e Laplace mechanism is the workhorse of differential privacy, applied to many instances where numerical data is processed. However, the Laplace mechanism can return semantically impossible values, such as negative counts, due to its infinite support. ere are two popular solutions to this: (i) bounding/capping the output values and (ii) bounding the mechanism support. In this paper, we show that bounding the mechanism support, while using the parameters of the pure Laplace mechanism, does not typically preserve differential privacy. We also present a robust method to compute the optimal mechanism parameters to achieve differential privacy in such a se ing.

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Protection of On-line Social Networks (OSNs) resources has become a primary need since today OSNs are the hugest repository of personal information on the Web. This has resulted in the definition of some access control models tailored to the protection of OSN resources. One of the key parameter on which access control decisions in OSNs should be based is represented by the trust between OSN users. A wellknown approach for the management of trust relationships is represented by trust negotiations [1], [2]. In this paper, we show how access control and trust negotiation can be combined in a framework for the protection of OSN resources. Moreover, we show how the outcome of a trust negotiation can be exploited to dynamically adjust the trust level between OSN users.

Providing functionalities that allow online social network users to manage in a secure and private way the publication of their information and/or resources is a relevant and far from trivial topic that has been under scrutiny from various research communities. In this work, we provide a framework that allows users to define highly expressive access policies to their resources in a way that the enforcement does not require the intervention of a (trusted or not) third party. This is made possible by the deployment of a newly defined cryptographic primitives that provides -among other things -efficient access revocation and access policy privacy. Finally, we provide an implementation of our framework as a Facebook application, proving the feasibility of our approach.