SOT: Secure Overlay Tree for Application Layer Multicast

1
SOT Secure Overlay Tree for Application Layer
Multicast

• Ken Yiu
• HKUST

2
Outline

• Introduction
• Related Work
• Issues on ALM security
• Two basic approaches
• Secure Overlay Tree (SOT)
• Simulation Results
• Conclusion

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Introduction

• Lack of widely deployed multicast-capable network
• Application Layer Multicast (ALM)
• No data confidentiality provided in ALM
• Applications that require secure multicast cannot
  apply ALM directly
• SOT a simple and efficient approach to provide
  data confidentiality

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Introduction Multicast Security

• Multicast data is encrypted by a shared symmetric
  key
• Forward Secrecy user cannot decrypt future
  multicast data after he leaves
• Backward Secrecy user cannot decrypt past
  multicast data before he joins
• Change keys (re-key) whenever there is a
  membership change

s
a
e
b
d
c
5
Related Work

• Security on IP multicast
• Logical Key Hierarchy (LKH)
• Iolus
• Assume multicast-capable network (IGMP, DVRMP,
  CBT, PIM-DM, etc.)
• Multicasting one re-key message takes only one
  communication overhead

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Related Work LKH

• Each user holds the keys on the path from its
  user key to the group key
• Re-key change all keys on the path
• Re-key message overhead (multicast)
• O(2 logk N) for join
• O(k logk N) for leave

Group key
kg
Key-encryption key (KEK)
k0
k1
User key
k00
k01
k10
k11
k000
k001
k010
k011
k100
k101
k110
k111
u0
u1
u2
u3
u4
u5
u6
u7
Re-key when u6 leaves Ek111k11,
Ek10k1, Ek11k1, Ek0kg, Ek1kg
Re-key when u2 joins Ek010k01, Ek01k01,
Ek0k0, Ek01k0, Ek0kg, Ekgkg
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Related Work Iolus

• Multicast group is divided into subgroups (each
  governed by a GSA and has its own subgroup key)
• Re-encryption required at GSIs
• Join/Leave only affects one subgroup (change
  subgroup key only)
• GSAs (special entities) are chosen a priori and
  statically configured
• No size bound on subgroups

GSI
GSI
GSC
GSI
GSI
GSI
Decryption and re-encryption required
GSC Group Security Controller GSI Group
Security Intermediary
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Issues on ALM security

• Multicast is accomplished by unicast connections
  between peers
• O(N) for each multicast re-key message
• O(N logk N) for each re-keying in LKH
• No GSAs in ALM
• Can their functionalities be moved to peers?
• How about large subgroups?
• Minimizing average nodal processing overhead on
  peers for data confidentiality

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Two basic approaches
Re-encryption EBCDABk

• Host-to-host encryption
• Large re-encryption overhead
• Whole group encryption
• Large re-keying overhead

A
D
EkdataEABk
EkdataECDk
B
C
EkdataEBCk
k random key generated by source XY secret
key shared by X and Y
A
D
EABg
ECDg
Egdata
EBCg
B
Egdata
C
Egdata
Re-keying EBCDABg
g group key shared by all users
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Secure Overlay Tree (SOT)

• Clustering peers into subgroups
• host-to-host between clusters
• whole group within clusters
• Balance two kinds of overhead and obtain the
  minimum total overhead by choosing appropriate
  optimal cluster size

A
B
Source
Ingress
f
EkdataEBk
b
a
h
EkdataEdek
e
d
EkdataEAk
c
g
Egress
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SOT (contd)

• Split/Merge mechanism is used to maintain cluster
  size within the bound (m/2 lt c lt 2m)
• Join/Leave only affects one cluster instead of
  the whole group
• Leaders are elected from each cluster to
  coordinate joining, merging and splitting
• Apply Internet coordinate system like GNP to
  obtain coordinates for clustering purpose
• SOT is a framework, existing ALM protocols can be
  used for implementation

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SOT architecture
S
I
L
E
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Simulation

• Setup
• GT-ITM TS topology 1024 routers
• Member Join Poisson process (avg. rate ?)
• Holding Time Exponential (mean T sec.)
• Random chosen data source (constant data rate R
  bps)

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Simulation (contd)

• Typical applications

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Results optimal cluster size
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Results avg. nodal processing overhead
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Results relative delay penalty
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Results physical link stress
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Conclusion

• Security schemes for IP multicast are not
  suitable for ALM
• SOT provides data confidentiality
• Based on clustering of peers
• Optimal cluster size
• Lower nodal processing overhead
• Comparable network performance

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Thank You !!