jmlr jmlr2013 jmlr2013-32 knowledge-graph by maker-knowledge-mining

32 jmlr-2013-Differential Privacy for Functions and Functional Data


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Author: Rob Hall, Alessandro Rinaldo, Larry Wasserman

Abstract: Differential privacy is a rigorous cryptographically-motivated characterization of data privacy which may be applied when releasing summaries of a database. Previous work has focused mainly on methods for which the output is a finite dimensional vector, or an element of some discrete set. We develop methods for releasing functions while preserving differential privacy. Specifically, we show that adding an appropriate Gaussian process to the function of interest yields differential privacy. When the functions lie in the reproducing kernel Hilbert space (RKHS) generated by the covariance kernel of the Gaussian process, then the correct noise level is established by measuring the “sensitivity” of the function in the RKHS norm. As examples we consider kernel density estimation, kernel support vector machines, and functions in RKHSs. Keywords: differential privacy, density estimation, Gaussian processes, reproducing kernel Hilbert space

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Summary: the most important sentenses genereted by tfidf model

sentIndex sentText sentNum sentScore

1 EDU Department of Statistics Carnegie Mellon University Pittsburgh, PA 15289, USA Editor: Charles Elkan Abstract Differential privacy is a rigorous cryptographically-motivated characterization of data privacy which may be applied when releasing summaries of a database. [sent-7, score-1.047]

2 We develop methods for releasing functions while preserving differential privacy. [sent-9, score-0.207]

3 Specifically, we show that adding an appropriate Gaussian process to the function of interest yields differential privacy. [sent-10, score-0.182]

4 When the functions lie in the reproducing kernel Hilbert space (RKHS) generated by the covariance kernel of the Gaussian process, then the correct noise level is established by measuring the “sensitivity” of the function in the RKHS norm. [sent-11, score-0.175]

5 As examples we consider kernel density estimation, kernel support vector machines, and functions in RKHSs. [sent-12, score-0.16]

6 Keywords: differential privacy, density estimation, Gaussian processes, reproducing kernel Hilbert space 1. [sent-13, score-0.327]

7 We want to release a summary of D without compromising the privacy of those individuals in the database. [sent-15, score-0.61]

8 One framework for defining privacy rigorously in such problems is differential privacy (Dwork et al. [sent-16, score-1.204]

9 Algorithms that preserve differential privacy have been developed for boosting, parameter estimation, clustering, logistic regression, SVM learning and many other learning tasks. [sent-22, score-0.693]

10 H ALL , R INALDO AND WASSERMAN A concept that has been important in differential privacy is the “sensitivity” of the output (Dwork et al. [sent-31, score-0.717]

11 To establish privacy a Gaussian process may be added to the function with noise level calibrated to the “RKHS sensitivity” of the output. [sent-35, score-0.511]

12 , dn ∈ Rd are a sample from a distribution with density f then we can estimate the density with the kernel density estimator f (x) = 1 n ∑W n i=1 ||x − di || , h x ∈ Rd , where W is a kernel (see, for instance, Wasserman, 2006) and h > 0 is the bandwidth parameter. [sent-44, score-0.326]

13 We may then want to release a “version” of the density estimator f in a way which fulfills the criteria of differential privacy. [sent-46, score-0.359]

14 With a differentially private density estimator in hand, a large sample of data may be drawn from that density. [sent-49, score-0.414]

15 The release of such a sample would inherit the differential privacy properties of the density estimator: see, in particular, Wasserman and Zhou (2010) and Machanavajjhala et al. [sent-50, score-0.84]

16 This is a very attractive proposition, since a differentially private sample of data could be used as the basis for any number of statistical analyses which may have been brought to bear against the original data (for instance, exploratory data analysis, model fitting, etc). [sent-52, score-0.355]

17 The preferred method for density estimation in statistics is kernel density estimation. [sent-57, score-0.152]

18 The methods developed in this paper lead to a private kernel density estimator. [sent-58, score-0.325]

19 We give methods which in essence permit the user to request the evaluation of the private function at arbitrarily many input points. [sent-63, score-0.269]

20 These points may be specified a-priori, or after the construction of the function, or even adaptively based on the outputs given, it makes no difference to the privacy guarantee. [sent-64, score-0.511]

