A Distributed Security Architecture for
Ad hoc Networks
Ratan Guha
, Mainak Chatterjee
and Jaideep Sarkar
School of Computer Science
University of Central Florida
Orlando, FL 32816-2362
Electrical and Computer Engineering
University of Central Florida
Orlando, FL 32816-2450
Abstract. Secure communication in ad hoc networks is an inherent problem be-
cause of the distributiveness of the nodes and the reliance on cooperation between
the nodes. All the nodes in such networks rely and trust other nodes for forward-
ing packets because of their limitation in the range of transmission. Due to the
absence of any central administrative node, verification of authenticity of nodes
is very difficult. In this paper, we propose a clusterhead-based distributed secu-
rity mechanism for securing the routes and communication in ad hoc networks.
The clusterheads act as certificate agencies and distribute certificates to the com-
municating nodes, thereby making the communication secure. The clusterheads
execute administrative functions and hold shares of network keys that are used
for communication by the nodes in respective clusters. Due to the process of
authentication, there are signalling and message overheads. Through simulation
studies, we show how the presence of clusterheads can substantially reduce these
overheads and still maintain secure communication.
1 Introduction
The absence of centralized administration or fixed network infrastructure is the essence
of peer to peer or ad hoc networks. Such networks can be established where there is no
infrastructure or in which the existing infrastructure does not meet requirements such
as deployment delay and costs. Though ad hoc networks provide huge benefit to ap-
plications like military operations, they at the same time fail to provide reliable and
secure communications. Most of the research in ad hoc networking have assumed non-
adversarial network setting or a trusted environment. Relatively little research has been
done in a more realistic setting in which an adversary may attempt to disrupt the com-
munication. A central issue concerning the design of any service in ad hoc networks is
not to rely on any centralized entities, because such entities would obviously be prone
to attacks, and also their reachability could not be guaranteed at all times for all partic-
ipants of the network. Therefore, it is not possible to implement a centralized, trusted
entity for managing network keys of the participants as performed in conventional wired
Guha R., Chatterjee M. and Sarkar J. (2005).
A Distributed Security Architecture for Ad hoc Networks.
In Proceedings of the 4th International Workshop on Wireless Information Systems, pages 81-91
DOI: 10.5220/0002570800810091
networks, where a central trusted authority is always capable of managing the network
and providing the entities of the network with keys for authentication purposes.
In this paper we propose a solution where we make the clusterheads as the distrib-
uted certificate authorities (CA) who will be vested with the charge of storing network
keys and then distributing those keys to the communicating nodes after their authentic-
ity has been confirmed by the clusterheads.
The rest of the paper is organized as follows. In section 2, we discuss the general
problem of securing ad hoc networks, with common attack techniques and methods to
tackle them. In section 3, we propose our clusterhead based architecture and present the
mechanism through which both clusterheads and ordinary nodes are authenticated. We
present the simulation model in section 4 and show how the overheads vary with the
number of hops. Conclusions are drawn in the last section.
2 Securing Ad hoc Networks
Unlike wired networks where an adversary must gain physical access to the network
wires or pass through several lines of defense at firewalls and gateways, attacks on the
wireless network can come from all directions and target at any node. The use of wire-
less links renders the network vulnerable to attacks ranging from passive eavesdropping
to active interfering. Moreover, mobile nodes with inadequate physical protection are
easy to be captured, compromised and hijacked. Thus attacks by a compromised node
from within the network are much damaging and hard to detect. In ad hoc networks,
the network algorithms rely on the cooperative participation of all nodes and the in-
frastructure. The lack of centralized authority means that the adversaries can exploit
this vulnerability for new types of attacks designed to break the cooperative algorithms.
The mobility of nodes on the other hand requires sophisticated routing protocols. Thus
security is an additional problem as it is hard to identify incorrect routing information
generated by compromised nodes or as a result of some topology changes. With slow
link, limited bandwidth, battery power constraints, nodes are inclined to adopt mech-
anisms like disconnected operation and location-dependent operations. In a nutshell,
wireless ad hoc networks will not have a clear line of defense and every node must be
prepared to encounter any adversary directly or indirectly.
