Abstract:
Assuming the existence of one-way functions, we show that there is no polynomial-time, differentially private algorithm A that takes a database D in ({0,1}^d)^n and outputs a "synthetic database" D all of whose two-way marginals are approximately equal to those of D. (A two-way marginal is the fraction of database rows x in {0,1}^d with a given pair of values in a given pair of columns.) This answers a question of Barak et al. (PODS `07), who gave an algorithm running in time poly(n,2^d) .
Our proof combines a construction of hard-to-sanitize databases based on digital signatures (by Dwork et al., STOC `09) with PCP-based Levin-reductions from NP search problems to finding approximate solutions to CSPs.
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