use to compromise the privacy of the data or, in other words, to increase the disclosure risk. Record linkage is a standard mechanism used to measure the disclosure risk of a microdata protection method. In this paper we present some improvements for the (standard) distance based record linkage. In particular, we test our improvements to evaluate the disclosure risk of rank swapping, which is higher than what was believed up to now. We will also present the results of the application of this approach to microaggregation. 2012-07-13T07:56:15Z http://hdl.handle.net/2117/15611 Title: Anonymous subscription schemes : a flexible fonstruction for on-line services access Authors: González Vasco, Maria Isabel; Heidarvand, Somayed; Villar Santos, Jorge Luis Abstract: In traditional e-cash systems, the tradeoff between anonymity and fraud-detection is solved by hiding the identity of the user into the e-coin, and providing an additional triggering mechanism that opens this identity in case of double spending. Hence, fraud detection implies loss of anonymity. This seems to be a somewhat natural solution when universality of the e-coin is required (i.e., the use of the coin is not determined at the time the coin is generated). However, much simpler protocols may suffice if we only want to prevent that payments for accessing certain services are over-used, even when users' anonymity is perfectly preserved. In this paper we propose a simple and efficient Subscription Scheme, allowing a set of users to anonymously pay for and request access to different services offered by a number of service providers. In our approach, the use of the token is completely determined at issuing time, yet this final aim remains hidden to the issuing authority. Moreover, fraud detection here implies no loss of anonymity; as we make access tokens independent of the owner in a quite simple and efficient way. On the other hand, if different usages of the same token are allowed, these are fully traceable by the service providers. 2012-03-19T10:50:53Z http://hdl.handle.net/2117/15610 Title: A fair and abuse-free contract signing protocol from Boneh-Boyen signature Authors: Heidarvand, Somayed; Villar Santos, Jorge Luis Abstract: A fair contract signing protocol is used to enable two mistrusted parties to exchange two signatures on a given contract, in such a way that either both of them get the other party’s signature, or none of them gets anything. A new signature scheme is presented, which is a variant of Boneh and Boyen’s scheme, and building on it, we propose a new signature fair exchange protocol for which all the properties of being optimistic, setup-free and abuse-free can be proved without random oracles, and it is more efficient than the known schemes with comparable properties. 2012-03-19T10:22:07Z http://hdl.handle.net/2117/14700 Title: A cryptographic solution for private distributed simple meeting scheduling Authors: Herranz Sotoca, Javier; Matwin, Stan; Meseguer González, Pedro; Nin Guerrero, Jordi Abstract: Meeting scheduling is a suitable application for distributed computation motivated by its privacy requirements. Previous work on this problem have considered some cryptographic and conceptually clear approach to solve a simple case of Meeting Scheduling, even achieving complete privacy. 2012-01-20T11:59:25Z http://hdl.handle.net/2117/12909 Title: Attribute selection in multivariate microaggregation Authors: Nin Guerrero, Jordi; Herranz Sotoca, Javier; Torra i Reventós, Vicenç Abstract: Microaggregation is one of the most employed microdata protection methods. The idea is to build clusters of at least k original records, and then replace them with the centroid of the cluster. When the number of attributes of the dataset is large, a common practice is to split the dataset into smaller blocks of attributes. Microaggregation is successively and independently applied to each block. In this way, the effect of the noise introduced by microaggregation is reduced, but at the cost of losing the k-anonymity property. The goal of this work is to show that, besides of the specific microaggregation method employed, the value of the parameter k, and the number of blocks in which the dataset is split, there exists another factor which can influence the quality of the microaggregation: the way in which the attributes are grouped to form the blocks. When correlated attributes are grouped in the same block, the statistical utility of the protected dataset is higher. In contrast, when correlated attributes are dispersed into different blocks, the achieved anonymity is higher, and, so, the disclosure risk is lower. We present quantitative evaluations of such statements based on different experiments on real datasets. 