1. A hierarchical key management scheme for secure group communications in mobile ad hoc networks Authors: Nen-Chung Wang and Shian-Zhang Fang Sources: The Journal of Systems and Software, accepted manuscript. Reporter: Chun-Ta Li ( 李俊達 )

2. 2 22 Outline  Motivation  The hierarchical key management scheme (HKMS)  Comments

3. 3 Motivation  Key management scheme in a MANET Improving security  Message encryption/decryption  Forward secrecy and backward secrecy Reducing the memory storage of keys  Clustering or hierarchical trees Frequent changes of the network topology (rekeying)  Members join or leave a group  Manage keys efficiently and reduce the amount of rekeying

4. 4 The hierarchical key management scheme  Notations  Key management (2-hop) ◙ Procedure 1: L1-head selecting ◙ Procedure 2: L2-head selecting public/private key

5. 5 The hierarchical key management scheme (cont.)  The node communications in different subgroups Subgroup 1 Subgroup 3 Subgroup 4 Subgroup 2 Subgroup 5 K c (2,3) K c (2,5) K c (3,4)

6. 6 The hierarchical key management scheme (cont.)  Encryption/decryption operation during data transmission Subgroup 1 Subgroup 2 L2GK 1,1,K DH,Data L1GK 1,K DH,Data L2GK 1,2,K DH,Data K c,K DH,Data L2GK 2,3,K DH,Data L1GK 2,K DH,Data L2GK 2,1,K DH,DataData

7. 7 The hierarchical key management scheme (cont.)  Subgroup key maintenance New node joining a subgroup  ◙ Step1: Sends a join request message ◙ Step2: Sends the join request message to the L2-head ◙ Step3: Sends a reply message ◙ Step4: Allowed to join the L2-subgroup ◙ Step5: L2-head regenerates an L2-subgroup key and sends it to all L2-subgroup nodes

8. 8 The hierarchical key management scheme (cont.)  Node leaving a subgroup (Case 1) The leaving of ordinary nodes  Step 1: Sends a leave message to the L2-head  Step 2: L2-head regenerates a new L2-subgroup key and sends it to all remaining nodes H1H1 H 1,1 H 1,2 Subgroup 1 Ordinary node Node leaving Ordinary node

9. 9 The hierarchical key management scheme (cont.)  Node leaving a subgroup (Case 2) The leaving of L2-heads H1H1 H 1,1 H 1,2 Subgroup 1 A Ordinary node Node leaving Ordinary node ◙ Step1: Sends a leave message to ordinary nodes and the L1-head ◙ Step2: Selects a new L2-head(A) by comparing the weight values of the ordinary nodes ◙ Step3: Sends the updated L2-subgroup information to the L1-head ◙ Step4: L1-head regenerates a new subgroup key and sends it to all the L2-heads ◙ Step5: L2-head regenerates a new subgroup key and sends it to all the ordinary nodes of L2-subgroup

10. 10 The hierarchical key management scheme (cont.)  Node leaving a subgroup The leaving of L2-heads H1H1 H 1,2 Subgroup 1 A Ordinary node

11. 11 The hierarchical key management scheme (cont.)  Node leaving a subgroup (Case 3) The leaving of L1-head H1H1 H 1,1 H 1,2 Subgroup 1 A Ordinary node Node leaving Ordinary node ◙ Step1: Sends a leave message to L2-heads ◙ Step2: Selects a new L1-head from L2-heads ◙ Step3: Selects a new L2-head from ordinary nodes of L2-subgroup ◙ Step4: All L2-heads send their L2- subgroup information to the new L1-head for registration ◙ Step5: L1-head regenerates a new subgroup key and sends it to all L2-heads ◙ Step6: L2-heads regenerate a new subgroup key and sends it to all ordinary nodes of L2- subgroup

12. 12 The hierarchical key management scheme (cont.)  Node leaving a subgroup The leaving of L1-head H1H1 H 1,2 Subgroup 1 A Ordinary node

13. 13 Comments  Rekeying in HKMS Join: m+1 asymmetric encryption/decryption Leave:  Case 1: m asymmetric encryption/decryption  Case 2,3: p asymmetric encryption/decryption m: number of nodes in L2-subgroup k: number of L2 heads p: total nodes in a subgroup (p=mk+1) H1H1 H 1,1 H 1,2 Subgroup 1 Ordinary node

14. 14 Comments (cont.) 1 23 Subgroup 1 Ordinary node 4 5 67 8 9 L1GK1 = H(1 ♁ 2 ♁ 3) L2GK 1,1 = H(L1GK1, H(4 ♁ 5 ♁ 6))L2GK 1,2 = H(L1GK1, H(7 ♁ 8 ♁ 9)) 1,2,3,4,5,6,7,8,9 4,5,67,8,9 5,6 4,6 4,5 7,8 7,9 8,9

15. 15 Comments (cont.)  Join 1 23 Subgroup 1 Ordinary node 4 5 67 8 9 L1GK1 = H(1 ♁ 2 ♁ 3) new L2GK 1,1 = H(L2GK 1,1, 10) L2GK 1,2 = H(L1GK1, H(7 ♁ 8 ♁ 9)) 1,2,3,4,5,6,7,8,9,10 4,5,6,107,8,9 5,6,10 4,6,10 4,5,10 7,8 7,9 8,9 10 4,5,6

16. 16 Comments (cont.)  Leave (Case 1) 1 23 Subgroup 1 Ordinary node 4 5 67 8 9 L1GK1 = H(1 ♁ 2 ♁ 3) new L2GK 1,1 = H(L2GK 1,1, 4) L2GK 1,2 = H(L1GK1, H(7 ♁ 8 ♁ 9)) 1,2,3,4,5,6,7,8,9,10 4,5,6,107,8,9 5,6,10 4,6,10 4,5,10 7,8 7,9 8,9 10 4,5,6

17. 17 Comments (cont.)  Leave (Case 2) 1 23 Subgroup 1 Ordinary node New L2-head Ordinary node 4 5 67 8 9 L1GK1 = H(1 ♁ 2 ♁ 4’) 1,3,4’,5,6,7,8,9,10 7,8,9 5,6,10 6,10 5,10 7,8 7,9 8,9 10 5,6 L2GK 1,1 = H(L1GK1, H(5 ♁ 6 ♁ 10))L2GK 1,2 = H(L1GK1, H(7 ♁ 8 ♁ 9))

18. 18 Comments (cont.)  Leave (Case 3) 1 23 Subgroup 1 Ordinary node 4 5 67 8 9 L1GK1 = H(1 ♁ 2 ♁ 3) new L2GK 1,1 = H(L2GK 1,1, 4) L2GK 1,2 = H(L1GK1, H(7 ♁ 8 ♁ 9)) 1,2,3,4,5,6,7,8,9,10 4,5,6,107,8,9 5,6,10 4,6,10 4,5,10 7,8 7,9 8,9 10 4,5,6

19. 19 Comments (cont.)  Leave (Case 3) 2 43 Subgroup 1 Ordinary node 10 5 67 8 9 L1GK1 = H(2’ ♁ 3 ’ ♁ 4’) L2GK 1,1 = H(L1GK1, H(4 ♁ 5 ♁ 6))L2GK 1,2 = H(L1GK1, H(7 ♁ 8 ♁ 9)) 2’,3’,4’,5’,6’,7’,8’,9’,10’ 5’,6’,10’7’,8’,9’ 5’,6’ 6’,10’ 5’,10’ 7’,8’ 7’,9’ 8’,9’