jmlr jmlr2011 jmlr2011-22 knowledge-graph by maker-knowledge-mining

22 jmlr-2011-Differentially Private Empirical Risk Minimization


Source: pdf

Author: Kamalika Chaudhuri, Claire Monteleoni, Anand D. Sarwate

Abstract: Privacy-preserving machine learning algorithms are crucial for the increasingly common setting in which personal data, such as medical or financial records, are analyzed. We provide general techniques to produce privacy-preserving approximations of classifiers learned via (regularized) empirical risk minimization (ERM). These algorithms are private under the ε-differential privacy definition due to Dwork et al. (2006). First we apply the output perturbation ideas of Dwork et al. (2006), to ERM classification. Then we propose a new method, objective perturbation, for privacy-preserving machine learning algorithm design. This method entails perturbing the objective function before optimizing over classifiers. If the loss and regularizer satisfy certain convexity and differentiability criteria, we prove theoretical results showing that our algorithms preserve privacy, and provide generalization bounds for linear and nonlinear kernels. We further present a privacypreserving technique for tuning the parameters in general machine learning algorithms, thereby providing end-to-end privacy guarantees for the training process. We apply these results to produce privacy-preserving analogues of regularized logistic regression and support vector machines. We obtain encouraging results from evaluating their performance on real demographic and benchmark data sets. Our results show that both theoretically and empirically, objective perturbation is superior to the previous state-of-the-art, output perturbation, in managing the inherent tradeoff between privacy and learning performance. Keywords: privacy, classification, optimization, empirical risk minimization, support vector machines, logistic regression

Reference: text


Summary: the most important sentenses genereted by tfidf model

sentIndex sentText sentNum sentScore

1 These algorithms are private under the ε-differential privacy definition due to Dwork et al. [sent-8, score-0.68]

2 We further present a privacypreserving technique for tuning the parameters in general machine learning algorithms, thereby providing end-to-end privacy guarantees for the training process. [sent-15, score-0.515]

3 Our results show that both theoretically and empirically, objective perturbation is superior to the previous state-of-the-art, output perturbation, in managing the inherent tradeoff between privacy and learning performance. [sent-18, score-0.7]

4 Thus, there is a great need for designing machine learning algorithms that also preserve the privacy of individuals in the data sets on which they train and operate. [sent-33, score-0.482]

5 For our privacy measure, we use a definition due to Dwork et al. [sent-41, score-0.458]

6 Their ε-differential privacy model is a strong, cryptographically-motivated definition of privacy that has recently received a significant amount of research attention for its robustness to known attacks, such as those involving side information (Ganta et al. [sent-43, score-0.916]

7 Algorithms satisfying ε-differential privacy are randomized; the output is a random variable whose distribution is conditioned on the data set. [sent-45, score-0.509]

8 A statistical procedure satisfies ε-differential privacy if changing a single data point does not shift the output distribution by too much. [sent-46, score-0.509]

9 An important aspect of our work is that we develop methods for end-to-end privacy; each step in the learning process can cause additional risk of privacy violation, and we provide algorithms with quantifiable privacy guarantees for training as well as parameter tuning. [sent-50, score-0.952]

10 In the context of privacy, we must guarantee the privacy of the holdout data as well. [sent-61, score-0.473]

11 We exploit results from the theory of differential privacy to develop a privacy-preserving parameter tuning algorithm, and demonstrate its use in practice. [sent-62, score-0.515]

12 Together with our training algorithms, this parameter tuning algorithm guarantees privacy to all data used in the learning process. [sent-63, score-0.5]

13 Guaranteeing privacy incurs a cost in performance; because the algorithms must cause some uncertainty in the output, they increase the loss of the output predictor. [sent-64, score-0.531]

14 Because the ε-differential privacy model requires robustness against all data sets, we make no assumptions on the underlying data for the purposes of making privacy guarantees. [sent-65, score-0.916]

15 However, to prove the impact of privacy constraints on the generalization error, we assume the data is i. [sent-66, score-0.473]

