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Differentially Private Recommendation Systems Jeremiah Blocki Fall 2009 18739A: Foundations of Security and Privacy

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Netflix $1,000,000 Prize Competition Queries: On a scale of 1 to 5 how would John rate “The Notebook” if he watched it? User/Movie….300The Notebook…. …………… John4Unrated MaryUnrated Sue25 Joe51 ……………

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Netflix Prize Competition Note: N x M table is very sparse (M = 17,770 movies, N = 500,000 users) To Protect Privacy: Each user was randomly assigned to a globally unique ID Only 1/10 of the ratings were published The ratings that were published were perturbed a little bit User/Movie….13,53713,538…. …………… 258,964(4, 10/11/2005)Unrated 258,965Unrated 258,966(2, 6/16/2005)(5, 6/18/2005) 258,967(5, 9/15/2005)(1,4/28/2005) ……………

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- predicted ratings - actual ratings

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Netflix Prize Competition Goal: Make accurate predictions as measured by Root Mean Squared Error (RMSE) - predicted ratings - actual ratings AlgorithmRMSE BellKor's Pragmatic Chaos0.8567 < 0.8572 Challenge: 10% Improvement0.8572 Netflix’s Cinematch (Baseline)0.9525

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Outline Breaking the anonymity of the Netflix prize dataset Recap: Differential Privacy and define Approximate Differential Privacy Prediction Algorithms Privacy Preserving Prediction Algorithms Remaining Issues

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Netflix FAQ: Privacy Q: “Is there any customer information in the dataset that should be kept private?” A: “No, all customer identifying information has been removed; all that remains are ratings and dates. This follows our privacy policy [... ] Even if, for example, you knew all your own ratings and their dates you probably couldn’t identify them reliably in the data because only a small sample was included (less than one-tenth of our complete dataset) and that data was subject to perturbation. Of course, since you know all your own ratings that really isn’t a privacy problem is it?” Source: How to Break Anonymity of the Netflix Prize Dataset (Narayanan and Shmatikov)

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De-Anonymizing the Netflix Dataset IMDB The Internet Movie Database (IMDB) is a huge source of auxiliary information! Challenge Even the same user, may not provide the same ratings for IMDB and Netflix. The dates also may not correspond exactly. Adversary Knows Approximate Ratings (±1) Knows Approximate Dates (14-day error) Source: How to Break Anonymity of the Netflix Prize Dataset (Narayanan and Shmatikov)

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De-Anonymizing the Netflix Dataset Does anyone really care about the privacy of movie ratings? Video Privacy Protection Act of 1988 exists, so some people do. Video Privacy Protection Act In some cases knowing movie ratings allows you to infer an individual’s political affiliation, religion, or even sexual orientation. Source: How to Break Anonymity of the Netflix Prize Dataset (Narayanan and Shmatikov)

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Privacy in Recommender Systems Netflix might base its recommendation to me on both: My own rating history The rating history of other users Maybe it isn’t such a big deal if the recommendations Netflix makes to me leak information about my own history, but they should preserve the privacy of other users It would be possible to learn about someone else’s ratings history If we have auxiliary information Active attacks are a source auxiliary information Calandrino, et al. Don’t review that book: Privacy risks of collaborative filtering, 2009.

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Recall Differential Privacy [Dwork et al 2006] Randomized sanitization function κ has ε- differential privacy if for all data sets D1 and D2 differing by at most one element and all subsets S of the range of κ, Pr[ κ (D1) S ] ≤ e ε Pr[ κ (D2) S ]

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Achieving Differential Privacy How much noise should be added? Intuition: f(D) can be released accurately when f is insensitive to individual entries x 1, … x n (the more sensitive f is, higher the noise added) Tell me f(D) f(D)+noise x1…xnx1…xn Database D User

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Sensitivity of a function Examples: count 1, histogram 1 Note: f does not depend on the database

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Sensitivity with Laplace Noise

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Approximate Differential Privacy Randomized sanitization function κ has ( ε, δ )-differential privacy if for all data sets D1 and D2 differing by at most one element and all subsets S of the range of κ, Pr[ κ (D1) ∈ S] ≤ e ε Pr[ κ (D2) S ] + δ Source: Dwork et al 2006

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Approximate Differential Privacy One interpretation of approximate differential privacy is that the outcomes of the computation κ are unlikely to provide much more information than ε-differential privacy, but it is possible. Key Difference: Approximate Differential Privacy does NOT require that: Range(κ (D1)) = Range( κ (D2)) The privacy guarantees made by (ε,δ)-differential privacy are not as strong as ε-differential privacy, but less noise is required to achieve (ε,δ)-differential privacy. Source: Dwork et al 2006

