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International Journal of Research in Computer Engineering and Electronics. Page # 1 ISSN 2319-376XVOl : 2 ISSUE :3 (June 2013)

IJRCEE@2013http://www.ijrcee.org

A Secure Model for Time Synchronization inBody Sensor Network

J. S. Lather

Abstract - In the recent years wireless sensor networks (WSN) have received a lot of attention due to wide range of applications such ashealth monitoring, agriculture, target tracking, environment monitoring, military operations, and scientific exploration in dangerousenvironments. The health monitoring system is one of emerging application area of WSN. A number of tiny wireless sensors nodes may beimplantable or attached to the cloths on the human body to create a wireless body sensor network (WBSN) that can continuously monitorvarious vital signs of patients and provide real-time feedback to the user and medical personnel. Time synchronization is an importantcomponent of sensor networks to provide a common clock time in all sensor nodes. Time synchronization protocols offer a mechanism forsynchronizing the local clocks of the nodes in a sensor network. In this paper a model for secure time synchronization in WBSN isproposed.

Index Terms : Wireless sensor network, Body sensor network, Time synchronization commas.

1 I NTRODUCTION

ireless sensor network (WSN) [1] is one of the fastestgrowing fields in the area of research. The sensor node

is fundamental unit of WSN. A WSN is a collection of tinysmall sensor nodes which are organized into a cooperativenetwork. Each sensor node consists of processing capabilityvia one or more microcontrollers, CPUs or DSP chips [2].These nodes have power of sensing, measuring, and collect theinformation from the environment and, based on the assignedtasks, they can transmit the sensed data to the designated useror system. The power in sensor nodes is depends on battery.The size of battery is directly proportional to power of battery.

The advances in battery technology are much slower than therecent advances that have taken place in the field of wirelesscommunications and networks.

In Section 2 and 3 Body Sensor Network (BSN) and Timesynchronization for BSN are discussed respectively. An ap-proach for time synchronization in WBSN is proposed in Sec-tion 4. Concluding remarks and future work are made in Sec-tion 5.

2 B ODY S ENSOR N ETWORK

The health care system is one of the emerging fields ofWSN. Body area sensor networks (BASNs) [3] is similar to

WSN. It consist of multiple interconnected nodes may be im-planted in human body or attached on wearable cloths, whichhave sensing, processing and communication capabilities. The

basic purpose of WNSN is to monitor the health of patientcontinuously whether he/she is at home or moving out side.

The sensor nodes continuously monitor vital signs such asglucose levels, heart beats, Electrocardiography (ECG), bodytemperature, etc. in real-time fashion as shown in Fig. 1. Ifthere is sudden change in vital signs, the sensor nodes aftercollecting the vital information about patient forwarded tohealthcare center and relative of patients in real time fashionto take care of patient.

Figure 1: Human body equipped with sensor nodes

W

J. S. Lather is currently working as Associate Professor, Department o

Electrical Engineering National Institute of Technology, Kurukshetra INDIA .

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International Journal of Research in Computer Engineering and Electronics. Page # 2 ISSN 2319-376X

VOL :2 ISSUE : 3 (June: 2013)


The three tier architecture of WBSN [3] is shown inFig. 2. The tier-1 is responsible communication of data amongsensor nodes in body area network only, tier-2 provides com-munication between body area sensor nodes and access

points, and the tier-3 of architecture provides communicationbetween access points to medical server, emergency services,patient family members etc. The information communicatedbetween tier-1 and tier-3 must be secure because it containsvital information about the patient.

Figure 2: Architecture in WBSN

3 T IME S YNCHRONIZATION IN BSN

Time synchronization among sensor nodes is one of thecritical activities in BSN. The sensor nodes in BSN are measur-ing different health parameters. The clock of each sensor nodein BSN must be synchronized to real clock i.e. clock of medicalserver. Time synchronization [4], [5], [6] is also important toensure accurate coordination and minimize energy consump-tion. The sensors nodes are distributed on the body measuretime using local clocks and have different sample frequencies[5]. The lifetime of the BSN platforms is depends on by verysmall batteries of each sensor node that comply with userscomfort. The advancement in battery is very slow as com-pared to sensor nodes. As battery decay may show differenttime as compared to real (actual) time. Local time readings ofsensors nodes can easily mismatch with other sensor nodesand the medical server clock because of random phase shiftsand drift rates of clock oscillators. The error will be cumulateas the sensors operate for long periods and thus sensor nodeclocks will lose synchronization. Time synchronization processmay suffer from external attack, internal attack or both. Exter-nal attacks are those in which an (external) attacker manipu-lates the communicating signals between pairs of nodes andcauses the nodes to desynchronize, or to remain unsynchro-nized even after a successful run of the synchronization proto-col. Fig. 3 shows the WBSN along with medical server takeninto consideration. The medical server sends a synchroniza-tion signal after a regular time interval to a trusted node inWBSN.