16 In addition to privacy guarantees, we also provide generalization bounds for this algorithm. [sent-78, score-0.473]

17 We also demonstrate the impact of end-to-end privacy on generalization error. [sent-87, score-0.473]

18 (2009) show that releasing R2 -values computed on high-dimensional genetic data can lead to privacy breaches by an adversary who is armed with a small amount of auxiliary information. [sent-95, score-0.519]

19 , 2006), spanning several communities, that uses privacy models other than differential privacy. [sent-98, score-0.495]

20 , 2008) considers the problem of privacy-preserving SVM classification when separate agents have to share private data, and provides a solution that uses random kernels, but does provide any formal privacy guarantee. [sent-102, score-0.68]

21 An alternative line of privacy work is in the secure multiparty computation setting due to Yao (1982), where the sensitive data is split across multiple hostile databases, and the goal is to compute a function on the union of these databases. [sent-103, score-0.472]

22 Differential privacy, the formal privacy definition used in our paper, was proposed by the seminal work of Dwork et al. [sent-107, score-0.458]

23 (2006b), and has been used since in numerous works on privacy (Chaudhuri and Mishra, 2006; McSherry and Talwar, 2007; Nissim et al. [sent-108, score-0.458]

24 Unlike many other privacy definitions, such as those mentioned above, differential privacy has been shown to be resistant to composition attacks (attacks involving side-information) (Ganta et al. [sent-112, score-0.979]

25 Some follow-up work on differential privacy includes work on differentially-private combinatorial optimization, due to Gupta et al. [sent-114, score-0.495]

26 In addition, Dwork and Lei (2009) explore a connection between differential privacy and robust statistics, and provide an algorithm 1072 D IFFERENTIALLY P RIVATE ERM for privacy-preserving regression using ideas from robust statistics. [sent-129, score-0.495]

27 In general, output perturbation methods alter the output of the function computed on the database, before releasing it; in particular the sensitivity method makes an algorithm differentially private by adding noise to its output. [sent-134, score-0.632]

28 In the classification setting, the noise protects the privacy of the training data, but increases the prediction error of the classifier. [sent-135, score-0.495]

29 In contrast, our approach takes into account how the value of the regularization parameter might change due to privacy constraints. [sent-141, score-0.458]

30 2 Assumptions on Loss and Regularizer The conditions under which we can prove results on privacy and generalization error depend on analytic properties of the loss and regularizer. [sent-176, score-0.495]

31 Strong convexity plays a role in guaranteeing our privacy and generalization requirements. [sent-181, score-0.487]

32 For our privacy results to hold we will also require that the regularizer N(·) and loss function ℓ(·, ·) be differentiable functions of f. [sent-182, score-0.557]

33 In some cases we can prove privacy guarantees for approximations to these non-differentiable functions. [sent-184, score-0.458]

34 3 Privacy Model We are interested in producing a classifier in a manner that preserves the privacy of individual entries of the data set D that is used in training the classifier. [sent-186, score-0.498]

35 The notion of privacy we use is the ε-differential privacy model, developed by Dwork et al. [sent-187, score-0.916]

36 (2006b) (see also Dwork (2006)), which defines a notion of privacy for a randomized algorithm A (D ). [sent-188, score-0.458]

37 The algorithm A provides differential privacy if for any set S , the likelihood that A (D ) ∈ S is close to the likelihood A (D ′ ) ∈ S , (where the likelihood is over the randomness in the algorithm). [sent-191, score-0.495]

38 The following definition of differential privacy is due to Dwork et al. [sent-193, score-0.495]

39 Definition 3 An algorithm A provides ε p -differential privacy if for any two data sets D and D ′ that differ in a single entry and for any set S , exp(−ε p )P(A (D ′ ) ∈ S ) ≤ P(A (D ) ∈ S ) ≤ exp(ε p )P(A (D ′ ) ∈ S ), (3) where A (D ) (resp. [sent-206, score-0.471]

40 From this definition, it is clear that the A (D ) that outputs the minimizer of the ERM objective (1) does not provide ε p -differential privacy for any ε p . [sent-210, score-0.525]

41 Note that the privacy parameter here does not depend on the sensitivity of the of the classification algorithm. [sent-283, score-0.55]

42 1 Compute fpriv = argmin Jpriv (f, D ) + 2 ∆||f||2 . [sent-289, score-0.721]

