tree based scalable secure group communication

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Bonfring International Journal of Research in Communication Engineering, Vol. 1, Special Issue, December 2011 1

ISSN 2250110X | 2011 Bonfring

Abstract--- In order to establish a group communication,

a common key must be available with all the members of the

group. The group key can be used for encrypting databetween the group members or restricting access to the

resources intended for group members only. Each member in

a group has a unique key referred as member key, used for

decrypting data in a group. The group key is distributed by

group key server, which changes the group key time to time

called as group rekeying. It is mandatory that the group key

changes after a new user has joined and an existing user

departed periodically. The existing system analyse the

Bursty behaviour and operation. Burstiness is an important

behavior in Secure Group Communication (SGC).

Performing bursty operation, which may accumulate the

simultaneous leave and join as a single operation, thus

reduces the frequency of key distribution and reduces time

complexity. But in the existing system the aggregate

operation will occur only in rare condition so it may not

perform the key reduction in all cases as well as it perform

less scalability and security. To achieve better scalability,

security and key reduction a new group key management

protocol based on the Chinese Remainder Theorem and a

hierarchical tree is proposed, in which each node contains a

key and a modulus. The Keys and modulus are constructed as

a tree and maintained by the key server. The key server

shares the keys with each member on the path from its leaf to

the root. The keys on its path from the leaf to the root need to

be updated in the protocol, when a member joins or leavesthe group but all modulus must be kept fixed. To update the

keys on the tree, the key server generates a new key for each

update node and encrypts it with its children keys on its path

from the leaf to the root. Thus the new scalable protocol

increases the security, scalability issues when the group size

goes up to millions of members and reduces the key.

Keywords--- Re-Keying, Scalability, Group key

Management.

I.INTRODUCTION

HE advances in communication and networkingtechnologies have paved ways for people to share and

disseminate information. Along with the growing exchange

of information, the security of communications has drawn

increasing attention. An important aspect of communication

M. Rameeya, Assistant Professor, Department of Computer Science and

Engineering, Mepco Schlenk College of Engineering. E-mail:

[email protected]

S. Oswalt Manoj, Assistant Professor, Department of Computer Science

and Engineering, Sri Ramakrishna Institute of Technology. E-mail:

[email protected]

security is content confidentiality. Secure group

communication (SGC) is becoming more popular in the

Internet. Examples of such applications include videoconferencing, interactive group games, TV over internet, e-

learning, and public stock quote broadcasting. As an

important and mandatory building block for multicast

applications, multicast security has been extensively

researched in the past decades for protecting multicast

communications. The research on multicast security addresses

authentication, confidentiality, and access control, among

other areas, where group key management is a key

component. Even though Internet multicast capability

provides an efficient way for secure group communication

applications, the security of multicast applications is

guaranteed by cryptographic techniques. The most important

feature of SGC is group dynamics by which we mean that

members can join and/or leave a group at any time. To

achieve confidentiality in group communications, a key

known to all group members is used to encrypt the

communication content. This key is usually referred to as the

group key. In a group with dynamic membership, the group

key needs to be updated upon each users join to prevent the

new user from accessing the past communications. Similarly,

upon each users departure, the group key needs to be updated

to prevent the leaving user from accessing the future

communications. Thus group members need to agree upon

the same key management protocol for key establishment and

update.The biggest challenge caused by group dynamics is that

when member(s) join or leave a group, the group key(s) must

be changed in an efficient and scalable way to prevent the

joining/leaving member from decrypting the previous/future

messages.

In existing burstiness is an important behavior in SGC.

Performing bursty operation in one aggregate operation is

important for reduce the rekeying message. When the

frequency of membership changes is high, it becomes

necessary to reduce the cost of frequent key distributions. One

feasible way is to accumulate the joins and leaves for a certain

period of time, thus reducing the frequency of keydistributions. This can be considered as another kind of bursty

behavior. Performing a bursty operation in one aggregate

operation is important for reducing the number of rekeying

messages, reducing the frequency of key distributions. This

may maximizes key management efficiency in secure, but

relatively dynamic, group communication. This technique is

based on logical key hierarchy. The aggregation of key

updates can reduce the cost of key distribution operations.

However, they are still vulnerable to scalability issues when

the group size goes up to millions of members and the re-key

Tree Based Scalable Secure Group CommunicationM. Rameeya and S. Oswalt

T


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messages require strong security protection such as signature.

