1. Key Establishment Scheme against Storage-Bounded Adversaries in Wireless Sensor Networks Authors: Shi-Chun Tsai, Wen-Guey Tzeng, and Kun-Yi Zhou Source: IEEE Transactions on Wireless Communications, Vol.8, No.3, p.p.1218-1222, 2009. Presenter: Yung-Chih Lu ( 呂勇志 ) Date: 2010/02/24 1

2. Outline Introduction Proposed Scheme Performance Evaluation Security Analysis Conclusion Comment 2

3. Introduction (1/3) Goal ◦ Key Establishment ◦ Saving resource  Computation cost  Communication cost  Storage Overhead 3

4. Introduction (2/3) Assume ◦ Storage-Bounded Adversary  Poor radio quality  Limited coverage area  Adversary’s storage is limited Adversary cannot pucks up all radio communications. 4

5. Introduction (3/3) Wireless Sensor Network 5 V: Sensor node β : Beacon node

6. Proposed Scheme Key Establishment with Beacon (KEB) 6 Random bits:100111101 101110 111101 Index:123456789 Index:456789 Index:125678  Step1: Beacon generates and broadcasts α random bits. Ex: α =9  Step2: Each sensor stores μ bits. Ex: μ =6  Step3: Each sensor exchanges indices.  Step4: Each sensor finds common bits.  Step5: Each sensor computes common bits. Ex: The common key is H(1110) broadcast exchange

7. Performance Evaluation Probability of Establishing Shared Keys ◦  Ex: ◦ Sensors will have 4k common bits on average ◦  Ex: 7 k: the length of the shared keys S, T: a sensor node μ : the number of randomly stored bits of a sensor node α : the number of broadcasted random bits (M. Meguerdichian, Cambridge University Press, 2005)

8. Security Analysis Security of Shared Keys ◦ ◦ Chernoff bound approximation ◦ ◦ Ex: k=128, δ =2/3, ε =1/2 ◦ = 1.125x10 -7 ◦ At least (1-2/3-1/4)512 ≈ 43 common bits are securely A: the stored bits by the adversary. B: the common stored bits by two neighboring sensor nodes α : the number of broadcasted random bits 8

9. Conclusion The proposed scheme don’t pre-load secrets to sensor nodes. High connectivity ratio. Resiliency against node capture. Shared key can be updated. 9

10. Comment Key Establishment without Beacon 10 101110 111101 Index:123456 10 01 Index:56 Index:12 exchange Shared key is H(0110)