  Location-based Services (LBSs) provide valuable services, with convenient
features for users. However, the information disclosed through each request
harms user privacy. This is a concern particularly with honest-but-curious LBS
servers, which could, by collecting requests, track users and infer additional
sensitive user data. This is the motivation of both centralized and
decentralized location privacy protection schemes for LBSs: anonymizing and
obfuscating LBS queries to not disclose exact information, while still getting
useful responses. Decentralized schemes overcome the disadvantages of
centralized schemes, eliminating anonymizers and enhancing users' control over
sensitive information. However, an insecure decentralized system could pose
even more serious security threats than privacy leakage. We address exactly
this problem, by proposing security enhancements for mobile data sharing
systems. We protect user privacy while preserving accountability of user
activities, leveraging pseudonymous authentication with mainstream
cryptography. Our design leverages architectures proposed for large scale
mobile systems, while it incurs minimal changes to LBS servers as it can be
deployed in parallel to the LBS servers. This further motivates the adoption of
our design, in order to cater to the needs of privacy-sensitive users. We
provide an analysis of security and privacy concerns and countermeasures, as
well as a performance evaluation of basic protocol operations showing the
practicality of our design.
