A Top-down Hierarchical Multi-hop Secure Routing Protocol for Wireless Sensor Networks

International Journal of Ad hoc, Sensor & Ubiquitous Computing( IJASUC ) Vol.1, No.2, June 2010.
10.5121/ijasuc.2010.1204 33
A Top-down Hierarchical Multi-hop Secure
Routing Protocol for Wireless Sensor Networks
M. P. Singh1
and Md. Zair Hussain2
1
Department of Computer Science and Engineering, NIT Patna
writetomps@gmail.com
2
Department of Computer Science and Engineering, MACET, Patna
mdzairhussain@gmail.com
ABSTRACT
This paper proposes a new top-down hierarchical, multi-hop, secure routing protocol for the wireless
sensor network, which is resilient to report fabrication attack. The report fabrication attack tries to
generate bogus reports by compromising the sensor nodes to mislead the environment monitoring
application executed by randomly deployed wireless sensor nodes. The proposed protocol relies on
symmetric key mechanism which is appropriate for random deployment of wireless sensor nodes. In the
proposed protocol, base station initiates the synthesis of secure hierarchical topology using top down
approach. The enquiry phase of the protocol provides assurance for the participation of all the cluster
heads in secure hierarchical topology formation. Further, this methodology takes care of failure of head
node or member node of a cluster. This protocol ensures confidentiality, integrity, and authenticity of the
final report of the monitoring application. The simulation results demonstrate the scalability of the
proposed protocol.
KEYWORDS
Algorithms, Design, Experimentation, Security, Cluster based wireless sensor network, Report fabrication
attack, Secure routing protocol
1. INTRODUCTION
The advancement in hardware technology (micro-electro-mechanical systems (MEMS)
technology & nano-electro-mechanical systems (NEMS) technology), and wireless
communications, has enabled the development of new category of computing devices which are
known as sensor nodes [4]. These devices integrate computation, communication and sensing
components. Aggregation of these sensor nodes into communication infrastructure leverages the
idea of wireless sensor network (WSN). WSNs, a specific kind of ad hoc network of resource
constrained sensor nodes, have enabled wide range of applications. These applications which
make use of several sensor nodes, are not suitably applicable to computing nodes on traditional
wired networks, wireless networks, and ad-hoc networks. The examples of such applications are
military surveillance, disaster detection & relief, space exploration, environmental monitoring,
habitat monitoring, acoustic detection, seismic detection, inventory tracking, medical
monitoring, smart spaces, process monitoring, structural health monitoring etc. [10, 13, 15, 23,
24, 26, 31, 33, 44, 45, 47, 51]. Some of these applications like monitoring of nuclear plants, sea
beds, remote locations in mountain ranges or deserts, volcanic eruptions etc. involve physically
challenging environments which are not conducive for human life. These monitoring
applications can be used to provide better services to humanity in terms of improved climate
control, security, and safety.
Sensor nodes which are being used in above mentioned applications are constrained in
computational speed, communication range, and storage capacity. This handicap or limitation of
sensor nodes makes the design of wireless sensor network a challenging task. The

34
aforementioned constraints of sensor nodes make the implementation of security mechanisms in
the wireless sensor network a challenging research issue.
The main concern of all possible applications of WSN is to fetch the data sensed/read by the
each node and transfer to the end user without losing data integrity, freshness etc. The
application with security requires more computation and/or communication in comparison to
other applications. Hence, it is important to design application which may provide the accurate
data from WSN against all odds, happens in WSN. The resource constrained nature of sensor
nodes poses the unique challenges to the design of WSNs for their applications [30, 37]. The
limited power of sensor nodes mandates the design of energy-efficient communication protocol
with or without security. According to [37], 3000 instructions could be executed at the same
energy cost as that of sending a bit for a distance of 100 meters using a radio. In another
observation [30], the power consumed by a Berkeley mote to transmit 1 bit data is equivalent to
the computation of 800 instructions. From the above two observations, it is noted that
communication cost is an important factor. This communication cost can be reduced
significantly by using cluster based communication as compared to one without clustering.
These are being taken care of in the proposed protocol in section 3.
”Some of the applications of wireless sensor networks like military surveillance, disaster
detection and relief, space exploration etc. require a secure communication channel for data
transfer from the sensor nodes to the base station. Without security mechanisms, such
applications may result in undesirable consequences.” The following examples are influenced
from [25]. In the military surveillance application, there are several attacks like denial of service
attack, eavesdropping attack (leaking of information to enemy), supply of misleading
information attack (there is no enemy movement) etc. In the disaster detection and relief
application, an adversary may send false information like bogus disaster warning causing huge
financial loss as a result of larger scale evacuation, and deployment of disaster equipments. In
the space exploration application, it is required that all the commands executed on space are
authenticated, and all the collected data are encrypted, and authenticated. Otherwise this may
cause a huge monitory loss as well as human life. The requirements of such applications
necessitate the security as one of the major concerns of wireless sensor network design. The
security attacks may be defended using strong cryptographic mechanisms [4, 14, 41]. But, the
cryptographic mechanisms developed for traditional wired network, traditional wireless network
and ad-hoc network may not be directly employable for the wireless sensor network due to
various reasons enumerated next:
1. The sensor nodes have scarce resources [5]. The implementation of cryptography algorithm
within a sensor node consumes extra memory, & processing time, which is certainly
overload condition. This overload consumes extra battery power. Thus security algorithm
for the wireless sensor network has to be developed in such a way that it consumes minimal
amount of battery power, and occupies minimal additional memory space of sensor node as
to allow seamless operation of the applications augmented with security.
2. Public key algorithms are not suitable for WSN because of computational complexity and
larger code size associated with such algorithms.
3. In most of the application, it is assumed that base station is the trusted entity that is base
station will not be compromised. It is also assumed that the base station is resource full
scalability is not a bottleneck for the WSN [41].
4. In-network aggregation in WSN reduces the communication overhead, and hence minimizes
the battery consumption. In-network aggregation is an important requirement for secured
the wireless sensor networks too [25].
5. Sensor nodes are often deployed in a hostile environment where the sensor nodes are open
to the physical attack. It is also one of the factors that require being paid attention in
designing the security mechanisms.
Because the above mentioned limitations/requirements, a new feasible security mechanism
needs to be developed to be used by the sensor nodes in a WSN.