21 However we note that in the case when the points are specified a-priori (for example, a grid over the domain) that the release of the function values corresponds to the release of a finite dimensional vector. [sent-65, score-0.226]

22 In this case we may regard this work as providing a conceptually clean way to determine the sensitivity of this vector so that standard finite dimensional privacy techniques may be applied. [sent-66, score-0.608]

23 After putting our contribution in the context of some related work, we introduce some notation and review the definition of differential privacy in Section 2. [sent-68, score-0.693]

24 We also give a demonstration of a technique to achieve the differential privacy for a vector valued output. [sent-69, score-0.757]

25 The theory for demonstrating differentially privacy of functions is established in Section 3. [sent-70, score-0.626]

26 In Section 4 we apply the theory to the problems of kernel density estimation and kernel SVM learning. [sent-71, score-0.16]

27 1 Related Work There are a few lines of research in the differential privacy literature that are related to this work. [sent-75, score-0.693]

28 In addition to the foundational papers mentioned above there has already been interest in the differentially private release of functions such as support vector machines. [sent-76, score-0.435]

29 (2010) and Chaudhuri and Monteleoni (2011) demonstrated that a private approximation to a nonprivate kernel support vector machine could be made by considering a certain finite dimensional projection of the original function. [sent-78, score-0.305]

30 Here we give an alternate method for the release of the classification function (or regression function) which avoids this approximation, although we do so by employing a weaker type of differential privacy. [sent-80, score-0.281]

31 Our method in essence results in an infinite dimensional private function which is not necessarily characterized by any finite dimensional vector. [sent-81, score-0.301]

32 (2010) give techniques which output a differentially private contingency table. [sent-86, score-0.376]

33 The second approach is to perform some other kind of density estimation on the input data in a way which admits the differential privacy, and then to sample that density to generate synthetic data. [sent-91, score-0.278]

34 So far differentially private density estimation is considered in Smith (2011) for parametric models and Wasserman and Zhou (2010) for non-parametric estimation. [sent-92, score-0.384]

35 In this work we give a technique which is useful for the differentially private estimation of a density in arbitrary dimension, and which achieves the same convergence rate (up to constants) as the non-private estimator. [sent-93, score-0.405]

36 It is worth noting that the availability of a differentially private synthetic data set allows the recipient to compute whatever function he wishes on the data. [sent-94, score-0.336]

37 This may seem to obviate the need for methods to compute private kernel machines and other functions. [sent-96, score-0.277]

38 On the other hand a kernel support vector machine learned on the original data may converge to a good classifier with a relatively small number of samples, and so building a private version of that directly may lead to better classification performance. [sent-98, score-0.277]

39 Therefore although private synthetic data may be seen as a panacea for differential privacy, it is important to remember that it in essence taints all analyses with the curse of dimensionality. [sent-99, score-0.427]

40 Differential Privacy Here we recall the definition of differential privacy and introduce some notation. [sent-101, score-0.693]

41 We phrase the definition of differential privacy using this characterization of randomized algorithms. [sent-116, score-0.71]

42 Typically the above definition is called “approximate differential privacy” whenever β > 0, and “(α, 0)-differential privacy” is shortened to “α-differential privacy. [sent-118, score-0.202]

43 We note that an alternate notion of adjacency for databases also appears in some of the differential privacy literature. [sent-121, score-0.715]

44 The σ-field A is rarely mentioned in the literature on differential privacy but is actually quite / important. [sent-124, score-0.693]

45 (2006a) give a technique to achieve approximate differential privacy for general vector valued outputs in which the “sensitivity” may be bounded. [sent-131, score-0.757]

46 We review this below, since the result is important in the demonstration of the privacy of our methods which output functions. [sent-132, score-0.535]

47 In demonstrating the differential privacy, we make use of the following lemma which is simply an explicit statement of an argument used in a proof by Dwork et al. [sent-135, score-0.182]

48 dλ (4) H ALL , R INALDO AND WASSERMAN In our next result we show that approximate differential privacy is achieved via (4) when the output is a real vector, say vD = v(D) ∈ Rd , whose dimension does not depend on the database D. [sent-145, score-0.771]

49 i=1 We note that this is basically a re-statement of the well-known fact that the addition of appropriate Gaussian noise to a vector valued output will lead to approximate differential privacy. [sent-147, score-0.249]

