The power of synergy in differential privacy: Combining a small curator with local randomizers

Amos Beimel
Aleksandra Korolova
Kobbi Nissim
Or Sheffet
ITC 2020


Motivated by the desire to bridge the utility gap between local and trusted curator models of differential privacy for practical applications, we initiate the theoretical study of a hybrid model introduced by "Blender" [Avent et al. USENIX Security ’17], in which differentially private protocols of n agents that work in the local-model are assisted by a differentially private curator that has access to the data of m additional users. We focus on the regime where m is much smaller than n and study the new capabilities of the interaction in this (m,n)-hybrid model. We show that, despite the fact that the hybrid model adds no new capabilities for the basic task of simple hypothesis-testing, there are many other tasks (under a wide range of parameters) which can be solved in the hybrid model yet cannot be solved either by the curator or by the local-users separately. Moreover, we exhibit additional tasks where at least one round of interaction between the curator and the local-users is necessary - namely, no hybrid model without such interaction can solve these tasks.