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- 1. Reputation Systems II Sybil Attack, BlogRank, B2Rank, EigenRumor, MailRank, TrustRunk Yury Lifshits Caltech http://yury.name Caltech CMI Seminar March 4, 2008 1 / 22
- 2. Outline 1 Sybil Attack 2 Ranking Blogs 3 Reputations For Fighting Spam 4 Conclusions 2 / 22
- 3. 1 Sybil Attack 3 / 22
- 4. Sybil Attack Graph of trust-weighted edges n honest nodes + adversary overall trust value on attack edges (honest-malicious) is limited 4 / 22
- 5. Sybil Attack Graph of trust-weighted edges n honest nodes + adversary overall trust value on attack edges (honest-malicious) is limited Question: whether splitting adversarial node into many is beneﬁcial for acquiring higher reputation (rank)? 4 / 22
- 6. Negative Result Assume reputation scores remain the same under isomorphism. Is it sybilproof? 5 / 22
- 7. Negative Result Assume reputation scores remain the same under isomorphism. Is it sybilproof? Unfortunately, no. Attack strategy? 5 / 22
- 8. Negative Result Assume reputation scores remain the same under isomorphism. Is it sybilproof? Unfortunately, no. Attack strategy? Answer: double the graph. 5 / 22
- 9. Positive Results (1/3) General form of trust ﬂow reputations: r(x) = max trust(p) Ptx p∈Ptx Notation: t is pre-trusted node Pxy is a family of disjoint paths from t to x 6 / 22
- 10. Positive Results (2/3) Assumptions: 1 Extending path nonincreases the trust(p) 2 and trust are monotone to number of paths and edges values, respectively 3 Splitting a path into two does not increase value 7 / 22
- 11. Positive Results (2/3) Assumptions: 1 Extending path nonincreases the trust(p) 2 and trust are monotone to number of paths and edges values, respectively 3 Splitting a path into two does not increase value 4 = max 7 / 22
- 12. Positive Results (3/3) Under assumptions (1-3) sybil attack does not increase adversary’s reputation 8 / 22
- 13. Positive Results (3/3) Under assumptions (1-3) sybil attack does not increase adversary’s reputation Under assumptions (1-4) sybil attack does not increase adversary’s rank 8 / 22
- 14. Positive Results (3/3) Under assumptions (1-3) sybil attack does not increase adversary’s reputation Under assumptions (1-4) sybil attack does not increase adversary’s rank Proof? 8 / 22
- 15. SybilGuard (1/2) Assume number of attack edges is A = o( n/ log n) System is distributed, honest nodes follow the same protocol Can an honest node t identify (w.h.p.) 2A + 1 nodes in such a way that at most A of them are powered by adversary? 9 / 22
- 16. SybilGuard (2/2) For every node ﬁx a bijective mapping from in-edges to out-edges Take a walk from t of length at most n log n using bijection routing At some point make a random switch, than continue another n log n steps using backwalk routing Report a point. Repeat, until 2A + 1 points are collected 10 / 22
- 17. SybilGuard (2/2) For every node ﬁx a bijective mapping from in-edges to out-edges Take a walk from t of length at most n log n using bijection routing At some point make a random switch, than continue another n log n steps using backwalk routing Report a point. Repeat, until 2A + 1 points are collected Claim w.h.p. at most A reported nodes are malicious 10 / 22
- 18. 2 Ranking Blogs 11 / 22
- 19. Ranking Blogs: Factors Entities: blogs, posts, communities, comments, brand names, external websites Frineds, blogroll, subscriptions, hyperlinks, visitors, clicks, votes Time Tags 12 / 22
- 20. BlogRank Any ideas how to rank blogs? 13 / 22
- 21. BlogRank Any ideas how to rank blogs? Why not just PageRank? 13 / 22
- 22. BlogRank Any ideas how to rank blogs? Why not just PageRank? Wait a minute, for which graph? 13 / 22
- 23. BlogRank Any ideas how to rank blogs? Why not just PageRank? Wait a minute, for which graph? Linked blogs: Hyperlinks, blogrolls Common commentors/authors, tags, co-references to news 13 / 22
