By decentralizing control, P2P systems provide efficient, scalable data sharing. However, when sharing data for different purposes (e.g., billing, purchase, shipping, etc.), data privacy can be easily violated by untrustworthy peers wich may use data for other purposes (e.g., marketing, fraudulence, profiling, etc.). A basic principle of data privacy is purpose specification which states that data providers should be able to specify the purpose for which their data will be collected and used. In the context of P2P systems, decentralized control makes it hard to enforce purpose-based privacy. And the major problem of data disclosure is not addressed. Hippocratic databases provide mechanisms for enforcing purpose-based disclosure control within a corporation datastore. In this paper, we apply the Hippocratic database principles to P2P systems to enforce purpose-based privacy. We focus on Distributed Hash Tables (DHTs), because they provide strong guarantees in terms of access performance. We propose PriServ, a privacy service which prevents privacy violation by prohibiting malicious data access. The performance evaluation of our approach through simulation shows that the overhead introduced by PriServ is small.

P2P systems are increasingly used for efficient, scalable data sharing. Popular applications focus on massive file sharing. However, advanced applications such as online communities (e.g., medical or research communities) need to share private or sensitive data. Currently, in P2P systems, untrusted peers can easily violate data privacy by using data for malicious purposes (e.g., fraudulence, profiling). To prevent such behavior, the well accepted Hippocratic database principle states that data owners should specify the purpose for which their data will be collected. In this paper, we apply such principles as well as reputation techniques to support purpose and trust in structured P2P systems. Hippocratic databases enforce purpose-based privacy while reputation techniques guarantee trust. We propose a P2P data privacy model which combines the Hippocratic principles and the trust notions. We also present the algorithms of PriServ, a DHT-based P2P privacy service which supports this model and prevents data privacy violation. We show, in a performance evaluation, that PriServ introduces a small overhead.1 Organization for Economic Co-operation and Development. One of the world's largest and most reliable source of comparable statistics, on economic and social data. http://www.oecd.org/

Online peer-to-peer (P2P) communities such as professional ones (e.g., medical or research) are becoming popular due to increasing needs on data sharing. P2P environments offer valuable characteristics but limited guarantees when sharing sensitive or confidential data. They can be considered as hostile because data can be accessed by everyone (by potentially untrustworthy peers) and used for everything (e.g., for marketing or for activities against the owner's preferences or ethics). In this paper we propose PriServ, a privacy service located on top of distributed hash table (DHT) based P2P systems which prevents data privacy violations. Based on data owner privacy preferences, PriServ uses Hippocratic database principles, takes into account which operations will be realized on shared data (e.g., read, write, disclosure) and uses reputation techniques to increase trust on peers. Several simulation results encourage our ideas and a prototype of PriServ is under development 1 .

Abstract. Peer-to-Peer (P2P) systems have been very successful for large-scale data sharing. However, sharing sensitive data, like in online social networks, without appropriate access control, can have undesirable impact on data privacy. Data can be accessed by everyone (by potentially untrusted peers) and used for everything (e.g., for marketing or activities against the owner's preferences or ethics). Hippocratic databases (HDB) provide an effective solution to this problem, by integrating purposebased access control for privacy protection. However, the use of HDB has been restricted to centralized systems. This chapter gives an overview of current solutions for supporting data privacy in P2P systems, and develops in more details a complete solution based on HDB.Keywords: data privacy, P2P systems, DHT, Hippocratic databases, purposebased access control, trust. IntroductionData privacy is the right of individuals to determine for themselves when, how, and to what extent information about them is communicated to others [40]. It has been treated by many organizations and legislations that have defined well accepted principles. According to OECD 3 , data privacy should consider: collection limitation, purpose specification, use limitation, data quality, security safeguards, openness, individual participation, and accountability. From these principles, we underline purpose specification which states that data owners should be able to specify the data access purposes for which their data will be collected, stored, and used.With the advent of Online Social Networks (OLSN), data privacy has become a major concern. An OLSN is formed by people having something in common and connected by social relationships, such as friendship, hobbies, or coworking, in order to exchange information [11]. Many communities use OLSNs to share data in both professional and non-professional environments. Examples of professional OLSNs are Shanoir 4 , designed for the neuroscience community to 3 Organization for Economic Co-operation and Development. One of the world's largest and most reliable source of comparable statistics on economic and social data (http://www.oecd.org/). 4 www.shanoir.org/ 3 archive, share, search, and visualize neuroimaging data, or medscape 5 , designed for the medical community to share medical experience and medical data. There are also non-professional OLSNs for average citizens and amateurs in different domains such as Carenity 6 , designed for patients and their relatives to share medical information about them in order to help medical research. Another example is DIYbio 7 , dedicated to make biology accessible for citizen scientists, amateur biologists, and biological engineers, who share research results. The most popular OLSN, Facebook, with hundreds millions of users, enables groups of friends to share all kinds of personal information among themselves.Scalable data sharing among community members is critical for an OLSN system. Two main solutions have emerged for scalable data sharing: cloud computing a...

Although Cloud Computing has remarkably provided us easy accessible, manageable and maintainable resources at effective costs, but due to the fact that all or multiple users are allocated with the similar resources present security threats to the cloud subscribers. It is well known fact that in cloud paradigm, the data and applications are always under the control of third party, which give rise to serious concerns among the cloud subscribers. Cloud computing attracts the attention of research community due to its potential to provide tremendous benefits to the industry and cloud subscribers, but it lingers because of the security, privacy, and trust issues with Cloud subscribers. If Cloud Service Provider's (CSP's) are being able to provide efficient security tools, the utilization of services will rise exponentially and it will soon become globally accepted computing. This paper provides state of the art of major security challenges in the cloud paradigm and the countermeasures to counteract with the security breaches.