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Participatory sensing has been envisioned as a promising paradigm for rapidly sharing of environmental information by mobile smart phones that are always equipped with sensors. It invokes a large amount of mobile applications, e.g., participatory environmental monitoring, transportation and traffic management, and personal entertainments. As participators are usually volunteers, participatory sensing thus poses several fundamental security problems such as data trustworthiness, participator reputation evaluation, participator privacy protection and functional robustness against inner attacks or malicious participators. Although some schemes have already been proposed in recent years, none of them is able to solve the above problems in one solution, especially, in a lightweight manner. In this paper, we make the first attempt in this regard and propose a scheme called LibTip. We pinpoint and model several new attacks in participatory sensing that are launched from inner attackers such as bad-mouth attack and key leakage attack. The definitions for the security of the scheme are formally stated. The security and performance are thoroughly analysed or proved, which justifies the applicability of LibTip.

As cloud servers provide on-demand and on-line storage services, it is possible for mobile users to work any-time and any-where by using hand-held smart devices. Mobile users upload working documents into cloud servers and other users can cooperatively read and edit them. Cloud servers are usually assumed untrustworthy, thus uploaded files have to be encrypted for confidentiality. It thus arises a security problem for key management, e.g., key distributing, updating and revoking. In mobile cloud computing context, key management must be lightweight in terms of power consumption and flexible, because devices have energy constraints, as well as users and files dynamically changes. In this paper, we propose a lightweight and flexible scheme, called LIFE, for key management in mobile cloud computing without relying on any trusted third party. LIFE provides a uniform key management framework for both pairwise keys and group keys by making use of Chinese remainder theorem. LIFE can implicitly update or revoke group keys, which extra protects the privacy of group membership. In addition, LIFE can also guarantee forward and backward secrecy. The security and performance of LIFE are analysed extensively, which justifies its applicability.