  Social networking services are increasingly accessed through mobile devices.
This trend has prompted services such as Facebook and Google+ to incorporate
location as a de facto feature of user interaction. At the same time, services
based on location such as Foursquare and Shopkick are also growing as
smartphone market penetration increases. In fact, this growth is happening
despite concerns (growing at a similar pace) about security and third-party use
of private location information (e.g., for advertising). Nevertheless, service
providers have been unwilling to build truly private systems in which they do
not have access to location information. In this paper, we describe an
architecture and a trial implementation of a privacy-preserving location
sharing system called Albatross. The system protects location information from
the service provider and yet enables fine-grained location-sharing. One main
feature of the system is to protect an individual's social network structure.
The pattern of location sharing preferences towards contacts can reveal this
structure without any knowledge of the locations themselves. Albatross protects
locations sharing preferences through protocol unification and masking.
Albatross has been implemented as a standalone solution, but the technology can
also be integrated into location-based services to enhance privacy.