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Achieving Approximate Differential Privacy Key Differences: Use of the L2 instead of L1 norm to define the sensitivity of Δf Use of Gaussian Noise instead of Laplacian Noise Source: Differentially Private Recommender Systems(McSherry and Mironov)

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Differential Privacy for Netflix Queries What level of granularity to consider? What does it mean for databases D1 and D2 to differ on at most one element? One user (column) is present in D1 but not in D2 One rating (cell) is present in D1 but not in D2 Issue 1: Given a query “how would user i rate movie j?” Consider: K(D-u[i]) - how can it possibly be accurate? Issue 2: If the definition of differing in at most one element is taken over cells, then what privacy guarantees are made for a user with many data points?

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Netflix Predictions – High Level Q(i,j) – “How would user i rate movie j?” Predicted rating may typically depend on Global average rating over all movies and all users Average movie rating of user i Average rating of movie j Ratings user i gave to similar movies Ratings similar users gave to movie j Intuition: Δf may be small for many of these queries

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Personal Rating Scale For Alice a rating of 3 might mean the movie was really terrible. For Bob the same rating might mean that the movie was excellent. How do we tell the difference?

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How do we tell if two users are similar? Pearson’s Correlation is one metric for similarity of users i and j Consider all movies rated by both users Negative value whenever i likes a movie that j dislikes Positive value whever i and j agreee We can use similar metrics to measure the similarity between two movies.

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Netflix Predictions Example Collaborative Filtering Find the k-nearest neighbors of user i who have rated movie j by Pearson’s Correlation: Predicted Rating similarity of users i and j k most similar users

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Netflix Prediction Sensitivity Example Pretend the query Q(i,j) included user i’s rating history At most one of the neighbors ratings changes, and the range of ratings is 4. The L1 sensitivity of the prediction is: p = 4/k

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Similarity of Two Movies Let U be the set of all users who have rated both movies i and j then

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K-Nearest Users or K-Nearest Movies? Find k most similar users to i that have also rated movie j? Find k most similar movies to j that user i has rated? Either way, after some pre-computation, we need to be able to find the k-nearest users/movies quickly!

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Covariance Matrix (MxM) matrix Cov[i][j] measures similarity between movies i and j M ≈ 17,000 Movie-Movie Covariance Matrix (NxN) Matrix to measure similarity between users N ≈ 500,000 so this is difficult to pre-compute or even store in memory! User-User Covariance Matrix?

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Useful Statistics Global average rating Average rating for movie j User Ratings Re-centered about average Clamped (B=1) Source: Differentially Private Recommender Systems(McSherry and Mironov)

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What do we need to make predictions? For a large class of prediction algorithms it suffices to have: Gavg Mavg – for each movie Average Movie Rating for each user Movie-Movie Covariance Matrix (COV)

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Differentially Private Recommender Systems (High Level) To respect approximate differential privacy publish Gavg + NOISE Mavg + NOISE COV + NOISE Don’t publish average ratings for users Gavg, Mavg are very small so they can be published with little noise COV requires more care. Source: Differentially Private Recommender Systems(McSherry and Mironov)

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Covariance Matrix Tricks Trick 1: Center and clamp all ratings around averages. If we use clamped ratings then we reduce the sensitivity of our function.

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Covariance Matrix Tricks Trick 2: Carefully weight the contribution of each user to reduce the sensitivity of the function. Users who have rated more movies are assigned lower weight.

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Publishing the Covariance Matrix Theorem*: Pick then * Some details about stabilized averages are being swept under the rug

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Useful Statistics for Predictions Sum of all the ratings in the dataset Total Number of ratings in dataset Sum of all the ratings for movie j Number of ratings for movie j

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Covariance Matrix The ratings vectors are adjusted Centered about individual averages Clamped (to reduce noise)

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Results Source: Differentially Private Recommender Systems(McSherry and Mironov)

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Note About Results Granularity: One rating present in D1 but not in D2 Accuracy is much lower when one user is present in D1 but not in D2 Intuition: Given query Q(i,j) the database D-u[i] gives us no history about user i. Approximate Differential Privacy Gaussian Noise added according to L2 Sensitivity Clamped Ratings (B =1) to further reduce noise

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Questions Could the results be improved if the analysis was tailored to more specific prediction algorithms? What if new movies are added? Can the movie-movie covariance matrix be changed without comprising privacy guarantees? How strong are the privacy guarantees given that there are potentially many data points clustered around a single user?

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