43 2 c 2c The algorithm requires a certain slack, log(1 + nΛ + n2 Λ2 ), in the privacy parameter. [sent-290, score-0.458]

44 We observe that given any fixed fpriv and a fixed data set D , there always exists a b such that Algorithm 2 outputs fpriv on input D . [sent-344, score-1.419]

45 Because ℓ is differentiable and convex, and N(·) is differentiable, we can take the gradient of the objective function and set it to 0 at fpriv . [sent-345, score-0.789]

46 , (xn , yn ), there is a bijection between b and fpriv . [sent-351, score-0.728]

47 The relation (6) shows that two different b values cannot result in the same fpriv . [sent-352, score-0.702]

48 Furthermore, since the objective is strictly convex, for a fixed b and D , there is a unique fpriv ; therefore the map from b to fpriv is injective. [sent-353, score-1.442]

49 The relation (6) also shows that for any fpriv there exists a b for which fpriv is the minimizer, so the map from b to fpriv is surjective. [sent-354, score-2.106]

50 To show ε p -differential privacy, we need to compute the ratio g(fpriv |D )/g(fpriv |D ′ ) of the densities of fpriv under the two data sets D and D ′ . [sent-355, score-0.702]

51 We note that the Jacobian is defined for all fpriv because N(·) and ℓ are globally doubly differentiable. [sent-361, score-0.744]

52 Moreover, we define matrices A and E as follows: n A = nΛ∇2 N(fpriv ) + ∑ y2 ℓ′′ (yi fT xi )xi xT + n∆Id , i priv i E= i=1 2 ′′ T −yn ℓ (yn fpriv xn )xn xT n T + (y′ )2 ℓ′′ (y′ fT x′ n )x′ n x′ n . [sent-369, score-0.779]

53 Let G denote the map from fpriv to b in (6) under B = D , and H denote the map under B = D ′ . [sent-422, score-0.702]

54 1 1 Corollary 13 Let fpriv be the output of Algorithm 2 with ℓ = ℓHuber , c = 2h , and N(f) = 2 ||f||2 . [sent-424, score-0.753]

55 For 2 ′ which differ in the private value any set S of possible values of fpriv , and any pair of data sets D , D of one person (xn , yn ), e−ε p P(S | B = D ′ ) ≤ P(S | B = D ) ≤ eε p P(S | B = D ′ ). [sent-425, score-0.95]

56 If S \ C has positive measure we can 1083 C HAUDHURI , M ONTELEONI AND S ARWATE calculate the ratio of the probabilities for fpriv for which the loss is twice-differentiable. [sent-441, score-0.724]

57 For such fpriv the Jacobian is also defined, and we can use a method similar to Theorem 9 to prove the result. [sent-442, score-0.702]

58 Remark: Because the privacy proof for Algorithm 1 does not require the analytic properties of 2, we can also use Huber loss in Algorithm 1 to get an εg -differentially private approximation to the SVM. [sent-443, score-0.702]

59 It also serves as a reference against which we can compare the additional burden on the sample complexity imposed by the privacy constraints. [sent-464, score-0.458]

60 Proof Let ¯ frtr = argmin J(f), f f∗ = argmin J(f, D ), f and fpriv denote the output of Algorithm 1. [sent-478, score-0.979]

61 Therefore, from Lemma 16, with probability 1 − δ over the privacy mechanism, J(fpriv , D ) − J(f∗ , D ) ≤ 8d 2 log2 (d/δ)(c + Λ) . [sent-482, score-0.458]

62 Combining the preceeding two statements, with probability 1 − 2δ over the noise in the privacy mechanism and the data distribution, the second term in the right-hand-side of (9) is at most: 16d 2 log2 (d/δ)(c + Λ) log(1/δ) ¯ ¯ J(fpriv ) − J(frtr ) ≤ . [sent-486, score-0.486]

63 Given training data D , let f∗ be a classifier that minimizes J(f, D ) and let fpriv be the classifier output by Algorithm 1. [sent-491, score-0.775]

