The popularity of the Role-based Access Control (RBAC) model is determined by its flexibility and its adaptability in different contexts, easing the enforcement and the management of security policy. In some cases, different kinds of (cardinality) constraints are considered to adjust and adapt roles and their assignment to best represent the organization’s security policy. However, the process of role mining, whether based on an organizational scenario or on existing permission assignments, is a hard task, since the problem shows NP-hard computational complexity and in case of frequent policy updates, the dynamic adaptation of the roles can be challenging. Then, the only possibility of producing an RBAC model compliant with the security policy is to resort to heuristics, which may return an approximation of the optimal solution. In this paper, we propose an innovative approach to explore the space of the solution based on the bag of word value, which is commonly deployed in the field of document representation and knowledge extraction. We propose different heuristics and validate our approach reporting the results of the application to standard datasets, and providing an evaluation under different metrics and indicators. We show that our technique returns improved results and provides an alternative way to produce valid solutions for constrained RBAC.

Abstract Role-based access control (RBAC) has become a de facto standard to control access to restricted resources in complex systems and is widely deployed in many commercially available applications, including operating systems, databases and other softwares. The migration process towards RBAC, starting from the current access configuration, relies on the design of role mining techniques, whose aim is to define suitable roles that implement the given access policies. Some constraints can be used to transform the roles automatically output by the mining procedures and effectively capture the organization’s status under analysis. Such constraints can limit the final configuration characteristics, such as the number of roles assigned to a user, or the number of permissions included in a role, and produce a resulting role set that is effectively usable in real-world situations. In this paper, we consider two constraints: the number of permissions a role can include and the number of roles assigned to any user. In particular, we present two heuristics that produce roles compliant with both constraints and evaluate their performances using both real-world and synthetic datasets.

In online learning, the dropout phenomenon is a relevant issue to address with practical solutions. Several data sets stimulate original, and resolutive data analysis approaches, demonstrating the importance of the dropout phenomenon. This study proposes a novel approach to predicting massive online open course (MOOC) students at risk of dropout stressing the need to consider the temporal dimension in the data log. The proposal aims to build a data-driven decision support system able to identify students at risk of dropout based on the conceptualization of such students' behavior and its evolution along the time dimension. The primary theoretical model behind the proposed method is the formal concept analysis, and its temporal extension (i.e., temporal concept analysis) for analyzing timestamped data and carrying out a timed lattice. The main result of the paper is a method to extract behavioral patterns of MOOC students at risk of dropout. Such patterns are defined as Time-based Behavior Rules extracted from the aforementioned timed lattice obtained through the preprocessing of MOOC platform log files. The resulting rule set can be easily integrated for implementing educational DSS, as shown in the last part of the paper. The conducted experiments reveal promising results in terms of F-score and students' monitoring time.

In this paper, we propose an intelligent and private method to profile social network users. The scenario is constituted by the social network platform and an advertiser willing to expose its products to potentially interested users. The intelligence is in the use of text analysis services (like Wikify!) to extract knowledge from the social networks posts and get measure criteria to compare them, by using Rough Set Theory. The private layer guarantees that neither of the parties in the scenario can gain any advantage by knowing information (tastes, users topic adherence, etc...) of the other party.

Role Based Access Control (RBAC) models have been adopted in many organizations as the standard way to implement security policies and assign access to restricted resources to roles and roles to users. To capture the business relationships within the organization and efficiently migrate towards RBAC, several role mining techniques have been defined. Constraints on the resulting roles and assignments to users can be imposed to filter out inconsistent situations produced by the automatic algorithm and to better capture the status of the organization. In this paper we are interested in constraints on the number of permissions that can be included in a role and on the number of persons a role can be assigned to. We analyze the problem and propose a couple of heuristics. The heuristics have been applied to standard datasets to validate their performance.

Electronic information is increasingly often shared among entities without complete mutual trust. To address related security and privacy issues, a few cryptographic techniques have emerged that support privacy-preserving information sharing and retrieval. One interesting open problem in this context involves two parties that need to assess the similarity of their datasets, but are reluctant to disclose their actual content. This paper presents an efficient and provably-secure construction supporting the privacy-preserving evaluation of sample set similarity, where similarity is measured as the Jaccard index. We present two protocols: the first securely computes the (Jaccard) similarity of two sets, and the second approximates it, using MinHash techniques, with lower complexities. We show that our novel protocols are attractive in many compelling applications, including document/multimedia similarity, biometric authentication and genetic tests. In the process, we demonstrate that our constructions are appreciably more efficient than prior work.

