Sergej Zerr Daniel Olmedilla Wolfgang Nejdl Wolf Siberski Zerber<sup>+R</sup>: top-k retrieval from a confidential index. 439-449 2009 EDBT http://doi.acm.org/10.1145/1516360.1516412 conf/edbt/2009 db/conf/edbt/edbt2009.html#ZerrONS09