64 As 0 ≤ t ≤ 1, we can combine (12), (13), and (14) to obtain ∇J(tf∗ + (1 − t)fpriv , D ) ≤ Λ(∇N(f∗ ) − ∇N(tf∗ + (1 − t)fpriv )) + 1 yi (ℓ′ (yi (f∗ )T xi ) − ℓ′ (yi (tf∗ + (1 − t)fpriv )T xi ))xi n∑ i 1 ≤ (1 − t) fpriv − f∗ · Λη + · n · c n ∗ ≤ fpriv − f (Λη + c). [sent-505, score-1.459]

65 (15) From the definition of Algorithm 1, fpriv − f∗ = b, where b is the noise vector. [sent-506, score-0.717]

66 according to P, and if n > C max ||f0 ||2 log(1/δ) c||f0 ||2 d log( d )||f0 || δ , , ε2 εg ε p εg ε p g , then the output fpriv of Algorithm 2 satisfies P L(fpriv ) ≤ L∗ + εg ≥ 1 − 2δ. [sent-532, score-0.753]

67 Proof Let ¯ frtr = argmin J(f), f ∗ f = argmin J(f, D ), f and fpriv denote the output of Algorithm 1. [sent-533, score-0.979]

68 1088 D IFFERENTIALLY P RIVATE ERM Therefore, we can apply Lemma 19 to conclude that with probability at least 1 − δ over the privacy mechanism, J(fpriv , D ) − J(f∗ , D ) ≤ 4d 2 log2 (d/δ) . [sent-538, score-0.458]

69 Let f∗ = argmin J(f, D ), and let fpriv be the classifier output by Algorithm 2. [sent-548, score-0.772]

70 Proof By the assumption ε′p > 0, the classifier fpriv minimizes the objective function J(f, D ) + 1 bT f, n and therefore 1 J(fpriv , D ) ≤ J(f∗ , D ) + bT (f∗ − fpriv ). [sent-550, score-1.442]

71 We can therefore apply Lemma 7 with G(f) = J(f, D ) and 1 g(f) = n bT f to obtain the bound ||f∗ − fpriv || ≤ ||b|| 1 1 ∇( bT f) ≤ . [sent-553, score-0.702]

72 There exists a C1 such that if n > C1 max ||f0 ||2 log( 1 ) d log( d )||f0 || d log( d )||f0 ||2 δ δ δ , , 3/2 ε2 εg ε p g εg ε p , then the output fpriv of Algorithm 1 satisfies P L(fpriv ) ≤ L∗ + εg ≥ 1 − δ. [sent-568, score-0.753]

73 There exists a C2 such that if n > C max ||f0 ||2 log(1/δ) ||f0 ||2 d log( d )||f0 || δ , , ε2 εg ε p εg ε p g then the output fpriv of Algorithm 2 with c = 1 4 , satisfies P L(fpriv ) ≤ L∗ + εg ≥ 1 − δ. [sent-570, score-0.753]

74 There exists a C1 such that if n > C1 max ||f0 ||2 log( 1 ) d log( d )||f0 || d log( d )||f0 ||2 δ δ δ , , 1/2 ε3/2 ε ε2 εg ε p g h g p then the output fpriv of Algorithm 1 satisfies P L(fpriv ) ≤ L∗ + εg ≥ 1 − δ. [sent-581, score-0.753]

75 There exists a C2 such that if n > C max ||f0 ||2 log(1/δ) ||f0 ||2 d log( d )||f0 || δ , , ε2 hεg ε p εg ε p g then the output fpriv of Algorithm 2 with c = 1 4 , satisfies P L(fpriv ) ≤ L∗ + εg ≥ 1 − δ. [sent-583, score-0.753]

76 A crucial difficulty in terms of privacy is that this directly releases the private values x(i) of some individuals in the training set. [sent-593, score-0.746]

77 1092 D IFFERENTIALLY P RIVATE ERM Algorithm 3 Private ERM for nonlinear kernels ¯ Inputs: Data {(xi , yi ) : i ∈ [n]}, positive definite kernel function k(·, ·), sampling function K(θ), parameters ε p , Λ, D Outputs: Predictor fpriv and pre-filter {θ j : j ∈ [D]}. [sent-620, score-0.729]