We present an experiment relative to the use of Bluetooth wireless technology to provide network support for midlet applications accessing Web services. We refer to the most common architecture used to invoke Web services, where a client and a server exchange SOAP messages using HTTP as the transport protocol. To the best of our knowledge, there is no implemented support for executing a HTTP POST operation over a Bluetooth channel. Therefore, to guarantee the independence of the application from the type of communication channel used, in this paper, we deal with the problem of designing a framework allowing a Java application programmer to directly interface Web services from a mobile device using a Bluetooth connection. This paper presents a proof of concept of how Bluetooth technology can be used to design, develop, and deploy Web services-based applications. According to our experiments, programming interfaces like Blue Cove and kSOAP, despite being still under development, are mature enough to be used as the underlying technologies for Web services invocation over Bluetooth in a real world application

The number of online commercial transactions involving small amount of money is more and more increasing. For these transactions, different payment mechanisms from traditional ones are needed in order to reduce the overall costs. To answer to the growing demand for micropayment technologies, several commercial companies have recently started offering micropayment services, which offer reduced costs per transactions. In this work we present the design and the implementation of a micropayment system relying on Web service technology to conclude commercial transactions. The proposed system enables clients to access the restricted resources offered by the merchants and to pay for the received service by invoking the Web services exposed by a Payment Service Provider, acting like an online bank.

Self-healing key distribution schemes allow group managers to broadcast session keys to large and dynamic groups of users over unreliable channels. Roughly speaking, even if during a certain session some broadcast messages are lost due to network faults, the self-healing property of the scheme enables each group member to recover the key from the broadcast messages he has received before and after that session. Such schemes are quite suitable in supporting secure communication in wireless networks and mobile wireless ad-hoc networks. Recent papers have focused on self-healing key distribution, and have provided definitions, stated in terms of the entropy function, and some constructions. The contribution of this paper is the following: We analyze current definitions of self-healing key distribution and, for two of them, we show that no protocol can achieve the definition. We show that a lower bound on the size of the broadcast message, previously derived, does not hold. We propose a new definition of self-healing key distribution, and we show that it can be achieved by concrete schemes. We give some lower bounds on the resources required for implementing such schemes, i.e., user memory storage and communication complexity. We prove that the bounds are tight

Nowadays email has become the most widely used means in daily communication on the net and is increasingly used in place of ordinary mail. Certified email protocols aim to provide additional properties to the standard email service. In this paper we provide a novel optimistic protocol for certified email satisfying nine of the most important properties usually considered in the literature. We give a formal description of the protocol with the input/output automation (IOA) framework and provide a prototype implementation for the Windows platform.

A self-healing key distribution scheme enables a group of users to establish a group key over an unreliable channel. In such a protocol, a group manager, to distribute a session key to each member of the group, broadcasts packets along the channel. If some packet gets lost, the users are still capable of recovering the group key using the received packets, without requesting additional transmission from the group manager. A user must be member both before and after the session in which a particular key is sent and lost, in order to be recovered through "self-healing". In this paper we propose a new technique to do self-healing, and we provide a secure and efficient scheme.

A self-healing key distribution scheme enables a dynamic group of users to establish a group key over an unreliable channel. In such a scheme, a group manager, to distribute a session key to each member of the group, broadcasts packets along the channel. If some packets get lost, users are still capable of recovering the group key using the received packets, without requesting additional transmission from the group manager. A user must be member both before and after the session in which a particular key is sent in order to recover the key through "self-healing". This novel and appealing approach to key distribution is quite suitable in military applications and in several Internet-related settings, where high security requirements should be satisfied. We show a ciphertext-only attack that applies to a proposed scheme.

Information hiding techniques allow a player to hide secret information in some innocent-looking document. In this paper we present a novel approach to information hiding. We investigate the possibility of embedding information using the intrinsic entropy of some classes of cover-documents. In particular we provide algorithms for embedding any binary string in an image mosaic (i.e. an image consisting of a mosaic of smaller images). The algorithms presented allow different levels of security for the information hidden in the cover-document. We also show some techniques to reduce the amount of information the users have to secretly store.

