2. Securing Wireless Sensor Networks Wenliang (Kevin) Du Department of Electrical Engineering and Computer Science Syracuse University

3. Overview Overview of Wireless Sensor Networks (WSN). Security in wireless sensor networks. –Why is it different? Our work on key pre-distribution in WSN –Deployment-based scheme (INFOCOM’04) –Pair-wise Scheme (ACM CCS’03) Summary.

4. Wireless Sensors Berkeley Motes

5. Mica Motes Mica Mote: –Processor: 4Mhz –Memory: 128KB Flash and 4KB RAM –Radio: 916Mhz and 40Kbits/second. –Transmission range: 100 Feet TinyOS operating System: small, open source and energy efficient.

6. Spec Motes

7. Wireless Sensor Networks (WSN) Deploy Sensors

8. Applications of WSN Battle ground surveillance –Enemy movement (tanks, soldiers, etc) Environmental monitoring –Habitat monitoring –Forrest fire monitoring Hospital tracking systems –Tracking patients, doctors, drug administrators.

9. Securing WSN Motivation: why security? Why not use existing security mechanisms? –WSN features that affect security. Our work: –Two key management schemes.

10. Why Security? Protecting confidentiality, integrity, and availability of the communications and computations Sensor networks are vulnerable to security attacks due to the broadcast nature of transmission Sensor nodes can be physically captured or destroyed

11. Why Security is Different? Sensor Node Constraints –Battery, –CPU power, –Memory. Networking Constraints and Features –Wireless, –Ad hoc, –Unattended.

12. Sensor Node Constraints Battery Power Constraints –Computational Energy Consumption Crypto algorithms Public key vs. Symmetric key –Communications Energy Consumption Exchange of keys, certificates, etc. Per-message additions (padding, signatures, authentication tags)

13. Slow –1000 times slower than symmetric encryption Hardware is complicated Energy consumption is high Constraints (Cont.) Public Key Encryption ProcessorEnergy Consumption (mJ/Kb) RSA/E/VRSA/D/SAES MIPS R40000.8116.70.00115 MC68328428400.0130

14. Memory Constraints Program Storage and Working Memory –Embedded OS, security functions (Flash) –Working memory (RAM) Mica Motes: 128KB Flash and 4KB RAM

15. Objectives of Our Research Long-term Goals –Study how WSN’s constraints/features affect the design of security mechanisms. –Develop security mechanisms for WSN. Current Projects –Key Management Problems –Data Fusion Assurance

16. Key Management Problem

17. Deploy Sensors

18. Key Management Problem Secure Channels Deploy Sensors

19. Approaches Trusted-Server Schemes –Finding trusted servers is difficult. Public-Key Schemes –Expensive and infeasible for sensors. Key Pre-distribution Schemes

20. Loading Keys into sensor nodes prior to deployment Two nodes find a common key between them after deployment Challenges Memory/Energy efficiency Security: nodes can be compromised Scalability: new nodes might be added later Key Pre-distribution

21. Naïve Solutions Master-Key Approach Memory efficient, but low security. Needs Tamper-Resistant Hardware. Pair-wise Key Approach N-1 keys for each node (e.g. N=10,000). Security is perfect. Need a lot of memory and cannot add new nodes.

22. Eschenauer-Gligor Scheme Each node randomly selects m keys A BE Key Pool S DC When |S| = 10,000, m=75 Pr (two nodes have a common key) = 0.50

23. Establishing Secure Channels A C B

24. Our Improvement Over Eschenauer-Gligor Scheme Appeared in IEEE INFOCOM 2004

25. Observations and Objectives A B F Property: Pr(A, B) = Pr(A, F) Using deployment knowledge Our objective: Pr(A, B) >> Pr(A, F)

26. Modeling Deployment Knowledge Deployment points for a group of sensors A F I J

27. Probability Distribution Function of Each Deployment Group

28. Key Pre-distribution Scheme Key Pools

29. Key Sharing Among Key Pools   A BC F HI D G        Horizontal VerticalDiagonal

30. Local Connectivity

31. Network Resilience What is the damage when x nodes are compromised? –These x nodes contain keys that are used by the good nodes. –What percentage of communications can be affected?

32. Network Resilience

33. A Pairwise Key Pre-distribution Scheme Appeared in CCS’03: ACM Conference on Computer and Communications Security

34. Objectives Pairwise key pre-distribution scheme. –Each pair of sensor share a unique secret key –Can be used for Authentication Our Approach: –We use Blom Scheme to achieve Pairwise –We use Random Key Selection scheme to improve performance and resilience

35. Blom Scheme Public matrix G Private matrix D (symmetric). D G +1 N A G = (D G) T G = G T D T G = G T D G = (A G) T Let A = (D G) T

36. Blom Scheme X = A = (D G) T G(D G) T G i j i j K ji K ij N +1 N N Node i carries: Node j carries:

37. -secure Property Undesirable Situation: if u*G(i) + v*G(j) = G(k) then u*A(i) + v*A(j) = A(k) A T =D G +1 ij N G k ijk

38. -secure Property ANY +1 columns in G are linear independent. –Different from saying that G has rank +1 –Rank: there exist +1 linear independent columns Can tolerate compromise up to nodes. –Once +1 nodes are compromised, the rest can be calculated if these +1 columns are linear independent. How to find such a matrix G?

39. Vandermonde Matrix 1111 ss2s2 s3s3 sNsN s2s2 (s 2 ) 2 (s 3 ) 2 (s N ) 2 s (s 2 ) (s 3 ) (s N ) G =

40. Properties of Blom Scheme Blom’s Scheme –Network size is N –Any pair of nodes can directly find a secret key –Tolerate compromise up to nodes –Need to store +2 keys Challenge: Can we increase without increasing the storage usage.

41. Multiple Space Scheme (D 2, G) (D 1, G) (D , G) Key-Space Pool  spaces Two nodes can find a pairwise key if they carry a common key space!

42. How to select  and  ? If the memory usage is m, the security threshold (probablistic) m is To improve the security, we need to increase  /  2. However, such an increase affects the connectivity.

43. Measure Local Connectivity p local = the probability that two neighboring nodes can find a common key.

44. P local for different  and 

45. Security Analysis Network Resilience: –When x nodes are compromised, how many other secure links are affected?

46. Resilience (p = 0.33, m=200) Blom

47. Resilience (p = 0.50, m =200) Blom

48. Improvement: Using Two-hop Neighbors  = 7  = 2  = 31  = 2

49. Summary Security in WSN is quite different from traditional (Wired) network security. We have proposed two key pre-distribution schemes for WSN. Our schemes substantially improves the performance and network resilience.