35
The intruder may mount an attack by eavesdropping of wireless channel. So it is required to
secure the environment. Otherwise, intruder may mount various attacks like insider attack,
outsider attack. In addition, it might also be necessary to provide security mechanisms in order
to provide data confidentiality, data integrity, data authenticity, data freshness, and availability.
Each of these is explained in brief next.
Data Confidentiality: In the wireless sensor networks, the data confidentiality is related to the
following observations [9, 36].
—Sensed data should not be leaked from the sensor node of a wireless sensor network to the
adversary.
—Before sending the highly sensitive data like cryptography keys, establishment of a secure
channel is necessary.
Data Integrity: Data integrity ensures the originality of data. Adversary and/or harsh
communication environment hamper the data integrity.
Data Authenticity: In two party communications, a receiver has to ensure the sender identity
using data authenticity approach.
Data Freshness: Data freshness ensures that data is recent. This is required to defend against
the replayed attack. That is it ensures that no old data have been replayed. The data
confidentiality, and the data integrity do not ensure the data freshness.
Availability: Denial of service (DoS) attack does not allow using the network service. Hence, it
is required to make sure for availability of network service despite of denial of service attack.
Two broad categories of security attacks, namely outsider attack and insider attack [41] in the
wireless sensor networks are discussed in the following.
Outsider Attack [4, 48]: The outsider attack is the one in which the adversaries try to attack the
wireless sensor network without having the knowledge about the network. The security attacks
such as eavesdropping the communication channel (where the adversary obtains the sensed
values by hearing the communication channel), injecting malicious messages into the wireless
sensor network, and denial of service attack (where the adversary jams the communication
channel using signals of high strength) are the few examples of outsider attack.
Insider Attack [4, 19, 28, 32]: When the adversary compromises a sensor node, it gets hold of
the information stored in the sensor node such as the cryptographic keys, location of the sensor
node etc. Such information helps the adversary to launch the insider attacks in the wireless
sensor network. The security attacks such as report fabrication attack (RFA), node replication,
generation of bogus reports, Sybil attack (impersonate multiple network entities by obtaining
their identities) are the few examples of insider attack.
So, there is need to develop a security protocol considering the resource constraints, security
challenges, and security requirements. The same has been done in the section 3.
The rest of the paper is organized in following sections. Section 2 summarizes the related work
of security protocols. This section also presents their limitations. Section 3 describes the
assumptions, notations, and the detail explanation of all the phases of proposed protocol.
Section 4 explains the handling of failures of either cluster members or cluster heads. Section 5
presents the security analysis of proposed protocol, like secure cluster setup, secure hierarchical
topology setup, event detection, and secure reporting. Section 6 presents the analysis of memory
overhead, computation overhead, communication overhead, and scalability. Section 7 presents
implementation details and simulation results of with and without security. This section also
shows the time consumption for the security implementation and finally, section 8 concludes the
work and also presents the future works.
2. Related Work
This section presents the related work on security protocols for wireless sensor networks.
Localized encryption and authentication protocol (LEAP) [55] is a key management protocol
for the wireless sensor network. It supports in networking processing, while at the same time

36
providing security properties. In the LEAP, every node creates a cluster key, and distributes this
key to its immediate neighboring nodes using pairwise keys that it shares with each of its
neighboring nodes. This scheme has some drawbacks as follows. (i) The bootstrapping phase is
quite expensive. (Bootstrapping phase establishes a secure communication channel among
sensor nodes. These nodes may have some preloaded keys but does not have any information
regarding the neighboring nodes.) (ii) The storage requirements on each sensor node are also
nontrivial since every node has to store a number of pairwise, and cluster keys. This number is
proportional to the number of actual neighboring nodes. (iii) This scheme performs well against
the outsider attack in comparison to the insider attack.
The many other key management schemes for wireless sensor networks have been proposed in
[8, 12, 20, 21, 27, 36, 39]. Security mechanism for denial of service attack in the wireless sensor
networks has been discussed in [48].
A study on the effects of hello flood attack, and Sybil attack on different routing protocols such
as directed diffusion, GEAR, LEACH, TEEN, and PEGASIS is presented in [29].
SAC: Secure Adaptive Clustering protocol in wireless sensor network is successful in
preventing attacks caused by adversary like hello flooding and provides resilience to sensor
nodes captured by adversary [38].
A security mechanism for the Sybil attack in the wireless sensor network is presented in [19,
32]. In this attack, a malicious node assumes the identity of a large number of the sensor nodes
of the network. It does this by impersonating other sensor nodes or by claiming the false
identities. An approach to defend against the Sybil attack is resource testing [19]. Resource
testing verifies that whether or not each identity has as much of tested resource as is expected
from the identity. This approach is not suitable in case of the wireless sensor network because of
resource limitations. For example, an attacker can deploy a physical device with capabilities that
are several orders magnitude larger than those of the normal sensor nodes. Other defenses such
as radio resource testing, verification of set for random key pre-distribution, registration, and
position verification have been proposed in [32].
A security protocol, An intrusion-tolerate routing protocol for wireless sensor networks
attempts to design a routing protocol that can tolerate intrusions rather than detecting the
intrusion [17]. This is done by choosing the redundant paths from any sensor node to the sink.
But this allows only the base station to broadcast information to protect the routing protocol
from attack.
In trust routing for location aware sensor networks (TRANS) [46], each sensor node calculates
trust value of its neighboring nodes. Based these calculated trust value; a secure path is being
setup by bypassing the insecure locations.
Security protocols for sensor networks (SPINS) [35] uses two low level secure building blocks,
namely SNEP, and µTESLA. SNEP provides data confidentiality, two-party data authentication,
data integrity, and data freshness between the sensor nodes & the sink. µTESLA provides
authentication for data broadcast. SPINS uses Rivest Cipher 5 (RC5), is a kind of block cipher,
[1] because of its small code size. SPINS assumes that only base station is the only point of
trust. It does not allow node to node communication directly. Node to node communication
necessitates authentication via the base station. Hence, SPINS may not be suitable for relatively
larger size of the wireless sensor network, especially hierarchical wireless sensor networks.
The three levels of security for three different types of data are considered in [43]. Each node
has a set of master keys initially, but one of them is active at a given time. All three security
levels derive keys from the master key. For the first level of security, the master key is used for
the messages, which are infrequent. For the second level of security the network is divided into
hexagonal cells. All the member nodes in a cell share a unique location based key. The nodes at

