Stavros Papadopoulos Yin Yang Spiridon Bakiras Dimitris Papadias Continuous Spatial Authentication. 62-79 2009 SSTD http://dx.doi.org/10.1007/978-3-642-02982-0_7 conf/ssd/2009 db/conf/ssd/sstd2009.html#PapadopoulosYBP09