- 24. B2Rank B2Rank(x) = BlogReputation × PostQuality 14 / 22
- 25. B2Rank B2Rank(x) = BlogReputation × PostQuality BlogReputation is computed in PageRank style for blogroll graph with one change: Blogroll links are weighted by activity level (frequency of blogging and commenting) 14 / 22
- 26. B2Rank B2Rank(x) = BlogReputation × PostQuality BlogReputation is computed in PageRank style for blogroll graph with one change: Blogroll links are weighted by activity level (frequency of blogging and commenting) PostQuality is average for PageRank-style score of blog posts Post-to-post links are weighted by referring post activity and time difference 14 / 22
- 27. EigenRumor (1/2) Picture from “The EigenRumor Algorithm for Ranking Blogs” paper 15 / 22
- 28. EigenRumor (2/2) Notation: ¯: reputation score for posts r ¯ ¯ a, h: authority and hub scores for bloggers P, E: provision and evaluation matrices 16 / 22
- 29. EigenRumor (2/2) Notation: ¯: reputation score for posts r ¯ ¯ a, h: authority and hub scores for bloggers P, E: provision and evaluation matrices ¯ ¯ = αPT a + (1 − α)ET h r ¯ ¯ a = P¯, h = E¯ ¯ r r 16 / 22
- 30. EigenRumor (2/2) Notation: ¯: reputation score for posts r ¯ ¯ a, h: authority and hub scores for bloggers P, E: provision and evaluation matrices ¯ ¯ = αPT a + (1 − α)ET h r ¯ ¯ a = P¯, h = E¯ ¯ r r Solution: iterative algorithm for ¯: r T T ¯ = (αP P + (1 − α)E E)¯ r r 16 / 22
- 31. 3 Reputations For Fighting Spam 17 / 22
- 32. Combining Two Scores Hyperlink graph 18 / 22
- 33. Combining Two Scores Hyperlink graph Pre-trusted nodes 18 / 22
- 34. Combining Two Scores Hyperlink graph Pre-trusted nodes Spam nodes 18 / 22
- 35. Combining Two Scores Hyperlink graph Pre-trusted nodes Spam nodes Reputation propagates in a forward manner 18 / 22
- 36. Combining Two Scores Hyperlink graph Pre-trusted nodes Spam nodes Reputation propagates in a forward manner Spam score propagates backwards 18 / 22
- 37. Combining Two Scores Hyperlink graph Pre-trusted nodes Spam nodes Reputation propagates in a forward manner Spam score propagates backwards Compute spam scores a-la PageRank 18 / 22
- 38. Combining Two Scores Hyperlink graph Pre-trusted nodes Spam nodes Reputation propagates in a forward manner Spam score propagates backwards Compute spam scores a-la PageRank Reweight hyperlink graph and pre-trusted nodes 18 / 22
- 39. Combining Two Scores Hyperlink graph Pre-trusted nodes Spam nodes Reputation propagates in a forward manner Spam score propagates backwards Compute spam scores a-la PageRank Reweight hyperlink graph and pre-trusted nodes Compute reputations a-la PageRank 18 / 22
- 40. 4 Conclusions 19 / 22
- 41. Challenges Measurable objectives? Model for input data? Dynamic aspects of reputations? Digg-style ranking? Price of attack? Ranking in social networks? Ranking in RDF data? Billion dollar question: how to avoid arms race? 20 / 22
- 42. References K. Fujimura, T. Inoue, M. Sugisaki The EigenRumor Algorithm for Ranking Blogs A. Kritikopoulos, M. Sideri, I. Varlamis BlogRank: ranking weblogs based on connectivity and similarity features M.A. Tayebi, S.M. Hashemi, A. Mohades B2Rank: An Algorithm for Ranking Blogs Based on Behavioral Features A. Cheng, E. Friedman Sybilproof reputation mechanisms H. Yu, M. Kaminsky, P.B. Gibbons, A, Flaxman SybilGuard: defending against sybil attacks via social networks P.A. Chirita, J. Diederich, W. Nejdl MailRank: using ranking for spam detection Z. Gyongyi, H. Garcia-Molina, J. Pedersen Combating web spam with TrustRank M. Dalal Spam and popularity ratings for combating link spam 21 / 22
- 43. http://yury.name http://yury.name/reputation.html Ongoing project: http://businessconsumer.net 22 / 22
- 44. http://yury.name http://yury.name/reputation.html Ongoing project: http://businessconsumer.net Thanks for your attention! Questions? 22 / 22

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