37
the border of the cell store the keys of the cell to which they belong to allow traffic to pass
through. This model requires that the sensor nodes be able to discover their exact location,
which allows them to organize into cells, and produce a location based key. This is expensive.
The third level of security uses a weaker encryption with a focus on computational overhead.
The authors assume that the sensor nodes are tamper proof. Thus, the set of the master keys, and
other preloaded information cannot be revealed by compromising the node.
An interleaved hop-by-hop authentication scheme for filtering of injected false data in sensor
networks tries to make the cluster based wireless sensor network which will be resilient to
report fabrication attack [55]. Report fabrication attack is the insider attack. In this attack, a
compromised node or a set of compromised nodes are used to generate forged report for a non
existing event. This forged report can either misguide the system or deplete the resources of the
sensor nodes in the wireless sensor network. The report fabrication attack is a severe security
attack in reactive sensor networks. This attack is one of the most recent security attacks which
have been paid less attention in the wireless sensor network security literature [4, 25]. This
protocol tries to detect, and filter out the false data packets either at or en route to the base
station. There are some drawbacks with this approach which are as follows:
1. It assumes that all the clusters contain at least n+1 nodes (including itself). But when
the sensor nodes are randomly deployed, the number of nodes varies from cluster to
cluster [18, 42]. So, this approach is not appropriate for the random deployment.
2. It does not handle the unavailability of the nodes either due to the physical damage of
the sensor nodes or failure of the sensor nodes [35].
3. If a single node fails to report, then the report generated by the cluster becomes invalid.
4. It uses localized encryption and authentication protocol (LEAP) [54] for neighborhood
discovery, and for establishing the shared key among neighboring nodes which itself is
prone to security attacks [18].
5. It is energy expensive as it requires fixed path to be maintained from the cluster head to
the base station.
The proposed approaches in [41, 55] focus mainly on the enroute filtering framework. In this,
the forged reports will be dropped enroute from the source to the base station. Further, the node
that has dropped the forged report, generates alarm message about the cluster that has generated
the forged report. In this approach, the base station trusts the node which has generated the
alarm message. But there is a possibility of generation of bogus alarm messages about the lower
associated clusters by a compromised node.
The strength of the approaches to defend against the report fabrication attacks mainly depends
on the followings:
1. The ability of a wireless sensor network to suppress the generation of the false reports.
2. The ability of the base station to differentiate between the forged report & legitimate
report, and take an action against the neighborhood that has generated the forged report.
3. The ability of the wireless sensor network to restrict the scope of attacks of the
compromised nodes to a particular cluster in which they reside.
Due to the above reasons, the enroute filtering does not perform well in the cluster based
wireless sensor networks.
The security, and privacy of data centric wireless sensor network have been proposed in [40]. In
[50], an attempt was made to provide security solution for false data injection attack in the
wireless sensor network.
In [49], an attempt was made to provide effective security solution for the flat wireless sensor
networks. It uses the location dependent approach, where the sensor nodes need to be deployed
at predefined location depends on the requirement of the application. However, the solution is
not optimal for the cluster based networks [34].

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The studies on these protocols helped in comprehensive analysis of the problem of development
of security mechanism as presented in section 3. Authors also note that it is better to incorporate
the security mechanism during the design phase of the system development rather than
implementing the security mechanism on top of a developed system. The latter approach fails in
many cases as detailed in [48]. In this proposed paper security issues are taken up along the
development of clustering mechanism and routing protocol.
3. The Proposed Secure Hierarchical Multi-hop Routing Protocol
This section describes the working of the proposed top-down, hierarchical, multihop, secure
routing protocol in detail. First, the threat model under consideration is presented. Second, this
section notes the assumptions about WSN and its components. Third, it enumerates the
notations used. At last, it presents the algorithm of each phase with detail explanation.
3.1 Threat Model
If a sensor node is compromised then: All the information stored in sensor node will be revealed
to the adversary. The adversary can create the clones of the sensor node and deploy them across
the sensor network. It may illegitimately take multiple identities.
The adversary may either eavesdrop the communication channel in the sensor network or replay
the old messages or inject malicious messages into the sensor network or physically destroy the
sensor node.
3.2 Assumptions
A1: Each sensor node has a unique ID.
A2: Broadcast message sent by the cluster head is received correctly within finite time by all of
its one hop neighboring nodes.
A3: Network topology is static during algorithm execution that is sensor nodes are not mobile.
A4: Packet broadcast by the base station is correctly received by some of the cluster heads.
Discussion: As the hierarchy formation is initiated by the base station, the deployment of the
base station is assumed flexible to justify this assumption otherwise it would not be possible to
synthesize the hierarchical topology in the wireless sensor network.
A5: The base station is static, resourceful, and trusted entity.
Discussion: The base station may be a laptop or a PC class device. Mobility of the base station
is not allowed. The base station need to keep the information about all the sensor nodes in order
to avoid the report fabrication attack. Hence, the base station assumed to be resourceful. The
base station is an interface between wireless sensor network and other networks. The base
station is physically protected. Hence, the base station is assumed to be trust worthy. If the base
station is not static, the hierarchy of clusters would change. And hence the cluster head has to
establishment a path to the base station each time before sending the data. Further, if the base
station does not have enough resource, then memory overhead, computation overhead, and
communication overhead will make it unsuitable for larger size of wireless sensor network. If
the base station is not trust worthy, then this can result in a single point failure of the proposed
security protocol. In such a case, securing the wireless sensor network is of no interest.
A6: All the sensor nodes in the wireless sensor network have same capabilities in computation,
and have equal communication range initially.
Discussion: The sensor nodes are homogeneous in hardware capability. Design of clustering
protocol would change, and hence routing protocol based on the clustering protocol will also
change.
A7: The wireless sensor network will not be attacked for the first ’t’ second [18, 34, 54] that is
during the secure cluster setup phase, and secure hierarchical topology discovery phase.
Discussion: Assumption of tamper resistance to design a security protocol for wireless sensor
network is not correct [6, 43]. The proposed protocol assumes that the adversary node requires
minimum time, Tcompromise to compromise a sensor node whereas Ttopology time is required to setup