2.1 Types of Attacks on Ad hoc Networks
Attacks on ad hoc networks usually try to disrupt the routing protocols. Such attacks
fall in two categories: routing disruption and resource consumption [3]. In a routing
disruption attack, the attacker attempts to cause legitimate data packets to be routed in
a dysfunctional way. In a resource consumption attack, the attacker injects packets into
the network and attempts to consume valuable network resources as memory storage,
battery life and computation power. Both attacks are instances of denial-of-service at-
tack. An example of routing disruption attack will be when an attacker sends forged
routing packets to create a routing loop, causing packets to traverse nodes in a cy-
cle without reaching their destinations, consuming energy and the available bandwidth
thereby clogging the network. An attacker may also create a black hole, in which all
packets will be dropped. Therefore by creating a false route into the black hole all the
packets will be discarded by the attacker thereby depleting the network resources. The
attacker may also attempt to use detours or partition the network thereby preventing a
particular set of nodes to be part of a route. The attacker may also include itself into the
route and along with that add some other nodes in the route that otherwise would not
have been necessary and thereby using the resources more than required. In case of re-
source consumption attacks, the attacker tries to deplete the resources of the network. It
tries to inject extra packets into the network, which may consume even more bandwidth
or computational resources as other nodes process and forward such packets.
2.2 General Techniques for Authentication and Secret Sharing
In securing a network, goals like authenticity, integrity, confidentiality, non-repudiation
and availability are most important. Authentication of communicating entities is of par-
ticular importance as it forms the basis for achieving the other security goals: e.g.,
encryption is not worthwhile if the communicating partners have not verified their iden-
tities before. The reason being that if the communicating nodes are not authenticated
then a malicious node might join the network and participate in communication thereby
making the use of encryption futile. Authentication of entities and messages can be re-
alized in different ways using either symmetric (3DES, AES) or asymmetric (ElGamal,
RSA) [11] cryptographic algorithms. Symmetric algorithms depend on the existence of
a pre-shared key (which does not exist in the general case). Authentication by asym-
metric cryptography requires a secure mapping of public keys to the owner’s identities
which is often accomplished by by public key infrastructures (PKI). PKI’s use digitally
signed certificates to verify a key owner’s identity. Each user has to prove his identity
to a certification authority (CA) and after the authority has authenticated the user it pro-
vides him with a public key with which it can then communicate with other users who
in turn will have to go through the same process.
In contrast to fixed networks, a centralized PKI or even a centralized certification
authority is not feasible in ad hoc networks, due to the lack of infrastructure in these
kind of networks. Distributing the signing key and the functionality of a CA over a
number of different nodes by the means of secret sharing and threshold cryptography is
a possible solution to this problem, Secret sharing schemes realize confidentiality of a
cryptographic secret by spreading it across different entities. As secret sharing schemes
need no central authorities, they are predestined for ad hoc networks. One secret sharing
scheme is threshold cryptography: A trusted dealer divides a secret into n parts so that
the knowledge of k parts (k n) allows the reconstruction of the secret, which is not
possible with the knowledge of k 1 or fewer parts. This is called a (k, n) threshold
scheme [10]. In general, a trusted dealer is a central authority and thus another central
target for attacks. To avoid this, the participants have to construct the secret without
any central authority. The construction algorithm has to ensure that participants can
only transmit correct values and that each participant can verify both secret and shares,
which is called verifiable secret sharing [9]. Due to the movement of mobile nodes,
the topology of ad hoc networks changes frequently, and moreover, nodes can join or
leave the network at any time. Hence, an algorithm for distributing the same key to a
different set of participants is required. Such a refresh algorithm [5] can be triggered
periodically, event-based, or both. Another form of symmetric cryptographic algorithm
is TESLA [3] where the authors have authenticated the nodes using time. When using
this protocol each node will be knowing the time it takes to send a message to any other
intermediate node in the route. A time-stamped value will then be computed using a
Hash Function known to all the nodes and then after each passing interval these values
will be made public to all the nodes.
3 Proposed Clusterhead based Authentication Architecture
Clusterheads are certain nodes in the network that are selected to do some additional
jobs. They are responsible for the formation of clusters each consisting of a number
of nodes and maintenance of the topology of the network. A clusterhead does the re-
source allocation to all the nodes belonging to its cluster. Due to the dynamic nature
of the mobile nodes, their association and dissociation to and from clusters perturb the
stability of the network and thus reconfiguration of clusterheads is unavoidable. This is
an important issue since frequent clusterhead changes adversely affect the performance
of other protocols such as scheduling, routing and resource allocation that rely on it.
Choosing clusterheads optimally is an NP-hard problem [1]. However, there are a num-
ber of heuristics for choosing clusterheads depending on the objective that the network
wants to achieve.
