1. Security Mechanisms for Distributed Computing Systems A9ID1007, Xu Ling Kobayashi Laboratory GSIS, TOHOKU UNIVERSITY 2011/12/15 1

2. Background Distributed computing systems (DCSs) – Definition: A system where nodes share their computing power with each other to finish certain goals – Example: P2P systems (Skype), volunteer computing systems (SETI@home), Grid 2

3. 3 Background Example: Volunteer computing system – A system that utilizes the idling computing resources on the network to finish computing intensive tasks worker 1 worker 2worker 3worker 4 host Task 1Task 2Task 3Task 4Result 1Result 2Result 3Result 4 The structure of a typical volunteer computing system Task 1 Task 2 Task n

4. Background Categorization – Centralized DCSs (e.g., volunteer computing): Few servers and many clients. Only have server-client communication – Decentralized DCSs (e.g., P2P) : all nodes are equal and communicate with each other – Hybrid DCSs (e.g., skype) Most nodes are equal, and communicate with each other A few servers exist – Authorized DCSs: DCSs that contain trustful authorities (e.g., volunteer computing systems) – Unauthorized DCSs: DCSs that contain no trustful authority (e.g., P2P systems) 4

5. Background: Attack to DCSs False result attack (FRA) (for centralized DCSs) – One host node and multiple worker nodes – Host dispatches tasks to workers. Workers compute tasks and return returns to host – Malicious workers return incorrect results to host 5 worker 1 worker 2worker 3Malicious worker 4 host Task 1Task 2Task 3Task 41+1=2 1+1=3 Task 1 Task 2 Task n

6. Background: Attack to DCSs Sybil attack (SA) (For decentralized and hybrid DCSs) – A few malicious users controls many Sybil nodes (malicious nodes) to break the system protocol – Sybil nodes can launch various attacks 6 1+1=? 1+1=3 1+1=2 1+1=3 1+1=3! malicious user Sybil node Honest node

7. Background: Existing solution to the false result attack The host dispatches multiple tasks to each worker v These tasks contains some special tasks called quizzes The host checks the correctness of the answers of quizzes  Node v is honest only if the answers of the quizzes return by v are correct Problem: – A Quiz should satisfy: the correctness of the answer of a quiz should be easy to check – Unpractical: How to generate quizzes that satisfy this property is an open problem. 7 1+1=? 1+2=? 11*11=? (quiz) 1+1=3 1+2=3 11*11=3 (quiz) v host 11*11=121! v is malicious

8. Background: Existing solution to the Sybil attack Social network model based Sybil detecting (SSD) – Social network model: # of attack edges is small – SSD algorithms Assumption: The network topology of the DCS obeys SNM Functionality: For each honest node v, enable v to judge the types of other nodes Basic idea: the # of attack edges is small  communication between nodes of different types is weakened – My idea: attack edge detecting is important in design effective SSD algorithms Effective: high judging accuracy Detect the attack edges and cut them  communication between nodes of different types can be stopped! 8 Honest clusterSybil cluster Attack edges Attack edge

9. Objective Motivation: – For FRA: existing solutions are unpractical (Quiz) – For SA: Attack edge detecting technique can be used to design effective SSD algorithms Objective: Design effective security mechanisms to resist the false result attack and the Sybil attack on DCSs. 9

10. Approach – Design a practical false result attack resisting algorithm  Enable host to detect malicious workers – Design an effective attack edge detecting-based SSD algorithm for authorized DCSs  For each node v, enable v to know the types of other nodes – Design an attack edge detecting algorithm for unauthorized DCSs  For each node v and an incident edge e of v, enable v to know whether e is an attack edge or not 10 Honest nodesSybil nodes v e2 v1 v2 e1 v1 is honest, v2 is Sybil worker 1 worker 2worker 3worker 4 (Malicious) workers 1 are honest; worker 4 is malicious e1 is not AE, e2 is AE

11. Organization 1.Introduction 2.MSC: an Practical Spot Checking Mechanism for Resisting False Result Attack 3.SybilDetector: an Attack Edge Detecting Based Sybil Detecting Algorithm 4.RSC: an Attack Edge Detecting Algorithm for Sybil Resisting 5.Conclusion 11

12. Comments from Professor Sone Comment: Clarify the approaches( ‘detect the malicious nodes’ is too broad, there are many way to detect) Solution: – To detail the models of FRA and SA, respectively – To specify the research approaches – To specify the functionality of each approach 12 Approach (new) Design an practical and efficient false result attack resisting algorithm. Design an effective attack edge detecting-based SSD algorithm for authorized DCSs. Design an attack edge detecting algorithm for unauthorized DCSs. Approach (old) For false result attack: enable honest nodes to detect malicious nodes For Sybil attack: enable honest nodes to detect Sybil nodes

13. Comments from Professor Sone Comment: Clarify the performance metric (Define the performance metric in the first chapter. Define what is ‘effective’.) Solution: Define the performance metrics of MSC and SSD algorithms in Chapter 1 13

14. Comments from Professor Sone Comment : Clarify the innovational point: – Emphasize on the new idea rather than the algorithm Solution: – Point out that the attack edge detecting technique is the innovation point in chapter 1. – Change chapter 4 Old  RSSR: A Random Walk and Attack Edge Detecting Based Sybil Detecting Algorithm (emphasized RSSR (a SSD algorithm)) New  RSC: an Attack Edge Detecting Algorithm for Sybil Resisting (emphasize RSC (an attack edge detecting algorithm)) 14

15. Comments from Professor Sone Comment : The current social network model considers only two clusters. How to deal with the case of more clusters? Solution : – Discuss this problem in Section Related Work of Chapter 3. – In the case of more clusters, for each cluster, we have to know the type of at least one node this cluster. 15

16. Comments from Professor Sone Comment : How to deal with nodes changing types? Solution – Reputation system? (will be vulnerable to the Sybil attack) 16

17. Comments from Professor Suganuma Comment : Explain the baseline algorithms (SybilLimit) used for the performance comparison Solution: Explain the baseline algorithm (SybilLimit, SOHL) in detail in Section Related Work of Chapter 3 of the dissertation, and in the presentation of the next defense. 17

18. Comments from Professor Takizawa Comment: Clarify the model used (Does this system have trustful authority?). Solution: – Specify the models of FRA and SA FRA: centralized SA: decentralized or hybrid 18