39
clusters, and hierarchy, which is smaller than Tcompromise. That is the wireless sensor network will
not be attacked for the first Ttopology time same as [18, 34, 54]. This protocol believes that this is a
reasonable assumption for most of the wireless sensor networks, and adversaries same as [18,
34, 54]. Otherwise, it could be easy for adversary to compromise the deployed sensor nodes
and then capture the wireless sensor network. These assumptions are used along with threat
model in the secure routing protocol presented next.
3.3 Notations
cid: Cluster ID. Connected Cluster Head (CCH): Cluster head connected to partially or fully
formed hierarchical topology. mKE : Encryption using key Km. holdback: Holds randomly
generated value which is used for cluster head making decision. Ki,j : Shared key between nodes
i and j. levelstatus: Whether or not the cluster head is the part of hierarchical topology. MAC(k;
s): Message Authentication Code computation on message ‘s’ using key ‘k’. MN: Member nodes
of any cluster. RFT: Report Fabrication Threshold. status: Whether the node is cluster member
or cluster head.
3.4 Working of the Proposed Hierarchical Multi-hop Secure Routing Protocol
The proposed protocol can be divided into two phases namely, (1) Pre Deployment Phase, and
(2) Post Deployment Phase. Pre deployment phase takes place before the deployment of the
sensor nodes in the given target area. This phase initializes the parameters. These parameters are
useful for post deployment phase. In the pre deployment phase, each sensor node is preloaded
with keys, which are used for security mechanisms like encryption, and decryption. Post
deployment phase takes a secure hierarchical topology based on clusters. This phase also detects
events that occur in the given target area, and reports to the base station in secure. In nutshell,
the proposed protocol follows the steps as described below:
1. Before deployment of sensor nodes, each node initializes some keys in order to provide
security in WSN.
2. After deployment of sensor nodes, the proposed protocol divides WSN into non-
overlapping clusters (groups). Each cluster has one cluster head.
3. After that, the proposed protocol synthesizes the hierarchy of cluster heads.
4. After that, the proposed protocol send the report of any event, arises in any cluster, to
end user in hop-by-hop strategy that is cluster head to another cluster head towards the
end user. The report is being authenticated at each hop and integrity of the report is also
maintained during transmission.
5. All the communications done in all of the above steps, if any are secure
communications.
In next, the each phase is explained in detail.
Algorithm 1: Pre deployment Phase
1: Each sensor node initializes the following its own parameters
2: cluster ID, cid = null, status=null, levelstatus=false,
3: holdback value to some randomly generated number,
4: Nodeid (Ni), MasterKey (Km) NodeKey (Ki), ClusterKey (Ki
c
), RFT
Pre Deployment Phase: The pre deployment phase takes place before the sensor nodes are
deployed in the given target area. During this phase each sensor node 'i' is assigned with unique
identifier Ni ranging from 0 to n-1 (where n is the number of sensor nodes in the network). Each
node initializes its own cid (Cluster ID) to null value, randomly generated value into holdback,
status to null, and levelstatus to false. The parameter status is used for checking the status of the
sensor node. Sensor node plays its role either as a cluster head or as a cluster member. The
parameter levelstatus is used to check whether or not node is connected with hierarchical
topology. Each sensor node is preloaded with the following keys:

40
Master Key (Km): A key of 64 bits is shared among all the sensor nodes, and the base station.
This key is used during the secure cluster setup phase, and secure hierarchical topology setup
phase. In order to ensure the security, this key will be erased after hierarchical topology setup
phase.
Node Key (Ki): This is a secret key of 64 bits. Each sensor node shares this key with base
station. This key will be used for authentication purpose while reporting to the base station.
Cluster Key ( c
iK ): Each node 'i' is loaded with unique cluster key c
iK , and will be used by
the sensor nodes that will become cluster heads.
Report Fabrication Threshold (RFT) RFT is the parameter that determines the number of
nodes that have to endorse the occurrence of the event. It is determined based on the number of
nodes in the cluster as follows
m = ┌RFT * no. of nodes in the cluster ┐; if (no. of nodes)>2
m = 2; if (no. of nodes) ≤ 2……………………………………………………….(1)
The RFT value varies from 0.5 to 1.0 (which ensures that at least 50 percent of the cluster
members have to report the occurrence of event). If RFT is 1 then all the member nodes in the
cluster have to endorse the occurrence of the event. Depending on the security requirements of
the applications the RFT value can be set. Once all the sensor nodes are preloaded with the
required keys, the base station is informed about the sensor nodes, and key mappings. The
sensor nodes are deployed redundantly.
Post Deployment Phase: The post deployment phase is further divided into four phases
namely, (1) Secure cluster setup phase, (2) Secure hierarchical topology setup phase, (3) Secure
enquiry Phase, and (4) Event detection and secures reporting to base station. These phases are
presented in detail in Algorithm 2, 3, 4, and 5 given below.
Algorithm 2: Secure cluster setup phase
1: after every tc seconds, each sensor node decrements the holdback value by one
2: if (holdback == 0 && status == NULL) then
3: set status = cluster_head and cid = node id,
4: Initializes the packet.type=cluster_head_hello
5: broadcasts cluster_head_hello end if
6: On receiving the cluster_head_hello broadcast
7: if (status == NULL && holdback , 0 && packet.type== cluster_head_hello) then
8: cid = SendID, /*ID of node that has broadcast the cluster head hello*/
9: set status = cluster_member,
10: compute the shared key using hash function `h'
11: reply with cluster_hello_reply message to cluster head end if
12: cluster head on receiving cluster_hello_reply message
13: computes the shared key with cluster members using hash function `h'
Secure Cluster Setup Phase: This phase partitions the wireless sensor network into non-
overlapping groups, known as clusters. During this phase, only those sensor nodes with
holdback = 0, declare cluster heads themselves, and start broadcasting the message
cluster_head_hello to form the cluster.
mK
c
iid EKChelloheadclusterhelloheadcluster },,__{:__ ………(2)
where Cid is node id of the node broadcasting cluster_head_hello. Upon receiving the message,
the sensor node decrypts the message, and decides to become cluster member, if it is not a part
of any other cluster, and inform to the cluster head about its membership by sending the packet
of type cluster_hello_reply. They respond by
− Computing the shared key with the cluster head using secure hash function 'h '

