Less is More: Revisiting the Gaussian Mechanism for Differential Privacy

Differential privacy via output perturbation has been a de facto standard for releasing query or computation results on sensitive data. However, we identify that all existing Gaussian mechanisms suffer from the curse of full-rank covariance matrices. To lift this curse, we design a Rank-1 Singular Multivariate Gaussian (R1SMG) mechanism. It achieves DP on high dimension query results by perturbing the results with noise following a singular multivariate Gaussian distribution, whose covariance matrix is a randomly generated rank-1 positive semi-definite matrix. In contrast, the classic Gaussian mechanism and its variants all consider deterministic full-rank covariance matrices. Our idea is motivated by a clue from Dwork et al.'s seminal work on the classic Gaussian mechanism that has been ignored in the literature: when projecting multivariate Gaussian noise with a full-rank covariance matrix onto a set of orthonormal basis, only the coefficient of a single basis can contribute to the privacy guarantee. This paper makes the following technical contributions. The R1SMG mechanisms achieves DP guarantee on high dimension query results, while its expected accuracy loss is lower bounded by a term that is on a lower order of magnitude by at least the dimension of query results compared existing Gaussian mechanisms. Compared with other mechanisms, the R1SMG mechanism is more stable and less likely to generate noise with large magnitude that overwhelms the query results, because the kurtosis and skewness of the nondeterministic accuracy loss introduced by this mechanism is larger than that introduced by other mechanisms.