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

2. 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.

3. Wireless Sensors
Berkeley Motes

4. 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.

5. Spec Motes

6. Wireless Sensor Networks (WSN)
Sensors
Deploy

7. 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.

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

9. 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

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

11. 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)

12. Constraints (Cont.) Public Key Encryption
Slow
1000 times slower than symmetric encryption
Hardware is complicated
Energy consumption is high
Processor
Energy Consumption (mJ/Kb)
RSA/E/V
RSA/D/S
AES
MIPS R4000
0.81
16.7
MC68328 42
840
0.0130

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

14. 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

15. Key Management Problem

16. Key Management Problem
Sensors
Deploy

17. Key Management Problem
Sensors
Deploy
Secure Channels

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

19. Key Pre-distribution
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

20. Naïve Solutions Master-Key Approach Pair-wise 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.

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

22. Establishing Secure Channels

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

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

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

26. Probability Distribution Function of Each Deployment Group

27. Key Pre-distribution Scheme
Key Pools

28. Key Sharing Among Key Pools
Horizontal a B C A b b a D a a F
Vertical
Diagonal b a b G H I b a

29. Local Connectivity

30. 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?

31. Network Resilience

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

33. Objectives Pairwise key pre-distribution scheme. Our Approach:
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

34. Blom Scheme Public matrix G Private matrix D (symmetric). +1 D +1 GN
Let A = (D G)T
A G = (D G)T G = GT DT G = GT D G = (A G)T

35. Blom Scheme A = (D G)T G (D G)T G i j Kij i N Kji j N +1 NX j N
+1 N
Node i carries:
Node j carries:

36. -secure Property i k j Undesirable Situation: if
u*G(i) + v*G(j) = G(k)
then
u*A(i) + v*A(j) = A(k) G
+1 N AT
=D G i k j

37. -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?

38. Vandermonde Matrix G = 1 s s2 s3 sN (s2)2 (s3)2 (sN)2 s (s2) (s3)

39. 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.

40. Multiple Space Scheme Key-Space Pool  spaces (D1, G)  spaces
Two nodes can find
a pairwise key if they
carry a common key space!
(D, G)

41. 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.

42. Measure Local Connectivity
plocal = the probability that two neighboring nodes
can find a common key.

43. Plocal for different  and 

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

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

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

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

48. 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.