41
)||,(,
jKCKhK mid
c
iC jid
= ………………………….(3)
− Acknowledging the cluster_head_hello by sending cluster_hello_reply, so that cluster
head knows the members of its cluster, and computes the shared key as in Equation 3.
mjid KidCid ECjKMACCreplyhelloclusterreplyhellocluster )}|,(,,__{:__ ,
(4)
After fixed time tc, every node decreases its holdback value by one. By the end of this phase, the
wireless sensor network is divided into clusters. The member nodes are at one hop distance from
their cluster heads. Each cluster member computes a shared key with the cluster head. Cluster
head also does the same, and computes the ‘m' value (which determines the number of nodes
within the cluster that have to endorse the occurrence of a particular event).
Algorithm3: Secure hierarchical topology setup phase
1: Base station initiates this phase /*Top-Down Approach*/
2: Base station initializes the following parameters
3: Packet type= base_hello; levelstatus=TRUE; Level = 0,
4: Now, base station initiates the hierarchical topology setup phase by broadcasting
the base_hello message
5: cluster head(s) on receiving first base_hello message
6: if (packet.type== base_hello && status == cluster head && levelstatus==FALSE)
then
7: Set its own levelstatus=TRUE,
8: Increase the level count by one,
9: Remember the its parent address,
10: Initialize the packet.type=base_forward_hello
11: compute the shared key using hash function `h' and broadcast the
base_forward_hello packet to form the next level
12: send the base_reply end if
13: on receiving the base_reply, call Algorithm 6
14: cluster head(s) on receiving first base_forward_hello message
15: if (packet.type==base_forward_hello && status == cluster_head &&
levelstatus==FALSE) then
16: Enqueue the packets /*May receive packets from more than one cluster heads*/
18: Increase the level count by one & maintain the list of parents,
19: compute the shared key using hash function `h' and broadcast the
base_forward_hello packet to form the next level
20: send the base_reply end if
21: on receiving the base base_reply Algorithm 6
Secure hierarchical topology setup phase: Base station starts this phase by broadcasting the
packet of type base_hello. This finds out the cluster heads for the level 1 in hierarchical
topology formation. Base station is at level 0. Upon receiving the first base_hello message each
cluster head responds to it by
− Computing the shared key with the base station or the cluster head from which it has
received the base_hello as in Equation 3.
mKid EcounthopChellobasehellobase }_,,_{:_ …………………..(5)
− Sending the base_reply to the sender of the base_hello. Base_reply acts as an
acknowledgment to the base_hello. It also contains the payload holding details of the
cluster head, and the cluster members destined to the base station.

42
mjid
idCid
KidCid
id
KC
EyCjKMACCreplybasey
yCreplyforwardbasey
EyKMACyy
membersclustertheofsidyreplybase
}),|,(,,_{
,,__{
)},(,{
}'{:_
23
}12
001
0
,
←
←
←
←
………(6)
base_reply consists of all the values calculated in y0, y1, and y2 that is y3 is nothing but the
base_reply.
− Forwarding the base_hello to the downstream cluster heads by incrementing the
hop_count.
On receiving the base_reply the cluster head responds by computing the shared key as in
Equation 3, and forwards payload base_forward_reply to the base station. On receiving the
base_reply the receiving cluster head responds by computing the shared key as in Equation 3. In
turn, the cluster heads at level 1, broadcast the packet of type base_forward_hello to find out the
cluster heads for the level 2, and so on.
By end of this phase, wireless sensor network is organized as a connected graph and base station
learns about the secure topology of the wireless sensor network.
Algorithm 4: Secure enquiry phase
1: if (levelstatus=FALSE && status == cluster_head) then
2: the cluster head is not the part of hierarchical topology
3: Cluster head initiates this by sending scan_hello packet to its members, end if
4: on receive scan_hello
5: Members scans cluster heads which are already the part of hierarchical topology by
broadcasting the packet.type = scan,
6: on receiving scan
7: if (packet:type==scan && status==cluster_head && levelstatus==TRUE) then
8: compute the shared key using hash function ('h')
9: cluster heads reply by sending the packet of type=scan_reply, end if
10: Member nodes receive the packet and decide to join the hierarchical topology
11: if (packet.type== scan_reply && status==cluster_member && ReceiveID ==
NodeID) then
12: Enqueue the packets,
14: Increase the level count by one,
15: Maintain the list of parents,
16: compute the shared key using hash function ('h')
17: Forward the packet.type==scan_reply forward to its cluster head,
18: send scan_reply_ack towards base station end if
19: on receiving scan_reply_ack
20: if (receiving node of scan_reply_ack == cluster_head && levelstatus ==TRUE)
then
21: forward the scan_reply_ack towards to base station end if
22: After receiving the scan_reply forward packet
23: Enqueue the packets, Set its own levelstatus=TRUE, Increase the level count by
one, Maintain the list of parents,
24: send scan_reply_ack
32: on receiving scan_reply_ack
33: if (receiving node of scan_reply_ack == cluster_member && levelstatus ==

