Wireless Sensor Network, 2011, 3, 183-188
doi:10.4236/wsn.2011.36021 Published Online June 2011 (http://www.SciRP.org/journal/wsn)
Copyright © 2011 SciRes. WSN
A New Kind of Dynamic Key Protocol for Wireless
Sensor Network
Cai-Xia Zhang1,2, Liang-Lun Cheng1, Xiang-Dong Wang2
1The Faculty of Automa tion, Guangdong University of Technology, Guangzhou, China
2Foshan University, Foshan, China
E-mail: zh_caixia@163.com
Received May 1, 2011; revised June 2, 2011; accepted June 9, 2011
For the source limitations and vulnerabilities of the sensor nodes of Wireless Sensor Networks, we propose
the new kind of dynamic key protocol for wireless sensor network, using the unidirection of hash function
and the thinking of Hill to study the dynamic key matrix. Through theoretical analysis of some aspects, our
method can promote security, connectivity expansibility; the results show that this protocol reduces storage
space and communication energy consumption also.
Keywords: Wireless Sensor Network, Key Matrix, Key Protocol, Connectivity
1. Introduction
Wireless sensor networks have the characteristics of
self-organizing, flexibility, small size and low cost, so it
is widely used in military, environmental mon itoring and
forecasting, urban transport and other fields. Since wire-
less sensor networks are generally deployed in the unat-
tended wilderness area and nodes can be easily captured
to leak sensitive classified information, ensuring the se-
curity of wireless sensor networks is very important.
Typical sensor nodes suffering the constraints of com-
munications, computing, storage, energy and other per-
formance, can not be deployed sophisticated security
mechanism; and its low-cost properties makes some ex-
pensive safety feature implemented. Therefore, low
power, low storage capacity, and high-security type of
security management mechanism for the normal opera-
tion of wireless sensor networks is very important.
Key management is the key issue in the security of
wireless sensor network. The current key management
scheme is divided into symmetric key and asymmetric
key management system. Since the complexity of algo-
rithm, the asymmetric key can not be directly applied to
the resource-constrained wireless sensor networks.
Symmetric key management scheme can generally be
divided into the following three categories: methods
based on key distribution centers, pre-distribution and
packet-based clustering approach, the three methods will
be introduced as following. Literature [1], the Kerberos
protocol is the first agreement based on one of the key
distribution centre. Its basic idea is that the session key
for communication used by sensor is from the key dis-
tribution center which is responsible for generating key.
The key distribution centers and each sensor node in the
networks share a unique key and store all the share keys.
The advantage of such agreements is easy to achieve,
moreover, both the computational overhead and storage
requirements of sensor nodes are low, but the network
service relies on the key center excessively. If the center
is destroyed, the high network Jibei is broken, while the
network scalability is poor. Literature [2] first proposes
the concept of random key pre-distribution. In this
mechanism, the node first will randomly select a certain
number of key from the key pool to pre-loading and after
being deployed, the nodes will build the links according
to the probability of hav ing the same key. Nodes must be
preloaded sufficient number of key to guarantee the
connectivity probability, and spend a lot of communica-
tion overhead on the same node authentication key. For
storage, the energy limited nodes; it will be a great bur-
den. Literature [3-7] makes a more detailed analysis on
this basis and the mechanism extends. Literature [8]
proposes grid-based and Literature [9] proposes a com
Special funds for technology of Guangdong Province:
(2009A08020700 8) funded.
Special funds for technology development project of Foshan City:
(FZ2010030) funded.
Special funds for technology development project of Foshan City:
(2009033) funded.
bination of methods which is the further study for the
random key pre-distribution mechanism.
Literature [10,11] proposes a key management pro-
gram which combines network topology. It is a key
management program to expand and improve random
key pre-distribution and it is based on the cluster group
key management scheme. The network is better to cal-
culate and expand, to support large-scale networks and
network dynamic change, but hasn’t got a good solution
to distributing the nodes in the cluster after deleting
cluster head.
