Sergio Mascetti Claudio Bettini Dario Freni Longitude: Centralized Privacy-Preserving Computation of Users' Proximity. 142-157 2009 Secure Data Management http://dx.doi.org/10.1007/978-3-642-04219-5_9 conf/sdmw/2009 db/conf/sdmw/sdmw2009.html#MascettiBF09