Multicast Security

May 10, 2004

Sam Irvine

Andy Nguyen

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

Security Objectives

• Usual suspects:– Authentication

• How do we authenticate members within the multicast group?

– Confidentiality– Integrity– Exclusivity

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

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

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?

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

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

Group Creation

k

k1

k01

k0

k10 k11k00

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

k

k1

k01

k0

k10 k11k00

u1 u2 u3 u4

Revoke u1's access

k is the shared key for the multicast group

Revoke u1's access

k

k1

k01

k0

k10 k11k00

u1 u2 u3 u4

Generate k', k0'

Revoke u1's access

k

k1

k01

k0

k10 k11k00

u1 u2 u3 u4

{k'}:k1

Revoke u1's access

k

k1

k01

k0

k10 k11k00

u1 u2 u3 u4

{k'}:k1

Revoke u1's access

k

k1

k01

k0

k10 k11k00

u1 u2 u3 u4

{k', k0'}:k01

Revoke u1's access

k

k1

k01

k0

k10 k11k00

u1 u2 u3 u4

{k', k0'}:k01

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)

Distributed key management

• Group Diffie-Hellman Key Exchange– N rounds, single key

• Distributed Logical Key Hierarchy– log(n) rounds– log(n) keys

Distributed Logical Key Hierarchy

m2 m3 m4m1

m1 and m2 agree on key k12

m3 and m4 agree on key k34

Key (12),(34)

m2 m3 m4m1

(m1,m2) and (m3,m4) agree on key k (12),(34)

k12 k34

• 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

• F0(X) = F(X)

• FL(X) = F(F

L-1(X))

Message authentication

Message authentication

• 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

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

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 1/2m

probability– Receivers can forge a MAC with 1/(2m-n)

probability

What haven't we talked about

• Routing table security– Unauthenticated clients cannot change the

routing topology– Can legitimate clients affect routing tables?

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