Task allocation or participant selection is a key issue in Mobile Crowd Sensing (MCS). While previous participant selection approaches mainly focus on selecting a proper subset of users for a single MCS task, multi-task-oriented participant selection is essential and useful for the efficiency of large-scale MCS platforms. This paper proposes TaskMe, a participant selection framework for multi-task MCS environments. In particular, two typical multi-task allocation situations with bi-objective optimization goals are studied: (1) For FPMT (few participants, more tasks), each participant is required to complete multiple tasks and the optimization goal is to maximize the total number of accomplished tasks while minimizing the total movement distance.(2) For MPFT (more participants, few tasks), each participant is selected to perform one task based on pre-registered working areas in view of privacy, and the optimization objective is to minimize total incentive payments while minimizing the total traveling distance. Two optimal algorithms based on the Minimum Cost Maximum Flow theory are proposed for FPMT, and two algorithms based on the multi-objective optimization theory are proposed for MPFT. Experiments verify that the proposed algorithms outperform baselines based on a large-scale real-word dataset under different experiment settings (the number of tasks, various task distributions, etc.).

Asymmetric group key agreement is a cryptographic primitive allowing a group of users to negotiate a common public encryption key while each of them holds a different secret private decryption key. Anyone (including outsiders) with the public encryption key can send encrypted messages to the group members, and then the group members can decrypt the messages. Authenticated key agreement protocols authenticate the identities of users to ensure that only the intended group members can establish a session in which the group members can communicate with each other. Dynamic asymmetric group key agreement concerns about the scenarios such as ad hoc networks in which the group members may join or leave at any given time. In this paper, we propose a one-round authenticated dynamic protocol for symmetric group key agreement. For efficiency reasons, we employ the identity-based public-key cryptography (IB-PKC) to authenticate users rather than the public key infrastructure and the certificate-less public-key cryptography. Our analysis shows that the proposals in the paper can resist active attacks and meet many desirable security attributes. Besides, our protocol allows users to join or leave the group at the same time. Furthermore, our protocol is round-optimal and has a quite good performance as compared with previous works.

Security of cloud storage has drawn more and more concerns. In the searchable encryption, many previous solutions can let people retrieve the documents containing single keyword or conjunctive keywords by storing encrypted documents with data indexes. However, searching documents with a phrase or consecutive keywords is still a remained open problem. In this paper, using the relative positions, we propose an efficient scheme LPSSE with symmetric searchable encryption that can support encrypted phrase searches in cloud storage. Our scheme is based on non-adaptive security definition by R.Curtmola and with lower costs of transmission and storage than existing systems. Furthermore, we combine some components of currently efficient search engines and our functions to complete a prototype. The experiment results also show that our scheme LPSSE is available and efficient.

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