43
TRUE) then
34: forward the scan_reply_ack to its parents towards the base station end if
Secure Enquiry Phase: After the secure hierarchical topology setup phase, there may be some
cluster heads which are not the part of hierarchical topology because such cluster heads are not
at the directly reachable from any other cluster heads. But member nodes of such clusters
connect the one or more CCHs. This phase handles such case of the wireless sensor networks.
Such cluster heads start the enquiry phase by sending a packet scan_hello to its member nodes.
Cluster heads know about its cluster members during cluster setup phase, and calculated the
share key during the secure cluster setup phase.
mKid EChelloscanhellosacn },_{:_ ……………………(7)
where Cid is node id of the node broadcasting scan_hello.
Now, the members nodes broadcast the packet of type scan to find out the reachable CCHs.
mKid ECscansacn },{: …………………………………….(8)
Only those CCHs, who receive $scan$ packet, compute the shared key with member node from
which it has received the $scan$ packet as in Equation 3, and reply by sending the packet of
type scan_reply.
mKid EcounthopCreplyscanreplysacn }_,,_{:_ .........................(9)
After checking the scan_reply packet, the member nodes accept this packet, and become the
member of the next level of the hierarchical topology by computing the shared key with the
node from which it has received the scan_reply as in Equation 3. Further, it sends the
scan_reply_ack toward base station. Further, the same member node sends the packet of type
scan_reply_forward to its cluster heads. The cluster heads accept this packet only from its
members. Now, these cluster heads connect themselves to the next level of their member nodes,
and send the packet scan_reply_ack towards base station.
mjid
idCid
KidCid
KC
EyCjKMACCackreplyscany
EyKMACyy
xyackreplyscan
}),|,(,,__{
)},(,{
}{:__
12
001
0
,
←
←
←
where x is node id of member nodes (which connect their cluster head to the partially formed
topology) in case of line number 19 of Algorithm 4, and id's of the cluster members in case of
line number 24 of Algorithm 4.
After the phases, namely cluster setup phase, hierarchical topology setup phase, and enquiry
phase (optional phase), none of the deployed sensor nodes of wireless sensor network possesses
master key, Km that is all the sensor nodes erase the master key, Km. An adversary may have
eavesdropped on all the communications during the above phases. But without Km , the
adversary cannot encrypt the false report to send it towards base station or decrypt any of the
messages. Hence, eavesdropping does not help adversary. However, after Tcompromise the
adversary may compromise sensor nodes and obtains the keys available with the compromised
nodes. This results in localizing the security impact. The false information injected by the
compromised node will be detected during reporting to base station.
Algorithm 5: Event Detection and Secure Reporting to Base Station
1: if the cluster member detects an event then
2: it sends report to its own cluster head end if
3: if the cluster head receives at least `m' reports from its members then
4: it prepares the final_report and routes it to base station in hop by hop authenticated
fashion end if

44
Event Detection and Reporting to Base Station: This phase reports the any event, that occurs
in any cluster, to the base station in multi hop. If any event occurs, each sensor node ‘i' senses
the event, and informs to its cluster head as in Equation 10.
},,{
)},(,,(,{
}inf_{
12
0)01
0
,,
yCreporty
EyKMACyKMACyy
oeventy
id
KCoi iidCiid
←
←
←
……………………(10)
The cluster head receives at least 'm' data from its members and takes MAC's (MAC's have been
computed for the base station). Further, cluster head computes XOR of all the MAC's in order to
reduce the communication overhead that is XORing all the MAC's rather than sending
individually. Now, Cluster head prepares the report, and includes its own identity in the report
as in Equation 11. Finally, cluster head sends this report to base station in hop by hop
authenticated fashion.
The base station on receiving the report checks for the source (that is the cluster head from
which the report has originated), and validates the report by computing the MAC. If the report
fails the validation check, the base station discards the report, otherwise the base station initiates
appropriate action. In next, handling of sensor nodes failure is presented.
},,_{
)},(,{
.........,inf_{
12
001
}210
yCreportfinaly
EyKMACyy
MACMACMACoeventy
id
KC
m
idCid
←
←
⊕⊕⊕←
……………(11)
Algorithm 6: Algorithm for base reply
1: if receiving node of base_reply == base_station then
2: compute the shared key using `h'
3: learn about the cluster using base_forward_reply
4: else if receiving node of base_reply == cluster_head && levelstatus == TRUE then
5: compute the shared key using the information in base reply
6: forward the base_forward_reply part of it to next hop towards the base station end if
4. Handling Node Failures
The sensor nodes are often deployed in hostile area, and left unattended. There is great
probability of physical damage to the sensor nodes. There is a need to handle the sensor node
failures effectively as the failure of single sensor node may lead to disastrous consequences.
Cluster Member Failure: In order to handle the cluster member failure, this protocol assumes
that the cluster heads frequently ping their cluster members, and keeps a track of them. If a
cluster member fails to respond to its cluster head, then the cluster head notifies it to the base
station so that the base station recomputes the `m' value, and send it to the cluster head.
Cluster Head Failure: As the cluster members contain the information about the cluster heads
within the communication range (the cluster member gathers this information during the secure
cluster setup phase). If cluster head becomes unavailable, the cluster members may execute
orphan adoption protocol as in [34].
5. Security Analysis
Secure Cluster Setup phase, Secure Hierarchical Topology setup phase, and Secure
Enquiry Phase: The security of these three phases, namely secure cluster setup phase, secure
hierarchical topology setup phase, and secure enquiry phase, is based on the assumptions that
the sensor nodes will not be tampered for the first 't' seconds after their deployment in the given
target area. The sensor nodes will not behave non-deterministically until and unless they are
tampered. So, the above assumptions [18, 54] ensure that only legitimate senor nodes become

45
part of the wireless sensor network. In order to ensure confidentiality all the messages are
encrypted using master key (Km), which is shared by all the nodes of the wireless sensor
network. The MAC (Messages Authentication Code) which ensures authenticity, and integrity
of the message, is used where ever required (for multi-hop communication). This work also
assumes that all the sensor nodes share a random counter value with base station as well as
single hop neighboring nodes, as it offers semantic security [52], and as well as protects against
replaying of messages.
Event Detection and Secure Reporting: During the normal of the network, the effect of the
sensor node compromise depends upon the type of the sensor node compromised. The
compromised node either may be cluster head or may be cluster member. If the cluster members
are compromised, then the adversary tries to generate the bogus data, and send it to the cluster
head. But as the valid event must be endorsed by at least m sensor nodes, it is violated in such
case. So if the cluster head receives less than m data then it detects the sensor node compromise
within the cluster, and takes an action either by re-computing and distributing the new keys or
by issuing a refresh command so that all the sensor nodes hash the keys [18]. From the above
approach, the adversary will only be able to generate a forged data once it has compromised at
least m sensor nodes in the cluster.
If the cluster head has been compromised then the adversary can generate bogus reports, and
route to the base station. As the base station verifies the authenticity of each report, the bogus
report can be easily identified by the base station as the base station is aware of each cluster
heads and their member during the secure hierarchical setup phase. The base station has a
unique shared key for each cluster heads along with their member nodes. The master key, plays
in important role in calculating shared key, is erased after the secure hierarchical setup phase. In
the proposed security protocol, member nodes of any cluster compute MAC based on node key
(Ki). The individual keys of the each deployed sensor node are shared with the base station. At
least m number member nodes send their MAC to their cluster head. Further, the cluster heads
compress the entire MAC into one MAC by using XOR operation. This compression scheme is
secure as XOR-MAC scheme is secure [7]. The cluster heads send the compressed MAC along
with the report and its cluster key toward base station. Now, base station again computes
individual MACs and applies XOR operation on these MACs to get one compressed MAC. If
this calculated compressed MAC is same as the received MAC then base station accepts the
compressed MAC otherwise rejects. The base station may respond to the bogus report by
revoking the cluster from the wireless sensor network as in [16]. The malicious cluster head
may also report about the unavailability of the cluster members unnecessarily. So the base
station does not isolate the sensor node as soon as it receives such report. It handles such
situation by declaring the sensor nodes as ORPHAN by unicasting the orphan status to it. So, if
the cluster member is available it may join other cluster using the orphan adoption protocol [34].
6. Overhead Analysis and Performance Analysis
This section analyzes the memory overhead/ storage requirement, computational and
communication overheads of the security scheme used in the proposed security protocol. It also
analyzes the scalable nature of the proposed security protocol.
Memory Overhead/Storage Requirement: Here, memory overhead represent the memory
space required to store the number of keys preloaded, and the key generated during execution of
the proposed security protocol. However, the memory required to store master key (deleted after
hierarchical topology setup), and temporary memory used during execution of the proposed
protocol are not taken into consideration. In the proposed protocol, all the nodes are
homogeneous whereas plays two different roles, namely as a member node and as a cluster
head. The member nodes need to keep three different keys, namely one node key (Ki), one
cluster key ( c
iK ), and one pairwise key with their cluster heads (Kcid,i). The cluster heads need
to keep one node key (Ki), one cluster key ( c
iK ), pairwise keys with their member nodes

