Additive Data Perturbation: the Basic Problem and Techniques

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Additive Data Perturbation the Basic Problem and
Techniques
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Outline

• Motivation
• Definition
• Privacy metrics
• Distribution reconstruction methods
• Privacy-preserving data mining with additive data
  perturbation
• Summary
• Note focus on the papers 10 and 11

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Motivation

• Web-based computing
• Observations
• Only a few sensitive attributes need protection
• Allow individual user to perform protection with
  low cost
• Some data mining algorithms work on distribution
  instead of individual records

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• Definition of dataset
• Column by row table
• Each row is a record, or a vector
• Each column represents an attribute
• We also call it multidimensional data

2 records in the 3-attribute dataset
A 3-dimensional record
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Additive perturbation

• Definition
• Z XY
• X is the original value, Y is random noise and Z
  is the perturbed value
• Data Z and the parameters of Y are published
• e.g., Y is Gaussian N(0,1)
• History
• Used in statistical databases to protect
  sensitive attributes (late 80s to 90s) check
  paper14
• Benefit
• Allow distribution reconstruction
• Allow individual user to do perturbation
• Publish the noise distribution

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Applications in data mining

• Distribution reconstruction algorithms
• Rakeshs algorithm
• Expectation-Maximization (EM) algorithm
• Column-distribution based algorithms
• Decision tree
• Naïve Bayes classifier

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Major issues

• Privacy metrics
• Distribution reconstruction algorithms
• Metrics for loss of information
• A tradeoff between loss of information and
  privacy

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Privacy metrics for additive perturbation

• Variance/confidence based definition
• Mutual information based definition

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Variance/confidence based definition

• Method
• Based on attackers view value estimation
• Knowing perturbed data, and noise distribution
• No other prior knowledge
• Estimation method

Confidence interval the range having c prob
that the real
value is in
Perturbed value

• Y zero mean, std ?
• is the important factor, i.e., var(Z-X) ?2
• Given Z, X is distant from Z in the Z/-? range
  with c conf
• We often ignore the confidence c and use ? to
  represent the
• difficulty of value estimation.

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Problem with Var/conf metric

• No knowledge about the original data is
  incorporated
• Knowledge about the original data distribution
• which will be discovered with distribution
  reconstruction, in additive perturbation
• can be known in prior in some applications
• Other prior knowledge may introduce more types of
  attacks
• Privacy evaluation need to incorporate these
  attacks

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• Mutual information based method
• incorporating the original data distribution
• Concept Uncertainty ? entropy
• Difficulty of estimation the amount of privacy
• Intuition knowing the perturbed data Z and the
  noise Y distribution, how much uncertainty of X
  is reduced.
• Z,Y do not help in estimate X ? all uncertainty
  of X is preserved privacy 1
• Otherwise 0lt privacy lt1

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• Definition of mutual information
• Entropy h(A) ? evaluate uncertainty of A
• Not easy to estimate ? high entropy
• Distributions with the same variance ? uniform
  has the largest entropy
• Conditional entropy h(AB)
• If we know the random variable B, how much is the
  uncertainty of A
• If B is not independent of A, the uncertainty of
  A can be reduced, (B helps explain A) i.e.,
  h(AB) lth(A)
• Mutual information I(AB) h(A)-h(AB)
• Evaluate the information brought by B in
  estimating A
• Note I(AB) I(BA)

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• Inherent privacy of a random variable
• Using uniform variable as the reference
• 2h(A)
• Make the definition consistent with Rakeshs
  approach
• MI based privacy metric
• P(AB) 1-2-I(AB), the lost privacy
• I(AB) 0 ? B does not help estimate A
• Privacy is fully preserved, the lost privacy
  P(AB) 0
• I(AB) gt0 ? 0ltP(AB)lt1
• Calculation for additive perturbation
• I(XZ) h(Z) h(ZX) h(Z) h(Y)

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Distribution reconstruction

• Problem Z XY
• Know noise Ys distribution Fy
• Know the perturbed values z1, z2,zn
• Estimate the distribution Fx
• Basic methods
• Rakeshs method
• EM esitmation

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Rakeshs algorithm (paper 10)

• Find distribution P(XXY)
• three key points to understand it
• Bayes rule
• P(XXY) P(XYX) P(X)/P(XY)
• Conditional prob
• fxy(XYwXx) fy(w-x)
• Prob at the point a uses the average of all
  sample estimates

Using fx(a)?
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• The iterative algorithm

Stop criterion the difference between two
consecutive fx estimates is small
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Make it more efficient

• Bintize the range of x
• Discretize the previous formula

x
m(x) mid-point of the bin that x is in Lt
length of interval t
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• Weakness of Rakeshs algorithm
• No convergence proof
• Dont know if the iteration gives the globally
  optimal result

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EM algorithm (paper 11)

• Using discretized bins to approximate the
  distribution
• Maximum Likelihood Estimation (MLE) method
• X1,x2,, xn are Independent and identically
  distributed
• Joint distribution
• f(x1,x2,,xn?) f(x1?)f(x2?)f(xn?)
• MLE principle
• Find ? that maximizes f(x1,x2,,xn?)
• Often maximize log f(x1,x2,,xn?) sum log
  f(xi?)

Density (the height) of Bin i is notated as ?i
x
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• Basic idea of the EM alogrithm
• Q(?,?) is the MLE function
• ? is the bin densities (?1, ?2, ?k), and ? is
  the previous estimate of ?.
• EM algorithm
• Initial ? uniform distribution
• In each iteration find the current ? that
  maximize Q(?,?) based on previous estimate ?,
  and z

zj upper(?i)ltY ltzj lower(?i)
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• EM algorithm has properties
• Unique global optimal solution
• ? converges to the MLE solution

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Evaluating loss of information

• The information that additive perturbation wants
  to preserve
• Column distribution
• First metric
• Difference between the estimate and the original
  distribution

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Evaluating loss of information

• Indirect metric
• Modeling quality
• The accuracy of classifier, if used for
  classification modeling
• Evaluation method
• Accuracy of the classifier trained on the
  original data
• Accuracy of the classifier trained on the
  reconstructed distribution

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DM with Additive Perturbation

• Example decision tree
• A brief introduction to decision tree algorithm
• There are many versions
• One version working on continuous attributes

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• Split evaluation
• gini(S) 1- sum(pj2)
• Pj is the relativ frequency of class j in S
• gini_split(S) n1/ngini(S1)n2/ngini(S2)
• The smaller the better
• Procedure
• Get the distribution of each attribute
• Scan through each bin in the attribute and
  calculate the gini_split index ? problem how to
  determine pj
• Find the minimum one

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• An approximate method to determine pj
• The original domain is partitioned to m bins
• Reconstruction gives an distribution over the
  bins ? n1, n2,nm
• Sort the perturbed data by the target attribute
• assign the records sequentially to the bins
  according to the distribution
• Look at the class labels associated with the
  records
• ? Errors happen because we use perturbed values
  to determine the bin identification of each
  record

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When to reconstruct distribution

• Global calculate once
• By class calculate once per class
• Local by class at each node
• Empirical study shows
• By class and Local are more effective

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Problems with paper 10,11

• Privacy evaluation
• Didnt consider in-depth attacking methods
• Data reconstruction methods
• Loss of information
• Negatively related to privacy
• Not directly related to modeling
• Accuracy of distribution reconstruction vs.
  accuracy of classifier ?

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Summary

• We discussed the basic methods with additive
  perturbation
• Definition
• Privacy metrics
• Distribution reconstruction
• The problem with privacy evaluation is not
  complete
• Attacks
• Covered by next class