Fernando Esponda Elena S. Ackley Paul Helman Haixia Jia Stephanie Forrest Protecting data privacy through hard-to-reverse negative databases. 403-415 2007 6 Int. J. Inf. Sec. 6 http://dx.doi.org/10.1007/s10207-007-0030-1 db/journals/ijisec/ijisec6.html#EspondaAHJF07