50 2 The Implications of Approximate Differential Privacy The above definitions provide a strong privacy guarantee in the sense that they aim to protect against an adversary having almost complete knowledge of the private database. [sent-166, score-0.762]

51 Specifically, an adversary knowing all but one of the data elements and having observed the output of a private procedure, will remain unable to determine the identity of the data element which is unknown to him. [sent-167, score-0.275]

52 , dn−1 ), and the private database by D = (d1 , . [sent-173, score-0.275]

53 First note that before observing the output of the private algorithm, the adversary could determine that the private database D lay in the set {(d1 , . [sent-177, score-0.566]

54 Thus, the private database comprises his data with one more element. [sent-181, score-0.275]

55 The above result follows immediately from noting that the rejection region of the test is a measurable set in the space and so obeys the constraint of the differential privacy. [sent-187, score-0.182]

56 Below, we demonstrate the approximate differential privacy of measures on function spaces, by considering random variables which correspond to evaluating the random function on a finite set of points. [sent-199, score-0.693]

57 We focus on the creation of algorithms for which the differential privacy holds over the field of cylinder sets, in the sense that, for all D ∼ D′ ∈ D , P( fD ∈ A) ≤ eα P( fD′ ∈ A) + β, ∀A ∈ F0 . [sent-215, score-0.744]

58 First we note that satisfying (7) implies that the release of any finite evaluation of the function achieves the differential privacy. [sent-217, score-0.281]

59 710 D IFFERENTIAL P RIVACY FOR F UNCTIONS The claimed privacy guarantee follows from (7). [sent-235, score-0.511]

60 In principle, if it were possible for a computer to release a complete description of the function fD then this result would demonstrate the privacy guarantee achieved by our algorithm. [sent-252, score-0.61]

61 Therefore in the case of continuous functions, differential privacy over F0 hence leads to differential privacy throughout the Borel σfield. [sent-256, score-1.386]

62 In summary, we find that if every finite dimensional projection of the released function satisfies differential privacy, then so does every countable-dimensional projection. [sent-257, score-0.259]

63 We now explore techniques which achieve the differential privacy over these σ-fields. [sent-258, score-0.693]

64 2 Differential Privacy via the Exponential Mechanism A straightforward means to output a function in a way which achieves the differential privacy is to make use of the so-called “exponential mechanism” of McSherry and Talwar (2007). [sent-260, score-0.717]

65 D∼D′ McSherry and Talwar (2007) demonstrate that such a technique achieves the α-differential privacy, which is strictly stronger than the (α, β)-differential privacy we consider here. [sent-266, score-0.532]

66 The technique given above can be interpreted as outputting a discrete random variable, and fulfilling privacy definition with respect to the σ-field consisting of the powerset of G. [sent-270, score-0.532]

67 This implies the privacy with respect to the cylinder sets, since the restriction of each cylinder set to the elements of G corresponds some subset of G. [sent-271, score-0.613]

68 Therefore, with some additional technical machinery which we will illustrate next, it is possible to move from differentially private measures over vectors to those over functions. [sent-280, score-0.336]

69 We first demonstrate the technical machinery which allows us to move from finite dimensional distributions to distributions on the function space, and then we give differentially private measures on function spaces of one dimension. [sent-288, score-0.364]

70 Then the release of ∆c(β) G α is (α, β)-differentially private (with respect to the cylinder σ-field F ) whenever   fD (x1 ) − fD′ (x1 )   . [sent-305, score-0.391]

71 Thus for the vector obtained by evaluation of f at those points, differential privacy is demonstrated by Proposition 3 since (8) implies the sensitivity bound (5). [sent-322, score-0.746]

72 Combining this with the above gives the requisite privacy statement for all A ∈ F0 . [sent-340, score-0.511]

73 4 Functions in a Reproducing Kernel Hilbert Space When the family of functions lies in the reproducing kernel Hilbert space (RKHS) which corresponds to the covariance kernel of the Gaussian process, then establishing upper bounds of the form (8) is simple. [sent-343, score-0.194]

74 Corollary 9 For { fD : D ∈ D } ⊆ H , the release of fD = fD + ∆c(β) G α is (α, β)-differentially private (with respect to the cylinder σ-field) whenever ∆ ≥ sup fD − fD′ H . [sent-369, score-0.391]

