Ken C. K. Lee Wang-Chien Lee Hong Va Leong Baihua Zheng OPAQUE: Protecting Path Privacy in Directions Search. 1271-1274 2009 ICDE http://doi.ieeecomputersociety.org/10.1109/ICDE.2009.218 conf/icde/2009 db/conf/icde/icde2009.html#LeeLLZ09