We introduce the problem of Private Linear Transformation (PLT). This problem includes a single (or multiple) remote server(s) storing (identical copies of) K messages and a user who wants to compute L linear combinations of a D-subset of these messages by downloading the minimum amount of information from the server(s) while protecting the privacy of the entire set of D messages. This problem generalizes the Private Information Retrieval and Private Linear Computation problems. In this work, we focus on the single-server case. For the setting in which the coefficient matrix of the required L linear combinations generates a Maximum Distance Separable (MDS) code, we characterize the capacity-defined as the supremum of all achievable download rates, for all parameters K, D, L.In addition, we present lower and/or upper bounds on the capacity for the settings with non-MDS coefficient matrices and the settings with a prior side information.
I. INTRODUCTIONThis work introduces the problem of Private Linear Transformation (PLT). This problem includes a single (or multiple non-colluding or with limited collusion capability) remote server(s) storing (identical copies of) a dataset consisting of K data items; and a user who is interested in computing L linear combinations of a D-subset of data items. The goal of the user is to perform the computation privately so that the identities of the data items required for the computation are protected (to some degree) from the server(s), while minimizing the total amount of information being downloaded from the server(s). The PLT problem generalizes the problems of Private Information Retrieval (PIR) (see, e.g., [1]-[7]) and Private Linear Computation (PLC) (see, e.g., [8]-[10]), which have recently received a significant attention from the information and coding theory community. In particular, PLT reduces to PIR or PLC when L = D or L = 1, respectively. In this work, we focus on the single-server setting of PLT.This problem setup appears in several practical scenarios such as linear transformation for dimensionality reduction in Machine Learning (ML), see, e.g., [11] and references therein. Consider a dataset with N data samples, each with K attributes. Consider a user that wishes to implement an ML algorithm on a subset of D selected attributes, while protecting the privacy of the selected attributes. When D is large, the D-dimensional feature space is typically mapped onto a new subspace of lower dimension, say, L, and the ML algorithm operates on the new L-dimensional subspace instead. A commonly-used technique for dimensionality reduction is linear transformation, where an L×D matrix is multiplied by the D × N data submatrix (the submatrix of the original K × N data matrix restricted to the D selected attributes). Thinking of the rows of the K × N data matrix as the K messages, the labels of the The authors are with the