75 Examples We now give some examples in which the above technique may be used to construct private versions of functions in an RKHS. [sent-372, score-0.242]

76 Then fD − fD′ = 1 n(2πh2 )d/2 ′ Kxn − Kxn and fD − fD′ 2 = H 1 n(2πh2 )d/2 1 ≤2 n(2πh2 )d/2 If we release 2 ′ ′ ′ K(xn , xn ) + K(xn , xn ) − 2K(xn , xn ) 2 . [sent-387, score-0.267]

77 √ c(β) 2 G fD = fD + αn(2πh2 )d/2 where G is a sample path of a Gaussian process having mean zero and covariance K, then differential privacy is demonstrated by corollary 9. [sent-388, score-0.715]

78 For the differentially private function it is easy to see that E ( fD (x) − f (x))2 dx = O h4 + Therefore, at least in terms of rates, no accuracy has been lost. [sent-392, score-0.336]

79 0 x Figure 1: An example of a kernel density estimator (the solid curve) and the released version (the dashed curve). [sent-403, score-0.183]

80 1 N ON -I SOTROPIC K ERNELS The above demonstration of privacy also holds when the kernel is replaced by a non-isotropic Gaussian kernel. [sent-416, score-0.567]

81 In this case the kernel density estimate may take the form fD (x) = 1 n(2π)d/2 |H|1/2 n ∑ exp i=1 1 − (x − xi )T H −1 (x − xi ) , 2 x ∈ T, where H is a positive definite matrix and |H| is the determinant. [sent-417, score-0.158]

82 So long as H is fixed a-priori, privacy may be established by adding a Gaussian process having mean zero and covariance given by 1 K(x, y) = exp − (x − y)T H −1 (x − y) . [sent-419, score-0.533]

83 2 As above, the sensitivity is upper bounded, as 1 fD − fD′ 2 ≤ 2 H d/2 |H|1/2 n(2π) Therefore it satisfies the (α, β)-DP to release √ c(β) 2 G, fD = fD + αn(2π)d/2 |H|1/2 2 . [sent-420, score-0.152]

84 In order to do this it is necessary to find a differentially private version of h and then to employ the composition property of differential privacy. [sent-427, score-0.518]

85 This technique may be amenable to private analysis via the “exponential mechanism,” however it would evidently require that T be a compact set which is known a-priori. [sent-430, score-0.242]

86 To make a private version h j we may use the technique of Dwork and Lei (2009) in which a differentially private algorithm for the interquartile range was developed. [sent-434, score-0.595]

87 2 Functions in a Sobolev Space The above technique worked easily since we chose a particular RKHS in which we knew the kernel density estimator to live. [sent-436, score-0.155]

88 Thus for a family of functions in one dimension which lay in the Sobolev space H 1 , we may determine a noise level necessary to achieve the differential privacy by bounding the above quantity for the difference of two functions. [sent-445, score-0.728]

89 , xd ) = f1 (x1 ) · · · fd (xd ), fi ∈ H 1 [0, 1] . [sent-450, score-0.629]

90 We revisit the example of a kernel density estimator (with an isotropic Gaussian kernel). [sent-458, score-0.154]

91 (2010) noted that these bounds are useful for establishing the noise level required for differential privacy of support vector machines. [sent-469, score-0.693]

92 They are also useful for our approach to privacy in a function space. [sent-470, score-0.511]

93 0 x Figure 2: An example of a kernel density estimator (the solid curve) and the released version (the dashed curve). [sent-490, score-0.183]

94 The setup is the same as in Figure 1, but the privacy mechanism developed in Section 4. [sent-491, score-0.531]

95 In the bottom image are the same data points, with the predictions of the private kernel svm. [sent-538, score-0.277]

96 Conclusion We have shown how to add random noise to a function in such a way that differential privacy is preserved. [sent-577, score-0.693]

97 Interesting future work will be to address the issue of lower bounds for private functions. [sent-579, score-0.221]

98 Specifically, we can ask: Given that we want to release a differentially private function, what is the least amount of noise that must necessarily be added in order to preserve differential privacy? [sent-580, score-0.617]

99 A simple and practical algorithm for differentially private data release. [sent-703, score-0.336]

100 Differentially private recommender systems: building privacy into the net. [sent-733, score-0.732]


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