In summary, many scholars have done a lot of work in
order to better solve the limitations on security, connec-
tivity, scalability, node storage of the wireless sensor
network in the key management scheme. Based on the
analysis of the previous, this paper proposes a key man-
agement based on dynamic key matrix for wireless sen-
sor networks to reduce the amount of key storage nodes
and enhance connectivity and resistance to attack of the
net. Section 1 and Section 2 present the background, and
they give the mathematical models and scen ario assump-
tions that adopted by the scheme. Section 3 describes the
detailed process to establish key; the results of theoreti-
cal analysis are in Section 4; Section 5 is the conclusions
of this paper.
2. Prior Knowledge and Application of
2.1. Hill Cipher
The realization of this agreement needs to learn some
basic password Hill’s ideas and methods, then the fol-
lowing is a brief description of it. In the process of
transferring information, through the matrix operation,
the sender converts the plaintext into the ciphertext and
the receiver through the inverse matrix converts ciphertext
into plaintext, then the secure transmission of infor-
mation completes. Details process as follows:
(1) Divide plaintext that needs to be transferred into
the same size as several explicit groups, each character
of the group encrypted another character of the group;
(2) Every time encrypted one group, if we express
each M characters in the group known as the 1
2, the corresponding char acters in the ciphertext
will be known as , encryption algorithm:
cc c
1111221 1
cpkpk pk
112 2mmm m
cpkpk pk 
That C = PK, the key K is a reversible square m × m
(m indicates the groups’ size), if encrypt the plaintext P
by linear transformation, the receiver can decrypt to get
the plaintext information through P = CK1. To ensure
that recipients can express exactly, it requ ires key matrix
K to be reversible.
2.2. Unidirectional One-Way Hash Function
In the realization of this Agreement, it needs to express
the dynamic generation of key matrix information, while
ensure that even if the plaintext information is captured,
the key matrix can not be quickly derived under the
information. This can be achieved by using the one-way
hash function. One-way hash function maps an arbitrary
length message M to a function H that has fixed length
hash value h (set the length m) .The function
has the following characteristics: when M is given, it is
easy to calculate h; and when h is given, according to
, it is hard to inverse M; Finding two random
to make
 
is difficult.
The above characteristics are suitable for this agreement.
2.3. Application of Assumptions
In practical situations, WSN nodes are not at the risk of
being hijacked all the time. When nodes are deployed in
the initial stage, the node usually has the ability to resist
the hijacking within a certain period of time, because it
doesn’t perceived by the malicious attacker or the at-
tacker does not know the distribution of the regional
node in advance, As the WSN begins to work, the target
nodes expose gradually and the nodes are attacked, in-
creasing the possibility of being hijacked. The network
attacker can exist in various forms [12]. Key establish-
ment protocol proposed in this paper is based on this
assumption on the network deployment phrase. Nodes do
not need to preset the initial pair of keys, only according
to the add ress information of neighbor no des to establish
the initial key matrix and th e link connection.
There are two common modes of communication in
Wireless sensor networks, which are communication
between adjacent nodes and communication between
nodes and base stations. So to establish link key and base
stations can meet the needs of most applications and
Link key can be used as the basis for the establishment of
other types of keys. This paper focuses on the link’s
establishment process under the single-hop key. Neigh-
bor sensor nodes which are usually in the range of radio
communications can directly communicate with each
other and establish a secure connection. The protocol
proposed in this paper establishes link key in the case of
reducing unnecessary verification and overhead of
computing. Non-neighbor node needs to go through the
middle Node multi-hop section to communicate.
Copyright © 2011 SciRes. WSN
3. Dynamic Key Agreement Based on
Password Matrix
In this paper, the dynamic key agreement can be divided
into three stages: the stage of presetting initial informa-
tion, the stage of establishing link key, and the stage of
updating link key.
3.1. Pre-Initial Information Stage
Prior to deployment in the network, a trusted base station
will randomly preset a unique address an d a 1 × n public
key matrix K0 for each node. The K0 is used to generate
the link key matrix, an d it will be cleared after generating
the link key matrix. Then the corresponding information
will be loaded into each node. And just need to store its
address i and public key matrix, which will greatly save
storage space.