A (k,n)-threshold visual cryptography scheme (VCS) is a method to encode a secret image SI into n shadow images called shares such that any k or more shares enable the "visual" recovery of the secret image. However, by inspecting less than k shares one cannot gain any information on the secret image. The "visual" recovery consists of copying the shares onto transparencies and then stacking them. Any k shares will reveal the secret image without any cryptographic computation. In this paper we analyze the contrast of the reconstructed image for a (k,n)-threshold VCS. We define a canonical form for a (k,n)-threshold VCS and provide a characterization of a (k,,n)-threshold VCS. We completely characterize a contrast optimal (n-1,n)-threshold VCS in canonical form. Moreover, for $n\geq 4$, we provide a contrast optimal (3,n)-threshold VCS in canonical form. We first describe a family of (3,n)-threshold VCS achieving various values of contrast and pixel expansion. Then we prove an upper bound on the contrast of any (3,n)-threshold VCS and show that a scheme in the described family has optimal contrast. Finally, for k=4,5 we present two schemes with contrast asymptotically equal to 1/64 and 1/256, respectively.

The aim of a metering system is the accurate measure of the number of accesses to a Web page in order to have feedback on the effectiveness of the advertising on the net. At the present, there are no standard means to measure the exposure of Web pages as well as the impact of online advertising campaigns. Indeed "traditional" metering techniques are afflicted by hit inflation and hit shaving attacks. In this paper we propose a framework to accurately count the number of visits to a Web site relying on cryptographic primitives. In this way it is possible to avoid cheating by any of the agents in such a framework. Furthermore, a viable implementation of a tool to securely monitor a Web site is discussed.

A form of advertisement which is becoming very popular on the web is based on electronic coupon (e-coupon) distribution. E-coupons are the digital analogue of paper coupons which are used to provide customers with discounts or gift in order to incentive the purchase of some products. Nowadays, the potential of digital coupons has not been fully exploited on the web. This is mostly due to the lack of "efficient" techniques to handle the generation and distribution of e-coupons. In this paper we discuss models and protocols for e-coupons satisfying a number of security requirements. Our protocol is lightweight and preserves the privacy of the users, since it does not require any registration phase.

A one-restricted key agreement scheme is a method by which initially a trusted authority distributes private individual pieces of information to a set of users. Later, each member of any group of users of a given size, referred to as a conference, can compute a common key by exchanging messages over a broadcast channel all users have access to. Such schemes can be used to establish only one common key. In this paper we analyse τ-restricted key agreement schemes. Such schemes allow the computation of up to rτ common keys for τ distinct conferences. For certain values of the parameters the scheme that we propose distributes less information than the trivial one obtained by considering τ copies of a one-restricted scheme.

In dynamic threshold schemes the dealer is able (after a preprocessing stage) to allow sets of participants of a given cardinality to reconstruct different secrets (in different time instants) by sending them the same broadcast message. In this paper we provide a tight lower bound on the number of random bits needed by the dealer to set up a dynamic threshold scheme.

We provide new bounds on the expected length L of a binary one-to-one code for a discrete random variable X with entropy H. We prove that L/spl ges/H-log(H+1)-Hlog(1+1/H). This bound improves on previous results. Furthermore, we provide upper bounds on the expected length of the best code as function of H and the most likely source letter probability.

In this paper, we discuss methods by which a trusted authority can broadcast a message over a network, so that each member of a specified privileged subset of users can decrypt this message to compute a secret key. In contrast with previously constru

/spl acute/A secret sharing scheme permits a secret to be shared among participants in such a way that only qualified subsets of participants can recover the secret, but any nonqualified subset has absolutely no information on the secret. We derive new limitations on the information rate of secret sharing schemes, that measures how much information is being distributed as shares as compared to the size of the secret key, and the average information rate, that is the ratio between the secret size and the arithmetic mean of the size of the shares. By applying the substitution technique, we are able to construct many new examples of access structures where the information rate is bounded away from 1. The substitution technique is a method used to obtain a new access structure by replacing a participant in a previous structure with a new access structure.>