Ik Rae Jeong Jeong Ok Kwon Dowon Hong Dong Hoon Lee Constructing PEKS schemes secure against keyword guessing attacks is possible? 394-396 2009 32 Computer Communications 2 http://dx.doi.org/10.1016/j.comcom.2008.11.018 db/journals/comcom/comcom32.html#JeongKHL09