3.2. Link Key Establishing Stage
After presetting initial information, all nodes can be
deployed to the target monitoring area. Because the node
hasn’t preset the key information, the deployment can be
deployed randomly and not depending on the neighbor
node. At that time, the sharing link key matrix between
neighbors has not been established in the network, we
should adopt the following methods to create the initial
key matrix:
Each node i generates a random number noncei which
to be the characteristics of random numbers that required
in the process of establishing the link key. Using the
random numbers in the process of establishing the link
key is to introduce spatial differences, which can limit
the destruction that the initial key has been captured in
the local area, that is, i
ii nonce.
Suppose the node j also randomly generates a random
number noncej, after receiving the identity message of
the node i, it can get the hash valu e L whose fixed length
is n by the hash calculation.
ashginoncej nonceL
Express the hash value L as a n × 1 column vector L,
then do the matrix operations on the vector L and the
preset K0 of initial key. After that, the invertible n × n
order matrix of the link key ij
can be found,
0ij i
Meanwhile, the node i also receives the identity
message of node j and calculates the Lj by hash
calculation, then do th e operations on the results with the
initial key matrix K0 .After that, the reversible link key
also has been found. To ensure that the i and
j can produce the same link key ijji in the
circumstance that they only know the identify each other,
the following cond itions should been met
ijg ji
In order to en sure that differen t neighbor nodes do not
generate the same link key, the function must also be
ij gik
3.3. Link Key Update Phase
This Agreement used dynamic update key matrix. In this
protocol during each session using a key matrix, the
session ended, both sending and receiving updates
immediately shared session key matrix.
First, the sending node will divide the plaintext P
which is need to been sent into n equal segments
pp p
 (each length is n and if the last segment is
less than n, replace it with the space). Use the Table 1 to
change the segmented information into the corres-
ponding digital information , then
multiply it with the lin k key matrix ij
dd dD
(n × n order) in
the form of (T expressed to be the
transpose of Di) to get the ciphertext message. After that,
send the ciphertext to the receiving node and the
receiving node will decrypt by the use of the inverse
matrix of the link key matrix which is consulted and
established by the nodes to get
ij i
CK D
KC, then
separate T
into the appropriate n section and check
the corresponding Table 1 to get the plaintext.
Meanwhile, the sending node and receiving node use
the deferent plaintext information to update key matrix,
the renewal process is as follows:
(1) In the same time of sending ciphertext in formation,
the sending nodes utilize the plaintext information to
update key matrix. The receiving nodes receive the
ciphertext and convert it into plaintext, and in the
meantime, the receiving nodes transform plaintext
information into the new key matrix. That matrix is the
same as the new key matrix of the sending node using
the same approach as follows:
(2) The sending or receiving nodes divide the plaintext
Table 1. The integer code table of space and letter.
spaces A B C D E F
0 1 2 3 4 5 6
14 15 16 17 18 19 20
7 8 9 10 11 12 13
21 22 23 24 25 26
Copyright © 2011 SciRes. WSN
186 C. X. ZHANG ET AL.
information into n segments of unequal length randomly,
pp p
(3) Make use of the Hash function which is loaded in
advance to do the operation on each segments, and the n
message values of which length is n can be generated;
(4) Use the Table 1 to convert the n message values of
which length is n into the n integer values of which
length is n, so a new key n × n matrix can be got. If K is
not reversible, it will re-do the above operation;
(5) Finally, both the sending and the receiving apply
the new key matrix to replace the original matrix. From
the above process, we can see that the process of
information transmission and the communication over-
head of updating key exist simultaneously;
(6) Repeat the above operation when sending infor-
mation again. The key matrix of the link encryption and
decryption should be updated instantly, truly to be
“one-time close” and to enhance the resistance to capture
of the node.
3.4. Adding New Nodes
After the network deployment, new nodes often need to
be added because of the damage in the previous nodes or
the requirement to expand the network. Verification is
very necessary for how to prevent malicious nodes from
masquerading as a new node to join the network and how
the new nodes add to the network under the conditions of
lacking real-time dynamic link key matrix. Because the
pre-existing random key distribution mechanism has no
way to make a distinction between the legitimate new
nodes and the disguised new nodes that have possessed
the captured key, it is vulnerable to be attacked by the
node replication. This paper presents a new node
authentication technology based on the density function
hash. That authentication technology confirms whether
the nodes that will join in are the new nodes or not by
comparing the relationship between the plaintext
information of the new nodes and the key matrix. The
technology includes the following steps.