Previously work has been done in this field where clusterheads have been used as
certifying agencies [2]. In this paper, we do not try to propose a new clustering algo-
rithm. Instead, we assume that there is a clustering algorithm present which judiciously
selects some of the nodes as clusterheads. It can be noted that the clusterheads will
solely be used for certification and not for any other purpose like routing or resource al-
location. Even after the clusterheads are identified, the topology remains flat as opposed
to hierarchical nature in usual clusterhead based networks.
3.1 Network Architecture
We consider an ad hoc network consisting of nodes with equal transmission range.
When the network is initialized only trusted nodes are present. Of course, there would
be malicious nodes during the lifetime of the network. Since, our main focus is secure
communication, we do not investigate routing protocols and assume that some routing
algorithm is present which provides a secure route. Though we use the concept of clus-
terheads, they do not help in routing as such and there is also no provision for gateways
to allow inter-cluster communication. Two nodes although being in different clusters
can still communicate with each other if they are within the transmission range. How-
ever there has to be a mutual agreement between the two nodes to exchange certificates
through the clusterheads which maintain the public keys of all the nodes in its cluster.
Every clusterhead will also have information of secret keys with which it can commu-
nicate with all other neighboring clusterheads.
Caching: In a session between a source-destination pair, if it so happens that the pair
belongs to different clusters then after the key exchange is done between the cluster-
heads, it will store the information about the public keys of the node that lies in the
other cluster in its cache. This caching feature will help the clusterheads in reducing
the message overhead if in future there exists a route that involves the node from the
other cluster. However this information will not be kept in the cache forever, but will be
cleared automatically after a given time period.
Clustering: This process involves partitioning the network to identify certain nodes
which will become clusterheads and act as the certificate authorities. Each node has the
information about its neighboring nodes with the help of beacon signals. We chose the
simplest of clustering algorithms to partition the network into clusters since our prime
concern is not clustering efficiently but demonstrate the use of clusterheads as adminis-
trators that can help in securing the network efficiently. We assume the highest degree
[8] clustering algorithm in which the node with the highest degree is chosen to be the
clusterhead. Once a clusterhead is found, all its neighbors join the cluster. The cluster-
ing algorithm is invoked till all the nodes have a clusterhead to attach to or itself is a
clusterhead. This of course does not guarantee a connected network. Figure 2 shows a
few nodes that are isolated.
Inheriting Authority: The clusterheads having been elected are then given the infor-
mation with which they can create certificates and are also given the information of the
public keys of all the other nodes in the network and the private key of the certificate
authority itself. Moreover the clusterheads are given pairs of shared secret keys with
which they can communicate with neighboring clusterheads securely. Each clusterhead
will then assign a pair of public and private keys to all other nodes of the network at the
time of formation of the network. The nodes also have the capability to generate new
public-private key pairs upon approval from the certificate authority.
C = E {Time , ID , KU }
C = E {Time , ID , KU }
B 2 bB
Fig.1. Protocol followed in key exchange
3.2 Authentication Mechanism
Idea to use a distributed certification authority based on a shared certification key and
threshold cryptography for securing ad hoc networks was first proposed by Zhou and
Haas [12]. It was further developed in the COCA system [13], where a general dis-
tributed authentication service was proposed. Recently similar work was been done
by Bechler et. al.[2]. Our approach is based on the same underlying principles, but
introduces several new concepts like a cluster-based network structure where the clus-
terheads are only responsible for the authentication mechanism. Clusterheads would
authenticate new nodes entering the network and are also able to discharge their re-
sponsibilities to newly formed clusterheads. However such a situation will arise only
when the network topology changes due to the mobile nature of the nodes. Let us now
discuss the securing scheme.
The approach of certificate authorities by Kohnfelder [11] suggested the use of cer-
tificates that can be used by nodes to exchange keys without contacting a public-key
authority. Each certificate containing a public key and other information is created by
the certificate authority and is given to the node with matching private keys. A node will
convey its key information to the other nodes by transmitting the certificate. The other
nodes can then verify that the certificate was actually created by the certificate author-
ity and not by any other malicious node. There are few requirements for this scheme to
work, which are given below.
Any node can read a certificate to determine the identity and public key of the certifi-
cate’s owner.
Any node can verify whether the certificate originated from the certificate authority.
Only the certificate authority can update the certificates.
