CSC 774 Advanced Network Security

1. CSC 774 Advanced Network Security Topic 5.3 Group Key Distribution Acknowledgment: Slides on LKH were originally provided by Dr. Wensheng Zhang at Iowa State. CSC 774 Adv. Net. Security

2. Outline • Overview of group key distribution • A naïve solution • Iolus: A Framework for Scalable Secure Multicasting • Logical key hierarchy (LKH) CSC 774 Adv. Net. Security

3. Group Key Distribution • Group session keys are determined by the group manager • Usually used for large groups. Group key manager Group members CSC 774 Adv. Net. Security

4. A Naïve Solution • Use a separate secure unicast connection from the group manager to EACH group member. • Requirement • Each client shares a unique key with the controller. • Poor scalability: • n secure unicast connections • n secret keys CSC 774 Adv. Net. Security

5. Problems Specific to Group Communication • “1 affects n” problem • The actions of one member affects the entire group Group key manager Old members New member joins CSC 774 Adv. Net. Security

6. Problems Specific to Group Communication (Cont’d) • “1 does not equal n” problem • Cannot deal with the group as a whole • Must consider the conflicting demands of members on an individual basis Group members Group key manager Example: Cannot use the old group key to distribute the new group key. Member leaves CSC 774 Adv. Net. Security

7. Iolus • Divide a large group into smaller groups • Introduce entities that manage and connect the subgroups • Group security controllers (GSC) • Control the entire group • Group security intermediaries (GSI) • Control the subgroups on behalf of GSC • GSC and GSI are both referred to as group security agent (GSA) • With GSC as the root, GSAs form a hierarchy of subgroups • A lower-level GSA is a member of the group headed by the higher-level GSA CSC 774 Adv. Net. Security

8. Iolus (Cont’d) CSC 774 Adv. Net. Security

9. Iolus (Cont’d) • Joins • GSA generates KGSA-MBR • Store this key along with other information • Send KGSA-MBR to the new member in a secure channel • Generate a new group key K’G • Send {K’G}KG to the group • Send K’G to the new member in a secure channel CSC 774 Adv. Net. Security

10. Iolus (Cont’d) • Leaves • Generate a new group key K’G • Send K’G to each member MBR individually in the secure channel encrypted with KGSA-MBR CSC 774 Adv. Net. Security

11. Iolus (Cont’d) • Data transmission • Data retransmitted within each subgroup CSC 774 Adv. Net. Security

12. Iolus (Cont’d) • Iolus for group key management • Replace the data with the group key in data transmission CSC 774 Adv. Net. Security

13. Group key Group Controller Logical entities N: number of members D: tree degree ln ( N ) depth + 1 ln ( d ) members Key Tree Approaches • Two types of keys • SEKs (Session Encryption Key) • KEKs (Key Encryption Key) • A Group Controller constructs a tree based hierarchy of KEKs CSC 774 Adv. Net. Security

14. Logical Key Hierarchy (LKH) • Keys are organized in a (logical) hierarchical tree • Group key is located at the root • Key encryption keys are the non-root, non-leave nodes • Each member corresponds to one leave node • Updates the group key and the key encryption key by means of the encryption of key-nodes • Rekey with only O(logN) messages CSC 774 Adv. Net. Security

15. K0 GKCs N secure channels K11 K12 K21 K22 K23 K24 K31 K32 K33 K34 K35 K36 K37 K38 M1 M2 M3 M4 M5 M6 M7 M8 LKH (Cont’d) • Initialization CSC 774 Adv. Net. Security

16. K0 K’0 K34 { K0’} K34 { K11’} K34 { K22’} K21 { K0’} K21 { K11’} GKCs K12 { K0’} K11 K’11 K12 K21 K22 K’22 K23 K24 K31 K32 K34 K35 K36 K37 K38 M1 M2 M3 M4 M5 M6 M7 M8 LKH (Cont’d) • Member leave Rekeying Messages ln ( N ) ln ( d ) CSC 774 Adv. Net. Security

17. Rekeying messages ln ( N ) ln ( d ) K0 K’0 K31 { K21’} K31 { K11’} K31 { K0’} K21 { K21’} K11 { K11’} K0{ K0’} GKCs K11 K’11 K12 K21 K’21 K22 K23 K24 K31 K32 K33 K34 K35 K36 K37 K38 M1 M2 M3 M4 M5 M6 M7 M8 LKH (Cont’d) • Member join CSC 774 Adv. Net. Security

18. User, Key, or Group Oriented Rekeying • User-oriented re-keying • Grouping re-keying messages by users • Less but bigger messages • Key-oriented re-keying • Grouping re-keying messages by keys • More but smaller messages • Group-oriented re-keying • Putting all re-keying messages together to generate a big, fat message • Only one gigantic message CSC 774 Adv. Net. Security

19. Example • User oriented • Key oriented • Group oriented CSC 774 Adv. Net. Security