(1) New nodes initialize. Before deployment, the base
station will be presorted stochastic plaintext information
and the Hash function in the new node; and then spread
the new node to a predetermined area;
(2) According to the mean in the stage of establishing
link key, th e new no de will eng ender n × n key matrix by
doing the Hash operation on the plaintext.
(3) The old node will do operation on the plaintext
information and agree to establish links with the new
node if the key matrix is the same as the one sent by the
new n node; and then resend the plaintext to the new
node randomly and build the link key matrix.
In the above process, even if the malicious nodes
intercept plaintext message, it can not learn the new
node’s Hash function in a short time and therefore can
not generate the key matrix, so the new node can add
4. Theoretical Analysis
4.1. Safety Analysis
The physical capture and attacks of node in sensor net-
works is the most serious attack. In order to gain security
information, the attacker can read or change the infor-
mation in the node’s memory. What the node saves in
this agreement is the key matrix obtained after the Hash
function, and due to the unidirection of the Hash function,
it is impossible to export the original information from
the key matrix in the calculation. The key in the agree-
ment update with the information sending. Even if the
node has been captured, there is new information com-
munication between the adjacent node from the time of
capturing to reading the security info rmation, and the key
has updated already. In this agreement, as the result of
updating the key matrix dyn amically, the malicious node
can not obtain the dynamic information and has been
isolated outsi d e the netw or k of the sensor.
4.2. Load Analysis
The electric power of communication consumption nodes
is much more than the calculated consumption and thus it
requires the communication load in key management
protocol should be as small as possible.
The calculation load and communication load of this
agreement will be analyzed. In this Agreemen t, when the
order n of the dynamic key matrix is greater, the amount
of computing the ciphertext message and generating a
new key matrix will be greater, communication load in
the network will be greater and the confidentiality of the
system will be relatively better. In addition, because the
dynamic update features of the key matrix can ensure
network security, so you can reduce the calculation en-
ergy consumption and communication load which are
generated by updating key information by reducing the
number of the n key matrix, and also reduce the storage
overhead of the sensor nodes.
Suppose you want to send a plaintext message which
length is L and the order of dynamic key matrix is n, then
ij i
D, namely:
ij j
The computational load of the process of generating
ciphertext: look-up table for n times, do multiplicative
Copyright © 2011 SciRes. WSN
operations n2 times, do additive operator n(n-1) times;
updating the dynamic key requires n times Hash matrix
to calculate and N2 times operation on checking table.
Therefore, minimizing the order of the matrix under the
premise that security is guaranteed is of great signify-
cance in improving the op eration speed of this agreement,
reducing the computation time and lowering energy
Convert the information into a relatively short cipher-
text so that the co mmunication load is reduced while the
safety is ensured.
Each node only needs to maintain the communication
key of the adjacent neighbor in this protocol. Comparing
with the key pre-distribution scheme which need to store
a lot of key information, under the circumstances of
lower key storage cost, this Protocol can maintain the
full connectivity of sensor network in the range of com-
5. Conclusions
This article proposes a kind of dynamic key management
protocol based on password matrix in wireless sensor
network. It uses the update mechanism about dynamic
password matrix. Each transmission of information u sing
the different encrypted and unencrypted keys, effectively
prevents physical capture, eavesdropping and other at-
tacks and enhances the security of the system; the con-
nectivity of network nodes is good and radio range is
wide between adjacent nodes connected; as the agree-
ment only need to establish key matrix between the
nodes in the neighborhood, it can effectively reduce
space where the node store key; information delivery
process and key update communication cost are at the
same time so that it can reduce the overhead in the
transmission of session keys between nodes and save
energy consumption. Meanwhile, putting forward the
joining system about the new nodes makes the network
have better scalability, and when the new sensor node is
added to the network, the network key which has been
leaked will not affect the new join intrant sensor nodes.
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