1. Toward an Optimal Social Network Defense Against Sybil Attacks Haifeng Yu National University of Singapore Phillip B. Gibbons Intel Research Pittsburgh Michael Kaminsky Intel Research Pittsburgh

2. Haifeng Yu (National University of Singapore)2 Background: Sybil Attack  Sybil attack: A malicious user pretends to be many fake/sybil users  Context: Decentralized systems (e.g., p2p)  Sybil users can comprise a large fraction of all users  Foil defenses against malicious behaviors (e.g., byzantine consensus, fault-tolerant schemes in DHTs)

3. Haifeng Yu (National University of Singapore)3 Idea: Leveraging Social Network  Undirected graph  Identities = vertexes  Edges = strong trust  Colleagues, relatives, etc

4. Haifeng Yu (National University of Singapore)4 Basic Insight malicious users good users fake users Malicious user cannot create additional edges between good users and fake users attack edges

5. Haifeng Yu (National University of Singapore)5 Basic Insight Dis-proportionally small cut disconnecting a large number of users But cannot search for such cut brute- force

6. Haifeng Yu (National University of Singapore)6 A Peek into the Protocols: Random Routes malicious/fake users good users Verifier must cross this attack edge to intersect same intersection attack edge Suspect 1 Suspect 2

7. Haifeng Yu (National University of Singapore)7 SybilGuard [SIGCOMM’06] SybilLimit [PODC’07] unbounded # sybil nodes accepted (smaller is better) under attack edges unbounded between and