78 3 Privacy Guarantees Because the workhorse of Algorithm 3 is a differentially-private version of ERM for linear classifiers (either Algorithm 1 or Algorithm 2), and the points {θ j : j ∈ [D]} are independent of the data, the privacy guarantees for Algorithm 3 follow trivially from Theorems 6 and 9. [sent-628, score-0.458]

79 That is, given an f0 with some properties, we will choose regularization parameter Λ, dimension D, and number of samples n so that the predictor fpriv has expected loss close to that of f0 . [sent-638, score-0.742]

80 D ∞ ≤ C/D, so (22) Now given such an f p we must show that fpriv will have true risk close to that of f p as long as there are enough data points. [sent-653, score-0.716]

81 Then we have ¯ ¯ L(fpriv ) − L(f p ) ≤ (J(fpriv ) − J(frtr )) + Λ fp 2 2 2− Λ fpriv 2 2 . [sent-658, score-0.726]

82 2 ε2 Λn p Λn 1094 (26) D IFFERENTIALLY P RIVATE ERM 2 2 Plugging (26) into (23), discarding the negative term involving fpriv gives L(fpriv ) − L(f p ) ≤ 8 fp 2 2 2 2 D log (D/δ) n2 ε2 εg p 2 1 2 log δ fp +O and setting Λ = εg / f p εg . [sent-664, score-0.786]

83 Then by the definition of frtr , L(frtr ) + Since εg 2 · frtr 2 H f0 2 H εg frtr 2 H ≤ L(f ) + εg . [sent-689, score-0.564]

84 However, even though the output of an algorithm preserves ε p -differential privacy for a fixed Λ (as is the case with Algorithms 1 and 2), by choosing a Λ based on empirical performance on a validation set may violate ε p -differential privacy guarantees. [sent-701, score-0.985]

85 Since the value of Λ does not depend on the values in the private data set, this procedure will still preserve the privacy of individuals in the private data. [sent-706, score-0.926]

86 The last step is needed to guarantee ε p -differential privacy for individuals in the validation set. [sent-711, score-0.482]

87 Set fpriv = fi with probability qi = e−ε p zi /2 . [sent-729, score-0.72]

88 , Dm+1 are disjoint; moreover, the randomness in the privacy mechanisms are independent. [sent-750, score-0.458]

89 We first demonstrate how the accuracy of the classification algorithms vary with ε p , the privacy requirement. [sent-819, score-0.458]

90 2 Privacy-Accuracy Tradeoff For our first set of experiments, we study the tradeoff between the privacy requirement on the classifier, and its classification accuracy, when the classifier is trained on data of a fixed size. [sent-840, score-0.474]

91 The privacy requirement is quantified by the value of ε p ; increasing ε p implies a higher change in the belief of the adversary when one entry in D changes, and thus lower privacy. [sent-841, score-0.497]

92 Objective perturbation outperforms sensitivity, and objective perturbation for support vector machines achieves lower classification error than objective perturbation for logistic regression. [sent-1003, score-0.524]

93 In the low privacy regime, logistic regression under objective perturbation is better than support vector machines. [sent-1070, score-0.67]

94 In contrast, in the high privacy regime (low ε p ), support vector machines with objective perturbation outperform logistic regression. [sent-1071, score-0.67]

95 We consider privacy in the ε p -differential privacy model of Dwork et al. [sent-1139, score-0.916]

96 In general, for classification, the error rate increases as the privacy requirements are made more stringent. [sent-1147, score-0.458]

97 ” Our experiments, as well as theoretical results, indicate that objective perturbation usually outperforms the sensitivity methods at managing the tradeoff between privacy and learning performance. [sent-1149, score-0.741]

98 Our current method is based on a reduction to the linear case, using the algorithm of Rahimi and Recht (2008b); however, this method can be statistically inefficient, and require a lot of training data, particularly when coupled with our privacy mechanism. [sent-1173, score-0.48]

99 We believe the present study is a starting point for further study of the differential privacy model in this relatively new subfield of machine learning. [sent-1180, score-0.495]

100 Demanding high privacy requires sacrificing utility, which in the context of classification and prediction is excess loss or regret. [sent-1183, score-0.48]


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