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Preserving Privacy in Semantic-Rich Trajectories of Human Mobility

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  1. 1. PRESERVING PRIVACY IN SEMANTIC-RICH TRAJECTORIES OF HUMAN MOBILITY Anna Monreale, Roberto Trasarti, Dino Pedreschi, Chiara Renso KDDLab, Pisa Vania Bogorny Univ. Santa Catarina, Brasile 1 Knowledge Discovery and Delivery Lab (ISTI-CNR & Univ. Pisa) www-kdd.isti.cnr.it ANONIMO MEETING, Pisa, 20,21 settembre 2010 SPRINGL 2010, San Jose, November 2, 2010
  2. 2. How the story begins… 2 Semantic trajectories represent the important places visited by people Semantic trajectories represent the important places visited by people This information can be privacy sensitive! We should find a good generalization of the visited places… preserving semantics! But how? This information can be privacy sensitive! We should find a good generalization of the visited places… preserving semantics! But how? Can we use a taxonomy of places to generalize and find anonymous datasets? Let’s ask help to Anna, Dino and Roberto! Can we use a taxonomy of places to generalize and find anonymous datasets? Let’s ask help to Anna, Dino and Roberto!
  3. 3. Semantic Trajectories  Availability of trajectory data increases  From raw trajectories to new forms of trajectory data with richer semantic information: semantic trajectories  Semantic trajectories represents moving objects traces as sequences of stops and moves  A semantic trajectory can be represented as the sequence of stops, e.g. <Home, Work, ShoppingCenter, Gym>
  4. 4. Semantic Trajectory and Privacy  Data owner should not reveal personal sensitive information  Disclosure of personal sensitive information puts the citizen’s privacy at risk.  Hiding personal identifiers may not be sufficient  Need for new privacy-preserving DT techniques  Privacy by Design  Natural trade-off between privacy quantification and data utility  Analysis results should not be altered significantly  Privacy has to be maximized
  5. 5. Semantic Trajectories Analysis and Privacy Issues  Analyzing datasets of semantic trajectories may cause privacy issues  A place allows to infer personal sensitive information of an individual  Example: From the fact that a person has stopped in an oncology clinic, an attacker can derive private personal information about the health of such person. 5
  6. 6. Semantic Trajectories Analysis and Privacy Issues k-anonymity is not enough for a robust protection When individuals with similar trajectories stop in the same sensitive place, we can easily infer the individual sensitive information. Example: #U1 <Park, Restaurant, Oncology Clinic> #U2 <Park, Restaurant, Oncology Clinic> This dataset is 2-anonymous but the attacker can infer that the user has been to the Oncology Clinic!!! 6
  7. 7. The Privacy Framework  Anonymizes dataset of semantic trajectories  Based on semantic generalization and the notion of c-safety - similar to the notion of l- diversity in relational, tabular data  It is based on: a taxonomy of places, the notion of quasi identifier places and sensitive places.  Preserves patterns mining results
  8. 8. Quasi-identifier and Sensitive stops8  The taxonomy of places  Represents important places and their semantic categories in a given domain  quasi-identifier places: can be used to infer the identity of the user  sensitive places: can disclose sensitive information about the user  In general we don’t have an apriori classification since it depends on the application and the context
  9. 9. Privacy place taxonomy 9
  10. 10. Privacy Model 10  Adversary Knowledge:  how we anonymize the data  the privacy place taxonomy describing the levels of abstraction  the user U is in the dataset  a quasi-identifier place sequence SQ visited by the user U   Attack Model:  Given SQ, the attacker builts a set of candidate semantic trajectories containing SQ and tries to infer the sensitive places visited by U.  We denote by Prob(SQ,S) the probability that, given a quasi-identifier place sequence SQ related to a user U, the attacker infers the sequence of sensitive places S visited by the user.