Figure 3: Architecture for Time Synchronization in BSN

In the given paper, sender-receiver based protocol fortime synchronization in BSN is proposed. In sender-receiverbased approach all receivers should be synchronize with des-ignated sender. Sender-receiver approach basically includesthree steps [7].

i. The sender node periodically broadcasts a messagewith its local time as a timestamp to all intended re-ceivers. Corresponding to message the receiver replyback to sender.

ii. The message delay between the sender and receiver iscalculated by measuring the total round-trip time(RTT) from the time a receiver requests a timestampuntil the time it actually receives a response.

iii. If delay is less than RTT, all receivers then synchron-ize with the sender using the timestamp they receivefrom the sender.

The sender-receiver protocol suffers from external attack-er. The greater RTT than expected shows there is an externalattacker has captured the packet and then replaying [5] itagain.

For example, if at time T1, Sensor node S sends a sy n-chronization packet to receiver node R. Node R receives thispacket at T 2, where T 2 =T1+d+. Here, and d represent theoffset between the two nodes and end-to-end delay respective-ly. At time T 3, sensor node R replies back to sender node San acknowledgement packet. This packet contains the valuesof T2 and T 3. Node S receives the packet at T 4. Similarly, T4 isrelated to T 3 as T4 = T3+d- . Node S can calculate the clockoffset and the end-to-end delay as:Offset () = ((T 2 T1) (T4 T3))/2 (1)Delay (d) = ((T 2 T1) + (T4 T3))/2 (2)

Figure 4: Replay attack by external attacker

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If an external attacker [5] performs pulse-delay attack, the eq-uations will change to: T 2* =T1+ + d+ and T 4*= T3- + d + . Here is t he pulse-delay introduced by the attacker [3]. In presenceof pulse delay clock offset and the end-to-end delay will be

Offset () = ((T 2 T1) (T4 T3) + )/2 (3)Delay (d) = ((T 2 T1) + (T4 T3) + )/2 (4)

4 P ROPOSED P ROTOCOL FOR BSN

In this section a protocol for secure time synchronizationfor BSN in tier-1 architecture of WBSN is proposed. The proto-col will synchronize the local clocks of all sensor nodes totrusted node participated in BSN.

Let us assume that group membership is known to trustednode and also all nodes reside in the power range of trustednode in WBSN. Let us consider MS is a Medical Server, which

is responsible for initialization of time synchronization signals,sends the synchronization packet to Trusted Node (TN) inBSN as shown in tier-2 and tier-3 communication architecture[3] of Fig 2. The MS always sends the synchronization messageto TN. The sending time of the challenge packet from MS isTMS and receiving time of challenge packet at TN is TTN. Afterreceiving the challenge packet the TN will reply back to MSvia sending response packet to MS. At MS will calculate delay(d) occurs. If calculated delay (d) at MS is less than expectedmaximal delay (d*) then MS assumes that it receives time syn-chronization packet without replay attack [5]. Otherwise, MSwill again send the challenge packet to trusted node until cal-culated delay is than or equal to maximal delay.

Let us consider TN is a trusted and non-malicioussensor node. The sending time of the challenge packet by nodeTN is represented by T i (time measured by node TN) and re-ceiving time of packet by any other sensor node (all remainingnodes participating in BSN except TN) in BSN is TN is T j. TheTN and TN measured by two different clocks. T i is measuredby the local clock of node TN (i.e. C TN) whereas T j is measuredby the local clock of node TN (i.e. C TN).

The offset (or the difference between the local clocks)between the two n odes is represented by ij (calculated bynode TN with respect to node TN). The delay for the packettransfer from TN to TN is represented by d ij. The proposed

protocol in tier-1 architecture is explained in following steps:Step 1: Trusted Node (TN) broadcasts challenge packets con-taining its node identifier (TN), time at the TN node clock(CTN) and challenge nonce (N s) toall sensor nodes participatingin the WBSN. If there are n nodes in the group then in the firststep the number of messages transmitted is n-1 (because n th node is TN). In proposed protocol the TN is taken as sendernode.

Step 2: In this step of the protocol, every node TN (other than

TN), which have received the challenge packet acknowledges back to sender node TN, known as response packet [8].This packet contains triples {T j , N s , TN}, where Tj is the receipttime of the challenge packet from node TN, Ns is nonce sent

by TN and TN is node-id of sender respectively. It also con-tains Message Authentication Code (MAC) [9], [10], whichenables TN to authenticate the packet sent by TN in this step.The response packet also includes the sending time (Tj) fromnode TN . MAC is used to provide resiliency against externalattacker. So in this step n -1 MACs are calculated one for eachpair of TN and TN and then each TN sends messages to TN.A pair wise secret key (K ij) which is shared between nodes TNand TN is also used in the response messages.

