1. Multicast Security May 10, 2004 Sam Irvine Andy Nguyen

2. Multicast Overview Bandwidth-conserving technology that reduces traffic by simultaneously delivering a single stream of information to thousands of recipients (multicast group) Applications include video-conferencing, streaming audio, sending out stock quotes, etc. Scalable reliability, flow control, congestion control, security are all active areas of research

3. Security Objectives Usual suspects: –Authentication How do we authenticate members within the multicast group? –Confidentiality –Integrity –Exclusivity

4. Multicast Security Inherently more susceptible to attack –Many more opportunities and points for interception of traffic and attacks –Attacks affect many systems –Usually multicast address is well-known –Possible for attacker to pose as one of the many possible systems in the multicast group Solutions must be scalable and address the dynamic nature of membership

5. Unicast versus Multicast Security Security association defines a set of keying material in order to setup a secure link between two systems in a unicast protocol –Membership remains static throughout the session In multicast, the security association is among many people –Membership is dynamic throughout the session

6. Dynamic Membership Must ensure that a member is only allowed to participate when it is authorized to do so New members must not be able to access old multicast data (joins) Old members must not be able to access new multicast data (leaves) Multicast security protocol must be prepared to change the keying material on each and every join to insure integrity How do we do key management for dynamic security associations?

7. Key management solutions Centralized group key management protocols Decentralized Architectures –Management divided into subgroups Distributed Key Management protocols –No explicit key distribution center, members themselves handle key generation

8. Centralized Key Management Example Canetti et al. use one way function trees in conjunction with pseudo-random generators Each user holds log(n+1) keys Issuing a new keys takes log(n) sends

9. Group Creation kk1k01k0k10k11k00 u1 u2 u3 u4 {k00,k01,k0,k10,k11,k1,k} {k,k0,k00,k01} {k,k0,k00} {k,k0,k01}{k,k1,k10} {k,k1,k10,k01} {k,k1,k11} k is the shared key

10. kk1k01k0k10k11k00 u1 u2 u3 u4 Revoke u1's access k is the shared key for the multicast group

11. Revoke u1's access kk1k01k0k10k11k00 u1 u2 u3 u4 Generate k', k0'

12. Revoke u1's access kk1k01k0k10k11k00 u1 u2 u3 u4 {k'}:k1

13. Revoke u1's access kk1k01k0k10k11k00 u1 u2 u3 u4 {k'}:k1

14. Revoke u1's access kk1k01k0k10k11k00 u1 u2 u3 u4 {k', k0'}:k01

15. Revoke u1's access kk1k01k0k10k11k00 u1 u2 u3 u4 {k', k0'}:k01

16. Decentralized Architectures Example Iolus –Splits a large group into small subgroups –Group Security Controller at the top, Group security intermediaries manage subgroups –In order to update key for leaves, must send out new key encrypted with everyone’s secret key. Size of message is O(n) –Data path affected when sending out data (Translating data between groups)

17. Distributed key management Group Diffie-Hellman Key Exchange –N rounds, single key Distributed Logical Key Hierarchy –log(n) rounds –log(n) keys

18. Distributed Logical Key Hierarchy m2m3m4m1 m1 and m2 agree on key k12 m3 and m4 agree on key k34

19. Key (12),(34) m2m3m4m1 (m1,m2) and (m3,m4) agree on key k (12),(34) k12k34

20. Digital Signatures –RSA,DSA, Elliptic Curve –Very expensive to compute for each message Message Authentication Codes (MAC) –Given a shared key K, a positive integer L and a one way function F Compute F L (K + message), where F 0 (X) = F(X) F L (X) = F(F L-1 (X)) Message authentication

21. MAC exclusivity –If all receivers have the MAC key, than any receiver can fake a message Solution –Generate a set of m keys –Distribute n < m of the keys randomly to each receiver –Sender knows all m keys

22. Message authentication Solution (cont) –Sender computes m MACs and sends them with the message –Receivers verify the MAC for each of their known n keys –Senders cannot independently create all m MACs without collusion –Randomness prevents intentional collusion

23. Message authentication Sets of keys can reduce MAC length overhead –Use previous scheme with 1 alteration: MACs map to a single bit –Can arbitrarily forge a MAC with 1/2 m probability –Receivers can forge a MAC with 1/(2 m-n ) probability

24. What haven't we talked about Routing table security –Unauthenticated clients cannot change the routing topology –Can legitimate clients affect routing tables?

25. Differing multicast requirements 1-N multicasting –1 Sender, N receivers M-N multicast –M senders transmit to N receivers N-N full duplex communication –Any member can communicate to any other member