  11. 11. C-Safe Dataset We want to control the probability Prob(SQ, S)  A dataset ST is said c-safe wrt the place set Q if for every quasi-identifier place sequence SQ, we have that for each set of sensitive place S Prob(SQ,S) ≤ c with c ∈ [0,1].  Given a sequence of sensitive places S = s1, . . . , sh and a quasi-identifier sequence SQ the probability to infer S is the conditional probability: P(SQ,S) = P(S|SQ) 11
  12. 12. How we can obtain a c-safe dataset? 12 The CAST (C-safe Anonymization of Semantic Trajectories) algorithm guarantees that P(S|SQ) ≤ c for each sequence of S and SQ While (|S|>0) SL = { s ∈ S| length(s) = MaxLength(S)} While (|SL| >= m) 1. Compute the Cost of all possible group Gi of m sequences in SL as: CostGi = CostQGi + CostSGi. 2. Apply the generalization with the lower Cost storing the results in R. 3. Remove Gi from S and SL.
  13. 13. Example (1): The process 13 Consider the following set of sequences, and m=3 and c=0.45: S = {<S1, R2, H1, R1, C1, S2> <S3, D1, R1, C1, S2> <S1, P3, C2, D2, S2> …}
  14. 14. Example (2) CostQ 14 CostQ is the number of hops on the tree needed to generalize the sequences of Quasi-identifiers to a common one. Consider the group: <S1, R2, H1, R1, C1, S2> <S3, D1, R1, C1, S2> <S1, P3, C2, D2, S2> CostQ = 6 + 6 + 6 = 18 <Station,Place,Entertainment,S2 (H1,C1)> <Station,Place,Entertainment,S2 (C1)> <Station,Place,Entertainment,S2 (C2)>
  15. 15. Example (2) CostS 15 CostS is the number of hops on the tree needed to generalize the sequence of Sensible in order to obtain the c-safety. From the generalized group: <Station,Place,Entertainment,S2 (H1,C1)> <Station,Place,Entertainment,S2 (C1)> <Station,Place,Entertainment,S2 (C2)> CostS = 3 The Total Cost of this group is 21 hops, which is the lower combination <Station, Place, H1, Entertainment, Clinic, S2 > <Station, Place, Entertainment, Clinic, S2> <Station, Place, Clinic, Entertainment, S2>
  16. 16. Example (4): Why is C-safe <Station,Place,Entertainment,S2 (H1,C1)> <Station,Place,Entertainment,S2 (C1)> <Station,Place,Entertainment,S2 (C2)> SQ = Station, Place, Entertainment, S2 .⟨ ⟩ Probability of crack: P (SQ , H1 ) = 1/3 <c , P(SQ,C1) = 2/3 > c and P(SQ,C2) = 1/3 <c We need to generalize C1 to the higher representation level in the taxonomy: Clinic. The probability of C1 become 2/5 < c !!!! C-safe dataset: <Station, Place, H1, Entertainment, Clinic, S2 > <Station, Place, Entertainment, Clinic, S2> <Station, Place, Clinic, Entertainment, S2> 16
  17. 17. Experiments We found 6225 semantic trajectories with an average length equal to 5.2 stops. We run the sequential pattern algorithm and we measured the quality of the results with two measures:  the coverage coefficient  the distance coefficient. 17 The dataset contains trajectories of 17000 moving cars in Milan, in one week, collected through GPS devices.
  18. 18. Experiments: Quality of the analysis the coverage coefficient measures how many patterns extracted from the original dataset are covered (have a superclass in the taxonomy) by the patterns extracted in the anonymized dataset 18
  19. 19. Experiments: Coverage Coefficient19
  20. 20. Experiments: Quality of the analysis Distance coefficient represents the distance in terms of steps in the taxonomy to transform the patterns from the set extracted on the original dataset and the one from the anonymized dataset. 20
  21. 21. Experiments: Distance Coefficient
  22. 22. Conclusions and Future work  Improve the algorithm with better heuristics and that does not consider only groups of a fixed size.  More experiments with other mining algorithms  More utility measures for the evaluation of results  Another future research direction goes towards the exploitation of c-safe semantic trajectories dataset for semantic tagging of trajectories. How does the anonymization step 22

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