1. An Efficient Key Management Scheme in Hierarchical Sensor Networks,2005 Author: Xiao Chen and Jawad Drissi

2. 1. Outline Introduction Hierarchical sensor network model Key Management Scheme Communication by keys Analysis of the Effectiveness of the Scheme Conclusion

3. 1.1 Sensor network limitation Vulnerability of physical capture Limited memory resources Limited bandwidth and transmission power

4. 1.2 key management scheme design goals Resilience against sensor capture: 防止某一 sensor 被攻破，就使得節點全被 攻破 Scale : The key management scheme should still be working well with the increase of sensor numbers.

5. 1.3 Traditional key distribution scheme A single key for the whole network: This scheme has the problem of compromising the whole network when any sensor is captured. A pair-wise shared key between every pair of sensor: This scheme requires each sensor to store n-1 shared keys.

6. 2.Hierarchical sensor network model A C D E F A bubble represents a group. The dots within a bubble represent group members. Group member can communicate with each directly,such as A and B. The high commander C can directly communicate with A or B. In order to let two sensors communicate securely, symmetric- key cryptography is used. R B L1 L2 L3 L4

7. 3. Key Management Scheme 3.1 Group key : Each group should have a group key so that all the members in a group can use it to communicate with each other. The key should be known to their commander so that it can send a command to the group encrypted by this key. A B C EX. Ａ want to communication with B, it will encrypt the message using group key K G{AB} A→ {B} : {m} K G{AB}

8. 3.2 Down-level Group Key : All sensors (except leaf) should store the down-level group key of the group it directly leads. It can use the key to give commands to the group. C 的 down level group key K G{AB} = A 和 B 的 group key K G{AB} A B C

9. 3.3 Up-level Pair-wise Key: Each member (except root) in a group should have a private pair-wise key with its direct commander. A B C EX. B want to communicate with D, it will send the message to C using the up-level pair-wise key between B and C, and then C will relay the message to D using the up-level pair-wise key between C and D. B → {C} : {m} K BC ; C → {D} : {m} K CD. D

10. 3.4 Down-level Pair-wise Key : Each sensor (except leaf) should have a down-level pair-wise key with each of the group members it directly leads. A B C EX. C want to communicate with A, it will send the message using down-level pair-wise key between C and A. C → {A} : {m} K CA. Down-level group key K CA = Up-level group key K AC

11. EX. A C DE F A → {C} : {m} K AC using up-level pair-wise key C → {D} : {m} K CD using up-level pair wise key D → {E} : {m} K G {DE} using group key E → {F} : {m} K EF using down-level pair-wise key

12. 4. sensor addition ， deletion and replacement 3.1 sensor addition (a single sensor or a sensor with a subtree ) Step1:It will contact its direct commander. Step2:Its commander will generate a downlevel pair-wise key to be shared with the new sensor. Step3:send the new sensor its downlevel group key encrypted by the downlevel pair-wise key.

13. Example: Sensor addition N will join the group of C. D generate a down-level pair- wise key K DN to be shared with N ( 即為 N 的 up-level pair-wise key K ND ) 再用 down-level pair-wise key 把 down level group key 加密後傳給 N( 即為 N 之 group key) D → {N} : { K G {CN} }K DN A C DE F N

14. 4.2 Sensor deletion Step1:Its direct commander will generate a new group key. Step2:It will remove the down-level pair-wise key shared with this sensor from its memory. Step3:If the leveling sensor has a subtree, it will take all its off-springs out of the network.

15. Example : sensor deletion C DE F N N want to leave the network. D will generate a new group key K G {CH},and send it to sensors C and H. D→ {C} : { K G (CH} } K DC D→ {H} : { K G {CH} } K DH And then D will remove the down- level pair-wise key K DN from its memory. H

16. 4.3 Sensor replacement Step1:Its direct commander will generate a down- level pair-wise key with the new sensor. Step2:The commander changes the group key of the group it leads and send the new group key to all the group members. Step3:If the new sensor is not a leaf, after deployment, it will send each of its subordinates a new down-level pair-wise key for future communication and change their group key.

17. Example: Sensor replacement C D N H IJ M Sensor N needs to be replaced by a new sensor M. D generate a new down-level key K DM to shared with M. D changes the group key.K G {CHM} D → {C} : {K G {CHM} }K DC D → {H} : {K G {CHM} }K HC D → {M}: {K G {CHM} }K MC If M is not a leaf, M generate new down-level pair-wise keys K MI and K MJ M generate a new group key K G{IJ} R

18. 5. Analysis of the Effectiveness of the Scheme Suppose there are n sensors on the network, the depth of the tree is d, if the tree is a complete tree, the number of sensors in a group is approximately log d n.

19. 5.1 訊息傳送次數 沒有子樹有子樹 Addition11 DeletionLog d n ReplacementLog d n3log d n

20. 5.2 各點所存的 KEY 個數 Root log d n+1 Leaf2 Others log d n+3 A B C Example: R D R 包含 : group key K G {CD} downlevel key K RC,K RD A,B: Group key K G {AB}, uplevel Pair-wise key K AC, K BC C 包含 : group key K G {CD} Downlevel group key K G {AB} downlevel key K CA,K CB uplevel Pair-wise key K CR

21. 6. Conclusion bring down the storage size bring down the number of message sent in updating to O(log d N)

22. 7.Commends 若將替代之節點早已被攻破,commander 如 何識別跟預防 ? 一群點中如何去挑選 commander? 由於 sensor 的限制, 群無法容納太多的成員