2011-07-11T09:33:15Z http://hdl.handle.net/2117/12612 Title: Constant size ciphertexts in threshold attribute-based encryption Authors: Herranz Sotoca, Javier; Laguillaumie, Fabien; Ràfols Salvador, Carla Abstract: Attribute-based cryptography has emerged in the last years as a promising primitive for digital security. For instance, it provides good solutions to the problem of anonymous access control. In a ciphertextpolicy attribute-based encryption scheme, the secret keys of the users depend on their attributes. When encrypting a message, the sender chooses which subset of attributes must be held by a receiver in order to be able to decrypt. All current attribute-based encryption schemes that admit reasonably expressive decryption policies produce ciphertexts whose size depends at least linearly on the number of attributes involved in the policy. In this paper we propose the first scheme whose ciphertexts have constant size. Our scheme works for the threshold case: users authorized to decrypt are those who hold at least t attributes among a certain universe of attributes, for some threshold t chosen by the sender. An extension to the case of weighted threshold decryption policies is possible. The security of the scheme against selective chosen plaintext attacks can be proven in the standard model by reduction to the augmented multi-sequence of exponents decisional Diffie-Hellman (aMSE-DDH) problem. 2011-05-19T16:20:47Z http://hdl.handle.net/2117/11364 Title: Máxima seguridad para firmas digitales con verificación distribuida Authors: Herranz Sotoca, Javier; Ruiz Rodríguez, Alexandre; Sáez Moreno, Germán Abstract: Una de las opciones para proteger el nivel de anonimato o privacidad de un firmante es construir firmas digitales con verificación distribuida: se requiere la colaboración de un subconjunto autorizado de usuarios para verificar la (in)validez de una firma. En RECSI’08, se propuso un esquema de este tipo, pero que no alcanzaba el máximo nivel de seguridad. En este trabajo proponemos el primer esquema de firma digital con verificación distribuida que consigue seguridad máxima, en términos de infalsificabilidad y privacidad. Demostramos formalmente estas dos propiedades por reducción a problemas computacionales estándar, en el modelo del oráculo aleatorio. 2011-02-14T14:10:05Z http://hdl.handle.net/2117/10652 Title: Proceso de inversión de competencias genéricas en los nuevos planes de estudios de grado de la ETSETB de acuerdo con el modelo CDIO Authors: Sayrol Clols, Elisa; Sardà Ferrer, Joan; Sáez Moreno, Germán; Bragós Bardia, Ramon; Alarcón Cot, Eduardo José; Cabrera Beán, Margarita Asuncion; Calveras Augé, Anna M.; Comellas Colomé, Jaume; O'Callaghan Castellà, Juan Manuel; Pegueroles Vallés, Josep R.; Prat Viñas, Lluís Abstract: Los distintos marcos normativos que fijan las condiciones de contorno en la elaboración de los nuevos planes de estudios ponen especial énfasis en el aprendizaje basado en competencias y en la inclusión de determinadas competencias genéricas. Después de comparar diversos planes de estudio y listados de competencias, se han establecido las competencias genéricas que deben incluir los nuevos grados de la ETSETB de la UPC y se ha diseñado su estructura para favorecer su aprendizaje. Para ello se ha utilizado como paradigma el modelo CDIO (Conceive, Design, Implement, Operate). 2010-12-16T14:07:13Z http://hdl.handle.net/2117/8986 Title: Linear threshold multisecret sharing schemes Authors: Farras Ventura, Oriol; Gràcia Rivas, Ignacio; Martín Mollevi, Sebastià; Padró Laimon, Carles Abstract: In a multisecret sharing scheme, several secret values are distributed among a set of n users, and each secret may have a differ- ent associated access structure. We consider here unconditionally secure schemes with multithreshold access structures. Namely, for every subset P of k users there is a secret key that can only be computed when at least t of them put together their secret information. Coalitions with at most w users with less than t of them in P cannot obtain any information about the secret associated to P. The main parameters to optimize are the length of the shares and the amount of random bits that are needed to set up the distribution of shares, both in relation to the length of the secret. In this paper, we provide lower bounds on this parameters. Moreover, we present an optimal construction for t = 2 and k = 3, and a construction that is valid for all w, t, k and n. The models presented use linear algebraic techniques. 2010-09-21T08:54:22Z http://hdl.handle.net/2117/8407 Title: Partial symbol ordering distance Authors: Herranz Sotoca, Javier; Nin Guerrero, Jordi Abstract: Nowadays sequences of symbols are becoming more important, as they are the standard format for representing information in a large variety of domains such as ontologies, sequential patterns or non numerical attributes in databases. Therefore, the development of new distances for this kind of data is a crucial need. Recently, many similarity functions have been proposed for managing sequences of symbols; however, such functions do not always hold the triangular inequality. This property is a mandatory requirement in many data mining algorithms like clustering or k-nearest neighbors algorithms, where the presence of a metric space is a must. In this paper, we propose a new distance for sequences of (non-repeated) symbols based on the partial distances between the positions of the common symbols. We prove that this Partial Symbol Ordering distance satisfies the triangular inequality property, and we finally describe a set of experiments supporting that the new distance outperforms the Edit distance in those ecenarios where sequence similarity is related to the positions occupied by the symbols. 2010-07-27T08:02:58Z