Figure 1: A Tree with Nodes Containing Key and

Modulus.

In this paper, we propose a new group key based on the

Chinese Remainder Theorem and a hierarchical tree graph in

which each node contains a key and a modulus. The protocol

is designed to minimize re-key messages, bandwidth usage,

encryption, and signature operations. In the new protocol, the

keys and moduli are constructed as a tree and maintained by

the key server. The tree graph is similar to the tree graph in

the logical key hierarchy protocol but each node of the tree in

the new protocol is assigned two values: a key and a modulus.

The key server shares the keys with each member on the path

from its leaf to the root. The keys on its path from the leaf to

the root need to be updated in the protocol when a member

joins or leaves the group but all moduli must be kept fixed.

II.

RELATED WORK

One feasible way is to accumulate the joins and leaves

(Chang et al., 1999) for a certain period of time, thus

reducing the frequency of key distributions. There has been

extensive research focusing on group dynamics in

SGC(Burmester & Desmedt, 1999), (Caronni et al., 1998),

(Ingemarsson et al., 1982),(Iolus, 1997), (Molva & Pannetrat,

1999), (Noubir, 1998).The Local Key Hierarchy(LKH)

protocol(wong et al, 2000) they reduce the re-key message.

However most of them place the emphasis on single join and

single leave, i.e., reducing the number of rekeying messages

when a member joins/leaves. A key tree scheme (Caronni et

al, 1998),(Noubir ,1998) processing multiple joins and leaves

in aggregation is possible and will reduce the number of

rekeying messages.The best time to join a group is when a

member leaves, as the new member just need to replace the

position previously occupied by the leaving member, and all

tha keys held by the latter are updated(zou et al,

2002).Rekeying operation, allowing member to share the

keys(wong et al,2000). key-tree key management protocol

(Caronni et al., 1998), (Noubir ,1998 ),(Wong et al., 1998)

for secure group communication to situations with bursty user

arrival and departure patterns, especially when multiple joins

and multiple leaves occur at the same time.

The typical

schemes for SGC with the emphasis on key tree scheme

(Caronni et al., 1998), (Noubir ,1998),(Wong et al., 1998) .

In other schemes (Burmester & Desmedt, 1995), (Dondeti,

1999),(Tang et al., 1982),(Steer et al, 1990),(Steiner et al.,

1996) the group key is generated by uniform contributions

from all group members. Based on the structural organization

of group members, most schemes do not split members

whereas some schemes (Dondeti et al., 1999),(Dondeti,

1999),(Mittra, 1997) divide group members into distinct

subgroups, resulting in two levels of key management and

increasing the scalability. To achieve confidentiality in group

communications, a key known to all group members is used

to encrypt the communication content (Judge & Ammar ,

2002),(Canetti et al., 1999).

III.

BURSTY BEHAVIOUR

A. Existing Schemes for Secure Group Communication

We summarize and classify secure group communication

schemes in this section. Based on the number of senders,

SGC applications can be divided into two categories:

broadcast communication, i.e., one-to-many communication

and conference communication, i.e., many-to-many

communication. Schemes are suitable for both kinds of

applications. Based on how the group key is formed, some

schemes require a Group Controller (GC) which generates

group key and distributes the key to group members. In other

schemes, the group key is generated by uniform contributions

from all group members. The bursty operation is based on the

tree based key management in which when the frequency of

membership changes is high, it become necessary to reduce

the cost of frequent key distributions. One feasible way is to

accumulate the join and leave for a certain period of time,

thus reducing the rekeying process.

Based on the structural organization of group members,

most schemes do not split members whereas some schemes

divide group members into distinct subgroups, resulting in

two levels of key management and increasing the scalability.

In the later case, the subgroup manager may be a member of

the group or not and may be trusted or not. Based on the kind

of security, the SGC schemes may be classified as

unconditionally secure or computationally secure.

The members of the group are placed at leaf nodes of

the tree. The nodes in the tree are assigned keys. The key at

the root is the traffic encryption key (TEK) Every member is

assigned the keys along the path from its leaf to the root. In

the new protocol, the keys and moduli are constructed as a

tree and maintained by the key server. The nodes in the

different level of the tree are assigned with the different

moduli but each a pair of siblings at the same tree depth are

assigned with the same two moduli under the different


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parents.

