Security in Wireless Sensor Networks

1. Security in Wireless Sensor Networks Group Meeting Fall 2004 Presented by Edith Ngai

2. Outline • Wireless Sensor Networks (WSN) • Security in WSN • Key Management Approaches • Straight Forward Approach • Basic Probabilistic Approach • Deployment-based Approach • Conclusion • References

3. Wireless Sensor Networks • A sensor network is composed of a large number of sensor nodes • Sensor nodes are small, low-cost, low-power devices that have following functionality: • communicate on short distances • sense environmental data • perform limited data processing • The network usually also contains “sink” node which connects it to the outside world Berkeley Motes

4. Applications • WSN can be used to monitor the conditions of various objects / processes • Military: battlefield surveillance, biological attack detection, targeting • Ecological: fire detection, flood detection, agricultural uses • Health related: human physiological data monitoring • Miscellaneous: car theft detection, inventory control, home applications • Sensors are densely deployed either inside or very close to the monitored object / process

5. Security in WSN • Main security threats in WSN are: • Radio links are insecure – eavesdropping / injecting faulty information is possible • Sensor nodes are not temper resistant – if it is compromised the attacker obtains all security information • Protecting confidentiality, integrity, and availability of the communications and computations

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

7. Key Management: Goals • The protocol must establish a key between all sensor nodes that must exchange data securely • Node addition / deletion should be supported • It should work in undefined deployment environment • Unauthorized nodes should not be allowed to establish communication with network nodes

8. Sensors Deploy Key Management Problem Secure Channels

9. Approaches • Trusted-server schemes • Finding trusted servers is difficult • Public-key schemes • Expensive and infeasible for sensors • Key pre-distribution schemes

10. 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

11. Straight Forward Approach • Single mission key is obviously unacceptable • Pairwise private key sharing between every two nodes is impractical because of the following reasons: • it requires pre-distribution and storage of n-1 keys in each node which is n(n-1)/2 per WSN • most of the keys would be unusable since direct communication is possible only in the nodes neighborhood • addition / deletion of the node and re-keying are complex

12. Basic Probabilistic Approach • Proposed by Eschenauer and Gligor • Relies on probabilistic key sharing among nodes of WSN • Uses simple shared-key discovery protocol for key distribution, revocation and node re-keying • Three phases are involved: key pre-distribution, shared-key discovery, path-key establishment

13. 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

14. Establishing Secure Channels B A C

15. Observations and Objectives A B F Problem: How to pick a large key pool while maintaining high connectivity? (i.e. maintain resilience while ensuring connectivity)

16. Deployment-based Scheme • Proposed by Du, et. al (IEEE Infocom 2004) • Improves Random Key Predistribution (Eschenauer and Gligor) by exploiting Location Information • Studies a Gaussian distribution for deployment of Sensor nodes to improve security and memory usage

17. Deployment-based Scheme • Groups select from key group S (i,j) • Probability node is in a certain group is (1 / tn).

18. Step 1 : Key Pre-distribution - Key Sharing Among Key Pools - Horizontal a B C A b b a D F a a Vertical Diagonal a b b G H I b a

19. Step 1 : Key Pre-distribution - Key Sharing Among Key Pools - • Determining |Sc| • When |S| = 100,000, t = n = 10, a = 0.167, b = 0.083 |Sc| = 1770

20. Step 2: Shared-key Discovery • Takes place during initialization phase after WSN deployment. Each node discovers its neighbor in communication range with which it shares at least one key • Nodes can exchange IDs of keys that they poses and in this way discover a common key • A more secure approach would involve broadcasting a challenge for each key in the key ring such that each challenge is encrypted with some particular key. The decryption of a challenge is possible only if a shared key exists

21. Step 3: Path-key Establishment • During the path-key establishment phase path-keys are assigned to selected pairs of sensor nodes that are within communication range of each other, but do not share a key • Find secure path by using flooding method • Limit the lifetime of the flooding message to three hops to reduce flooding overhead • Share random key K by using secure path

22. Local Connectivity • With 100 keys, location management improves local connectivity from 0.095 to 0.687

23. 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?

24. Conclusion • Robust security mechanisms are vital to the wide acceptance and use of senor networks for many applications • Security in WSN is quite different from traditional (wired) network security • Various peculiarities of WSN make the development of good key scheme a challenging task • We have discussed several approaches to key management in WSN

25. References • I. F. Akyildiz, W. Su, Y. Sankarasubramaniam, and E. Cyirci. Wireless Sensor Networks: A Survey. Computer Networks, 38(4):393-422, 2002. • L. Eschenauer and V. Gligor. A Key-Management Scheme for Distributed Sensor Networks. In Proc. of ACM CCS’02, November 2002. • H. Chan, A. Perrig, and D. Song. Random Key Predistribution Schemes for Sensor Networks. In 2003 IEEE Symposium on Research in Security and Privacy. • W. Du, J. Deng, Y. Han, S. Chen, and P. Varshney. A Key Management Scheme for Wireless Sensor Networks Using Deployment Knowledge. IEEE Infocom 2004.