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"Privacy" and "utility" are words that frequently appear in the literature on statistical privacy. But what do these words really mean? In recent years, many problems with intuitive notions of privacy and utility have been uncovered. Thus more formal notions of privacy and utility, which are amenable to mathematical analysis, are needed. In this paper we present our initial work on an axiomatization of privacy and utility. We present two privacy axioms which describe how privacy is affected by post-processing data and by randomly selecting a privacy mechanism. We present three axioms for utility measures which also describe how measured utility is affected by post-processing. Our analysis of these axioms yields new insights into the construction of privacy definitions and utility measures. In particular, we characterize the class of relaxations of differential privacy that can be obtained by changing constraints on probabilities; we show that the resulting constraints must be formed from concave functions. We also present several classes of utility metrics satisfying our axioms and explicitly show that measures of utility borrowed from statistics can lead to utility paradoxes when applied to statistical privacy. Finally, we show that the outputs of differentially private algorithms are best interpreted in terms of graphs or likelihood functions rather than query answers or synthetic data.
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