46
(Kcid,j), and one pairwise key with either base station or another cluster head (Kcid,k). Here, i =
1...n (n is total number of deployed sensor nodes); j = 1...m (m is total number of member nodes
of the clusters; k is the id of cluster head which is parent in hierarchical topology.
The proposed protocol uses a PC of processor of Intel(R) Pentium4 with speed of 3.2 GHz, and
1 GB RAM for execution. TOSSIM (TinyOS simulator) executes the code of the proposed
protocol for MICA mote, MICA2 mote, and MICA2Dot mote. Fig. 1(a) shows the consumption
of RAM of these mote. The memory size of RAM in all three mote is 4Kbyte [2]. Fig 1(a).
shows that the code of the proposed protocol consumes less size of RAM for all three types of
motes in comparison to 4Kbyte.
Computational Cost: The computation cost arises mainly due to establishing pairwise key and
report authentication. Both require a number of encryption, and decryption.
Assume the total number of cluster head is C in the wireless sensor network that is there are C
clusters in the wireless sensor network. These cluster heads establish pairwise keys with their
member nodes. All the member nodes also establish a pairwise key with their cluster heads. The
establishment of pairwise keys uses encryption and decryption. The total number of encryptions
(a)Memory used by the code of the proposed (b) Energy consumption during cluster
Protocol setup phase
(c) Energy consumption during Hierarchical (d) Energy consumption during
setup phase reporting to base station.
Fig. 1. Overhead analysis and performance analysis
and decryptions depends on total number of cluster heads and their member nodes. Assume, Nj

47
is the total number of member nodes, where j = 1...m. The total number of encryptions during
cluster setup phase is Ne = C + ∑=
+=
m
j
je NCCN
1
* . The total number of decrypts during
cluster setup phase is ∑=
=
m
j
jd NCN
1
**2 .The total number of encryptions during hierarchical
topology setup phase is ∑∑
−
= =
+=
1
1 1
*21
l
i
n
j
je LH , where l is the total number of level of
hierarchical, and L is the total number of cluster heads at particular level. The total number of
decryptions during hierarchical topology setup phase is ∑∑
−
= =
+=
1
1 1
1 *2
l
i
n
j
jd LLH . The total
number of encryptions, and decryptions in the WSN are ec HC + and dd HC + respectively.
Report Authentication: In the proposed security protocol, each member nodes, cluster heads,
and forwarding nodes (the node forwards the message received from cluster heads at higher
level in the hierarchical) computes two MACs for an event occurred in the given target area.
These two MACs use node key and pairwise key as a MAC key.
Communication overhead: The communication overhead is much higher than that of
computation overhead. This protocol uses the following consumption rates similar as [53]:
Mica2 mote consumes 10mA current when the node is in idle, and receiving state whereas it
consumes 13mA for transmitting. Based on the battery voltage, 3V and data rate, 19.2Kbps the
Mica mote consumes 16.25µJ/byte for transmission, 12.25µJ/byte for reception. This protocol
uses RC5 block cipher [22] for MAC and hash computation. Both computation take about 0.5ms
and consumes about 15 µJ. The default size of packet in TinyOS is 36 byte. The size of packet
with authentication is 37 byte, and the size of packet with encryption, and authentication is 41
byte [22]. Fig. 1(b) shows that the consumption of battery power of the cluster head during
cluster setup phase, which is the summation of consumption on encryption of cluster heads’
hello packet & then transmit the same of size 36 byte, on receiving the acknowledgment from
member nodes, and on hash function to compute pairwise key with their member nodes. The
total consumption of battery power of the cluster head gets change because of second, and third
terms of the summation, depends on total number of its member nodes. Fig. 1(b) show that the
consumption of the battery power of the cluster head increases by an approximately same
amount, 0.6mJ for each member node. Assume that the cluster of this cluster head has total
number of member nodes varying from 1 to 10. Each member node of the cluster consumes
battery power of an amount of 1.28mJ, which is summation of consumption on receiving packet
from cluster head (decrypt the message of size 36 byte), on hash function to compute pairwise
key (15µJ), and on encryption the acknowledge packet & then transmit the same of size 41 byte.
Assume that there is maximum six levels on any path from base station to last level. Fig. 1(c)
shows that the extra consumption of battery power of cluster heads on level 1 to level 5 during
hierarchical topology setup phase. The cluster heads at level 1 consumes more battery power
than level 2 to 5 and so on. The consumption of battery power is lowest at the level 5 in
comparison to higher level (level 4 to level 1).
Fig. 1(d) depicts the battery power consumption of a member node & its cluster head, and
forwarding node during report preparation and forwarding the same towards base station. The
each member nodes of the cluster consumes battery power of a amount of 688.75µJ (.07mJ) to
prepare a information of the event occurs in the cluster, and transmit the same to the its cluster
head. The cluster head consumes battery power of an amount of 1.10mJ (in case of 5 member
nodes) to prepare a final report and transmit to its parent (parent is the cluster head at lower
level). The forwarding nodes consumes battery power of an amount of 1.32mJ per report, which
is summation of battery power consumption on receiving & decryption of a report of size 41
byte, and on encryption and transmitting the same of size 41 byte. The forwarding node is the