The key server shares the keys with each member on the

path from its leaf to the root. The keys on its path from the

leaf to the root need to be updated in the protocol when a

member joins or leave the group but all moduli must be kept

fixed. To update the key, the key server generates a new key

and encrypts it with its children key on its path from the leaf

to the root.

IV.REKEYING STRATEGIES

A user who wants to join /leave a secure group sends a

join/ leave request to the key server, denoted by s. For a join

request from user u, we assume that group access control is

performed by server using an access control list provided by

the initiator of the secure group. A join request initiates an

authentication exchange between u and s. If user is not

authorized to join the group, server s sends a join-denied

reply to u. If the join request is granted, then a key is

distributed as a result of the authentication exchange by the

individual key ku of u. Key exchange between server s and

user u, and secure distribution of key kuto be shared by u and

s.

After each join or leave, a new secure group is formed.

Server s has to update the group's key graph by replacing the

keys of some existing k-nodes, deleting some k-nodes (in the

case of a leave), and adding some -nodes (in the case of a

join). It then securely sends rekey messages containing new

group/subgroup keys to users of the new secure group.

V.TREE BASED SCALABILITY OF SGC

Our new scalable group key management protocol is based

on the following: the Chinese Remainder Theorem and ahierarchical graph in which each node contains a key and a

modulus. The protocol is designed to minimize re-key

messages, bandwidth usage, encryption, and signature

operations. Chinese Remainder Theorem: Let m1, m2,

...mn be n positive integers where they are pairwise

relatively prime (i.e. gcd(mi,mj)=1 for ij , 1i, jn),

R1,R2, ...Rnbe any positive integers, and M=m1m2...mn.

Then the set of linear congruous equations XR1mod m1,

...XRnmod mnhave a unique solution as:

X= MyMRn

i

iii mod1

whereMi=M/mi and yi=M 1

i mod mi .

In the new protocol, the keys and moduli are constructed

as a tree and maintained by the key server. The tree graph is

similar to the tree graph in the LKH protocol but each node of

the tree in the new protocol is assigned two values: a key and

a modulus. Figure 1 depicts the key and modulus graph,

where TEK is a traffic encryption key, kijis a key encryption

key, and mijis a modulus.

A.

Moduli Maintenance

The key server needs to store 2log2n moduli and each

member needs to store log2n moduli but theydo not need to

keep the moduli secret. The sibling nodes in the tree graph

are assigned with two different moduli (i.e., mi1 and mi2

where i is the depth of the tree) and the nodes in the different

level of the tree are assigned with the different modulibut

each a pair of siblings at the same tree depth are assigned

with the same two moduli under the different parents .Thismeans there are only 2log2n different moduli in the tree

graph, i.e. mij(1ilog2n, j=1, 2) where i is the depth of the

node in the tree, and the nodes (except the root) on a path

from a leaf to the root and its direct children exactlycover all

moduli. In addition, all different moduli in the tree graph

should be pair wise relatively prime i.e., gcd(mij,mst)=1 and

each modulus should be bigger than thekey encryption value,

i.e mij>Eilk

(kst) where mij and kil belong to the same node

and kstbelongs to its parent node.

B.

Key Maintenance

The key server needs to store 2n-1 keys, and each member

needs to store log2n+1 keys. The key server shares the keys

with each member on the path from its leaf to the root. The

keys on its path from the leaf to the root need to be updated in

the protocol when a member joins or leaves the group but all

moduli must be kept fixed.

To update the keys on the tree graph, the key server

generates a new key for each update node and encrypts it with

its children keys on its path from the leaf to the root. For

instance, the key server needs to generate new keys {TEK,

kil} to update {TEK, kil} for the arrival of member to the

group.

The key server then calculates a lock L as follows and

multicasts the lock with the indices of keys to all valid

members.

L=1log

1

mod2 z

zt

sjsjst

n

s

MyMk

Where,

z=12/

2/

2

2

log

log

sn

sn

d

dif

snd 2log2/ is odd

,2

,1j if t 1 mod2, otherwise

M=2

1

log

1

,2

j

sj

n

s

m M ,/ sjsj mM y sjsjsj mM mod1

Each member decrypt the updated traffic encryption key

and related key encryption keys based on their own moduli

and keys.


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tree based scalable secure group communication

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