The certificate authority can also transfer its duties to any other node once it has
authenticated that the node is a true node.
Any node can also verify the currency of the certificate implying if the certificate is
of the latest version or not.
The general scheme [11] by which a certificate authority (CA) secures a system is
shown in figure 1. For node A, the certificate authority provides a certificate of the form
= E
[T, ID
, KU
where E
is the encryption algorithm of the certificate authority using its private
key, once node A has made a request for the certificate. T is the timestamp that validates
the currency of the certificate. ID
and KU
are the node-ID and the public key of
node A respectively. Once node A receives the certificate C
, it passes the certificate
to the node with which it wants to communicate (say B). On receiving C
, B will read
and verify the certificate as follows.
] = D
[T, ID
, KU
= (T, ID
, KU
The recipient node B will use the public key of the certificate authority KU
with the decryption algorithm D to decrypt the certificate. Because the certificate is
readable only using the certificate authority’s public key, it confirms that the certificate
came from the certificate authority and not any other malicious node. The parameters
and KU
provide the node B with the name and public key of the certificate’s
holder, which is node A.
The timestamp T validates the currency of the certificate and also secures a com-
munication even if a node As private key is known by an opponent. A generates a
new private-public key pair and applies to the certificate authority for a new certifi-
cate. Meanwhile, the opponent will replay the old certificate to B. If B then encrypts
messages using the compromised old public key, then the opponent will read those mes-
sages. With the use of the timestamp this kind of a situation will be taken care of as the
timestamp will be indicating the currency of the certificates. Thus we see that finally
at the end of the transmission the node B knows the public key of A without the other
nodes in the network knowing. In a similar fashion node B will also communicate with
the certificate authority and the same sequence will follow by which node A will know
the public key of node B. Once this process is complete, both nodes can communicate
either way by encrypting the messages with their public keys and decrypting with the
private keys.
Clusterheads (CA)
Ordinary nodes
Fig.2. Proposed clusterhead based architecture
3.3 Securing the Message Communication
In our architecture once the network has been formed and a route has been discovered
the authentication mechanism is initiated. From the route information, the clusterhead
identity for all the nodes participating in the route is obtained. Our main aim is to
secure the route on a hop by hop basis. The source first communicates with its next hop
neighbor in the route and then goes through the entire security protocol as explained
in section 3.2. For example, the source will first communicate with its clusterhead and
then request for a certificate for communication as
= E
[node ID].
Within this request, the node will also send the node-ID of the node with which it wants
to communicate. The source encrypts this message first with the public key of the cer-
tificate authority and then with its private key. The certificate authority will decrypt the
message first with the public key of the source and then with its private key as
] = D
[node ID]
= node ID
In this way the certificate authority can verify that the message actually came from
the source and not from any malicious node. Once the certificate authority knows the
node-ID of the node with which the source wants to communicate it scans through
its cluster to see if that node belongs to that cluster. Two scenarios might arise; the
node might be in the same cluster or in a different one. If it is in the same cluster the
problem becomes simple. However, if the node is not in the same cluster the problem
becomes complex as the certificate authority (say CA1) will have to find and exchange
information with the new clusterhead (say CA2) to which the other node belongs.
Let us now explain the communication process. As shown in figure 2 we observe
that there are both intra-cluster and inter-cluster routes. The source belongs to cluster-
head A whereas the destination is in cluster D. The route segments 3 and 7 demonstrate
inter-cluster routing. In case of segment 3 there will be a mutual handshaking between
clusterheads A and C, whereas for segment 7 there is a handshake between cluster-
heads C and D. This procedure of handshaking between two different clusterheads
starts with the clusterhead asking the source about the affiliation of the node in the
next hop. Once the clusterhead CA1 gets that information it will send a query with a
timestamp encrypted with its secret key to the new clusterhead via the source and its
next hop neighbor. The message is as follows.
= K
[N1, node ID, T 1]
The message will essentially ask if the next hop neighbor actually belongs to that clus-
ter. The message consists of the node-ID of the next hop neighbor, a timestamp T 1, and
a nonce N 1. The new clusterhead (CA2) after decrypting the message with the shared
secret key will authenticate that the message essentially came from another cluster-
head and not from any other malicious node. Once the query is verified, the clusterhead
searches its own cluster for the target node. If found, the clusterhead sends the public
key of that node. The message is encrypted with the shared secret key between the two
clusterheads and the message is as follows.