48
cluster head which receives a report from lower level. The cluster heads at the highest level,
and base station of the wireless sensor network are not a forwarding node.
Scalability: Each member nodes receive one cluster message to setup a pairwise key. Member
nodes need to keep only one node key, cluster key, and one pairwise key with their cluster
heads. Hence, memory overhead, computation overhead, and computation overhead of cluster
member nodes remains same as the number of sensor nodes increases. The memory overhead of
cluster heads depends on size of cluster, and total number of their member nodes. The total
number of member nodes in a cluster will be approximately same as number of sensor nodes
increases but the density of sensor nodes per unit area remains same. Hence, memory overhead,
computation overhead, and computation overhead of cluster heads remains approximately same
as the number of sensor nodes increases. In the proposed security protocol, the base station need
to keep node key of all the deployed sensor nodes, information of cluster heads, and their
member nodes. Hence, memory overhead, computation overhead, and computation overhead of
base station increases as the number of sensor nodes increases. This is not a bottleneck in the
proposed security protocol as the base station is supposed to be resourceful.
7. Implementation Details and Simulation Results
The proposed secure hierarchical multi-hop routing protocol is implemented in NesC
programming language with the underlying operating system TinyOS and simulation is done by
using TOSSIM simulator. TOSSIM is a discrete event simulator for TinyOS. TinyOS is
operating system for sensor nodes. Instead of compiling a TinyOS application for a sensor node
(mote), users can compile it into TOSSIM, which runs on a personal computer. TOSSIM builds
directly from TinyOS code. This allows users to debug, test, and analyze algorithms in a
controlled environment. Fig. 2(a) shows the relation between the components of the proposed
protocol.
Boot
Timer
Report RawEvent
Comm.
Event
MsgAuthCode
(a) Components of the proposed protocol (b) Configuration file in TOSSIM
Fig. 2 Components and Configuration file of the proposed protocol
CluserC: ClusterC is the main component. It Contains the logic for initialization phase, cluster
setup phase, hierarchical topology setup phase, enquiry phase, and maintenance phase.
EventDetetionC: This component is responsible for monitoring the environment in which the
sensor nodes are deployed, for occurrence of events of interest.
MsgAuthC: This component computes MAC, and uses RC5 encryption algorithm to encrypt
the messages.
ReportC: This component prepares the report of the occurred event, and reports to the base
station.
Due to the resource constrained nature of sensor nodes, the cryptographic algorithms are not
only selected based on their strength, but also based on the energy conserving nature. As
evaluation of security mechanisms in wireless sensor networks identifies the RC5 encryption
algorithm as the right candidate for sensor nodes [22]. So, the proposed protocol uses RC5 for
implementation. In order to save the memory the same algorithm is used for performing all
MsgAuthCode
Main
ReportC
MsgAuthCodeC
EventDetectioC
ClusterC
MainC TimerC
GenericComm EventDetectionC

49
cryptographic primitives as in [36] that is the same algorithm has been used for all purposes
such as encryption, key computation, and MAC computation. Fig. 2(b) depicts the configuration
files of the application in TinyOS. For the simulation purpose, the value of tc is 700 milliseconds
that is after every 700 milliseconds holdback value of every sensor node decreases by one. The
simulation graph in Fig.3(a) shows the relationship between the number of sensor nodes, and
simulation time for with and without security in the top-down, hierarchical, multi-hop, secure
routing protocol for the wireless sensor network.
(a) No. of Nodes vs. Simulation Time (b) No. of Nodes vs. simulation time without
With and without security enquiry setup phase
Fig. 3 Simulation Results
It also shows that the implementation of security methodology consumes the significant amount
of simulation time. Fig. 3(a), and Table 1 analyze that hierarchical topology setup, enquiry
setup, and event detection & reporting to base station phases consume the significant amount
Table1: Number of nodes vs. simulation time for cluster setup phase
time. Fig. 3(b) shows the relationship between the number of sensor nodes and simulation time
for secure sensor network setup in the absence of the enquiry setup phase.
Analytical observations: The simulation results, shown in Fig 3(a), depend on the
deployment/distribution of sensor nodes in the target area. Variation in simulation time is
mainly due to time required for cluster setup phase, and enquiry phase that is initial holdback
value of each sensor node and how many cluster heads are not connected to secure hierarchical
topology after the completion of secure hierarchical topology setup phase.
8. Conclusion and Future Work
This paper has narrated the new top-down hierarchical, multi-hop, secure routing protocol for
the reactive hierarchical wireless sensor networks. This protocol is resilient to Report
Fabrication attack that is the presented protocol suppresses the bogus report generated either by
adversary node or by captured member node or by cluster head. The proposed solution relies on
symmetric key mechanism which is appropriate for random deployment of sensor nodes in the
given target area. This protocol divides the wireless sensor network into non-overlapping
clusters in which each cluster member is at most one hop distance from their cluster heads. In
Number of
Nodes
Simulation time for cluster setup phase (in Sec.) Difference
Col2 – Clo3without security with security
20 11.804 14.735 2.931
40 11.854 14.810 2.956
80 14.105 16.356 2.251
150 16.735 20.917 4.082f

50
the proposed protocol, all the sensor nodes participate in cluster formation either as a cluster
member or as a cluster head. Further, the proposed protocol also ensures the participation of all
the cluster heads of wireless sensor network in hierarchical topology formation. The proposed
protocol is scalable. The memory overhead, computation overhead, and communication
overhead of the proposed protocol has been evaluated. The battery power consumption of
member nodes, and cluster heads during cluster setup phase, hierarchical topology setup phase,
and report preparation and forwards towards base station have been also calculated. It also
ensures the confidentiality, integrity, and authenticity of sensed data, and reports.
The proposed routing protocol has been designed with the assumption of static sink
node, and sensor nodes. Hence, future work can be done to enhance the protocol
considering the mobility of sink node, and sensor nodes. In such a case, the proposed
security protocol also needs to be modified.
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