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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 )

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Outline Motivation The hierarchical key management scheme (HKMS) Comments

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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

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The hierarchical key management scheme Notations

Key management (2-hop)

◙ Procedure 1: L1-head selecting

◙ Procedure 2: L2-head selecting

public/private key

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The hierarchical key management scheme (cont.) The node communications in different subgroups

Subgroup 1

Subgroup 3

Subgroup 4

Subgroup 2

Subgroup 5

Kc(2,3)

Kc(2,5)Kc(3,4)

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The hierarchical key management scheme (cont.)

Encryption/decryption operation during data transmission

Subgroup 1

Subgroup 2

L2GK1,1,KDH,Data

L1GK1,KDH,Data

L2GK1,2,KDH,Data

Kc,KDH,Data

L2GK2,3,KDH,Data

L1GK2,KDH,Data

L2GK2,1,KDH,DataData

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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

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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

H1

H1,1 H1,2

Subgroup 1

Ordinary node

Ordinary node

Ordinary node

Node leaving

Ordinary node

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The hierarchical key management scheme (cont.) Node leaving a subgroup (Case 2)

The leaving of L2-headsH1

H1,1 H1,2

Subgroup 1

A

Ordinary node

Ordinary node

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

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The hierarchical key management scheme (cont.) Node leaving a subgroup

The leaving of L2-heads

H1

H1,2

Subgroup 1

A

Ordinary node

Ordinary node

Ordinary node

Ordinary node

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The hierarchical key management scheme (cont.) Node leaving a subgroup (Case 3)

The leaving of L1-head

H1

H1,1 H1,2

Subgroup 1

A

Ordinary node

Ordinary node

Ordinary nodeNode 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

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The hierarchical key management scheme (cont.) Node leaving a subgroup

The leaving of L1-head

H1

H1,2

Subgroup 1

A

Ordinary node

Ordinary node

Ordinary node

Ordinary node

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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-subgroupk: number of L2 headsp: total nodes in a subgroup (p=mk+1)

H1

H1,1 H1,2

Subgroup 1

Ordinary node

Ordinary node

Ordinary node

Ordinary node

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Comments (cont.)

1

2 3

Subgroup 1

Ordinary node

Ordinary node

Ordinary node

Ordinary node

4

5

6 7

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9L1GK1 = H(1♁2 ♁ 3)

L2GK1,1 = H(L1GK1, H(4♁5 6))♁ L2GK1,2 = H(L1GK1, H(7♁8 9))♁

1,2,3,4,5,6,7,8,9

4,5,6 7,8,9

5,6

4,6

4,5

7,8

7,9

8,9

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Comments (cont.) Join

1

2 3

Subgroup 1

Ordinary node

Ordinary node

Ordinary node

Ordinary node

4

5

6 7

8

9L1GK1 = H(1♁2 ♁ 3)

new L2GK1,1 = H(L2GK1,1, 10) L2GK1,2 = H(L1GK1, H(7♁8 9))♁

1,2,3,4,5,6,7,8,9,10

4,5,6,10 7,8,9

5,6,10

4,6,10

4,5,10

7,8

7,9

8,9

10

4,5,6

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Comments (cont.) Leave (Case 1)

1

2 3

Subgroup 1

Ordinary node

Ordinary node

Ordinary node

Ordinary node

4

5

6 7

8

9L1GK1 = H(1♁2 ♁ 3)

new L2GK1,1 = H(L2GK1,1, 4) L2GK1,2 = H(L1GK1, H(7♁8 9))♁

1,2,3,4,5,6,7,8,9,10

4,5,6,10 7,8,9

5,6,10

4,6,10

4,5,10

7,8

7,9

8,9

10

4,5,6

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Comments (cont.) Leave (Case 2)

1

2 3

Subgroup 1

Ordinary node

Ordinary node

New L2-head

Ordinary node

4

5

6 7

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9L1GK1 = 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

L2GK1,1 = H(L1GK1, H(5♁6 10))♁ L2GK1,2 = H(L1GK1, H(7♁8 9))♁

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Comments (cont.) Leave (Case 3)

1

2 3

Subgroup 1

Ordinary node

Ordinary node

Ordinary node

Ordinary node

4

5

6 7

8

9L1GK1 = H(1♁2 ♁ 3)

new L2GK1,1 = H(L2GK1,1, 4) L2GK1,2 = H(L1GK1, H(7♁8 9))♁

1,2,3,4,5,6,7,8,9,10

4,5,6,10 7,8,9

5,6,10

4,6,10

4,5,10

7,8

7,9

8,9

10

4,5,6

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Comments (cont.) Leave (Case 3)

2

4 3

Subgroup 1

Ordinary node

Ordinary node

Ordinary node

Ordinary node

10

5

6 7

8

9L1GK1 = H(2’♁3’ 4’)♁

L2GK1,1 = H(L1GK1, H(4♁5 6))♁ L2GK1,2 = 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’