Privacy-Preserving Distributed Zeroth-Order Optimization

We develop a privacy-preserving distributed algorithm to minimize a regularized empirical risk function when the first-order information is not available and data is distributed over a multi-agent network. We employ a zeroth-order method to minimize the associated augmented Lagrangian function in the primal domain using the alternating direction method of multipliers (ADMM). We show that the proposed algorithm, named distributed zeroth-order ADMM (D-ZOA), has intrinsic privacy-preserving properties. Unlike the existing privacy-preserving methods based on the ADMM where the primal or the dual variables are perturbed with noise, the inherent randomness due to the use of a zeroth-order method endows D-ZOA with intrinsic differential privacy. By analyzing the perturbation of the primal variable, we show that the privacy leakage of the proposed D-ZOA algorithm is bounded. In addition, we employ the moments accountant method to show that the total privacy leakage grows sublinearly with the number of ADMM iterations. D-ZOA outperforms the existing differentially private approaches in terms of accuracy while yielding the same privacy guarantee. We prove that D-ZOA converges to the optimal solution at a rate of $\mathcal{O}(1/M)$ where $M$ is the number of ADMM iterations. The convergence analysis also reveals a practically important trade-off between privacy and accuracy. Simulation results verify the desirable privacy-preserving properties of D-ZOA and its superiority over a state-of-the-art algorithm as well as its network-wide convergence to the optimal solution.