= K
[N1, N2, T 2, KU
where, N 1 is the nonce generated by CA1, N 2 is the nonce generated by CA2. T 2
is another timestamp and KU
is the public key of the target node. Once CA1
receives this message it will re-confirm to CA2 by sending another message including
the nonce generated by CA2 and will also send the public key of the source to CA2.
The message that CA1 sends will be as follows.
= K
[N2, T 3, KU
Thus CA2 knows the public key of the source.
Our architecture also proposes that once a clusterhead knows a public key of any
other node that does not belong to its cluster, then that key is stored for use in future
sessions. In this way after a period of time, the stored information about the public key
will bring down the message overhead. Once this handshaking procedure is done the
respective clusterheads will provide an encrypted certificate having the public keys of
the other node with which the respective node of the cluster wants to communicate as
explained in the protocol in section 3.2.
The clusterheads will essentially send a certificate to the node in its cluster, for
example in our case clusterhead A will send a certificate to the source as shown in
figure 2 as below
= E
[T 1, KU
Once the source receives the public key of the next-hop-neighbor with which it
has to communicate, it first decrypts the certificate with its private key and obtains the
public key of the next-hop-neighbor as shown below.
M = D
] = [T 1, KU
The next-hop-neighbor on the other hand will go through the same procedure and
receive a certificate bearing information about the public key of the source from its
Once the nodes receive each other’s public keys they start the message communica-
tion encrypted with the public key of the other node to which the message is sent and
the receiving node decrypts it with its private key as that is known to the nodes at the
time of setup of the network. This procedure propagates throughout the route until the
destination is reached.
4 Simulation Model and Results
We simulated the proposed clusterhead based authentication mechanism on a UNIX
based environment. Nodes were randomly scattered on a square grid of 100 × 100.
Clustering was performed and some routes were obtained based on some random source-
destination pairs. We implemented the DSR[6] routing algorithm to find a source des-
tination pair. Once the route between the source-destination pair is obtained the next
task is to ensure a secure message transmission between the source and destination.
For routes of various length, we calculated the message overhead. For the same route
length, the actual message overhead will depend on the distribution of the nodes along
the route, i.e., how many nodes belong to the same cluster and how many do not. More-
over there is a dependence of message overhead on time as the caching feature of the
clusterheads come into play as with time, the key informations of more and more neigh-
boring nodes will be cached.
4.1 Results
We base our results on the message overhead against the number of hops in a route
and show the variation in figure 3 and figure 4. The message overhead is the additional
bytes of information sent by either the clusterhead to nodes or vice versa in exchange
of certificates as well as the exchange of the encrypted data between the nodes. This
0 10 20 30 40 50 60 70 80 90 100
Message Overhead
5 hops
10 hops
Fig.3. Message overhead
2 4 6 8 10 12 14
Number of hops
Message Overhead
Fig.4. Message overhead
overhead will be more in case of inter-cluster message transmission as this would in-
volve clusterheads communicating amongst themselves for mutual authentication and
also for transferring the public keys. Figure 3 gives the overhead against a hop length
of 5 and 10 as time evolves. For both cases, there is a minimum overhead. However, at
times, the overhead spikes. This is because of mobility and re-authentication of nodes
in the route. Also, the fluctuation is due to the distribution of the nodes- within same
clusters or different ones.
We also observe that as time evolves there are few aberrations of the message over-
head and it tends to be at the minimum. This is because of the cache of the clusterheads
which slowly gathers information about the public keys of various nodes and become
richer in information content. The gathered information avoids extra overhead in inter-
cluster communication. It is obvious that the minimum message overhead for routes of
length 10 would be more than that of routes of length 5.
In figure 4, we observe that when the number of hops increases the message over-
head is almost linear. The reason being that with time the overhead against the number
of hops averages out as the caching reduces the overhead. So even if there are sudden
increases in the overhead, the average is considerably lower.
5 Conclusion
With ad hoc networks becoming increasing popular, securing communication in such
networks is gaining importance. Securing ad hoc networks is more challenging because
of the absence of an central authority and the distributiveness of the nodes. In this paper,
we propose a clusterhead based distributed authentication mechanism. The clusterheads
execute administrative functions and act as certificate agencies and distribute certificates
to the communicating nodes, thereby making the communication secure. Simulation
experiments were conducted to study the message overhead that would be incurred due
to the process of authentication. The use of cache to store public keys has shown to
better the performance in the long run.
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