Step 3: On receiving the response packet the sensor node TNcalculates the delay occurred (d), corresponding to challenge

response and if all the calculated delays for each node are lessthan a maximal delay (d*: It is maximum delay in Round TripTime (RTT)) then node TN calculates the offset for each nodeTN. If any nodes calculated delay is more than maximal delaythen TN assumes that response from TN is suffers from exter-nal attacker and dont sy nchronization signal to that TN .

4.1 P ROPOSED P ROTOCOL FOR T IME S YNCHRONIZA-TION IN BSN:

The proposed protocol is discussed as follows:

1. Broadcasting of challenge packet to all nodes in the groupde noted by TN.

2. TN (T i) TN(T j): TN, C TN, Ns; for all j=1, ....,n-1.

/* Node TN sends a challenge packet for synchroniza-tion, containing its node-id TN, Clock time of TN i.e. C TN,and nonce N s at time T i to all nodes in the group and nodeTN receive the packet at time T j */

3. Reply from all TN sensor node to TN

TN (T j) TN (T i) : TN, T j, m, M, ACKm = {T j, N s, TN }M = {MAC{ Kij}[TN, T j, T j, N s, TN, ACK]}

/* Node TN neighboring node of TN will send responsepacket to TN at time T j. The packet also contains receivingtime of challenge packet at TN i.e. T j from node TN withnonce N s. */

4. Compute d for each pair of TN and TN .

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/* calculate end-to-end delay and offset set between each sen-sor node from trusted node. */

d ij = [(T jTi)+(TiT j)]/2, for all j=1,..,n-1if all d ij d* then

ij=[(T jTi)(Ti-T j)]/2, for all j=1,...,n-1if ( ij=0)

thenTN is already synchronized with clock time of TNi.e. CTN.else

send the synchronization packet to TN from TN asin step1end if

elseNode TN is Malicious (external attacker)

thenRepeat step 1 to 3 again.

end if

5 CONCLUSIONS AND F UTURE W ORK

The WBSNs suffers from challenges such as limitedbattery power, memory limitation, limited computational ca-pability, cost and the physical size of the sensor nodes. Thetime synchronization in WBSN is a critical activity. In this pa-per, the secure time synchronization process in WBSN is in

Tier -1 architecture is proposed. The proposed protocol re-moves the chance of external attacks in time synchronizationin BSN. In this protocol, it is assume that transfer of time syn-chronization packet from medical server to trusted node issecure. In future the communication between medical serverand trusted node i.e. tier-3 architecture to tier -1 architecturemay implement with the help of Message AuthenticationCode (MAC) for secure time synchronization. The proposedprotocol can detect only external attackers but not internalattackers. In future the internal attacker can be identified.

REFERENCES

[1] Ghosal D., Mukherjee, B., Yick, J. Wireless sensor network su rvey,Computer Network, 2008, 52(12), 2292 2330.

[2] Friedemann Mattern, Kay Romer, The design space of wireless sensornet works, IEEE Wireless communications, 2004, vol. 11, no. 6, pp. 5461.

[3] Athanasios Vasilakos, Huasong Cao, Min Chen, Sergio Gonzalez, M.Leung , Victor C., Body Area Networks: A Survey, Mobile Networksand Applica tions, April 2011, Volume 16, Issue 2, pp 171-193.

[4] G. Cao, H. Song, S. Zhu Attack-resilient time synchronization for wire-less sensor networks, in Proceedings of the 2nd IEEE Intern ationalConference on Mobile Ad- Hoc and Sensor Systems (MASS 05), pp.765772.

[5] Ajay Agarwal, Arun Kumar Tripathi , An Approach towards TimeSynchroni zation Based Secure Protocol for Wireless Sensor Network,Networked Digital Technologies, Communications in Computer andInformation Science Volume 88, 2010, pp 321-332.

[6] Capkun S., Ganeriwal S., Popper C., Srivastava, M.B., Secure Time

Synchroni zation in Sensor Networks, ACM Transactions on Info rma-tion and System Security, 2008. Article No: 23, 11(4).[7] M. Manzo, S. Sastry, T. Roosta Time synchronization in networks, in

Pro ceedings of the 3rd ACM Workshop on Security of Ad Hoc andSensor Net works (SASN 05), pp. 107 116, November 2005.

[8] D. Sanchez, H. Baldus, A deterministic pairwise key pre -distributionscheme for mobile sensor networks, in Proceedings of the 1st Intern ational Conference on Security and Privacy for Emerging Areas inCommunications Networks (Se cureComm 05), pp. 277 288.

[9] C. Wang , K. Sun, P. Ning , Secure and resilient clock synchr onization inwireless sensor networks, IEEE Journal on Selected Areas in Comm unications, vol. 24, no. 2, pp. 395 408,2006.

[10] Chen, K., Li, H., Wen, M., Zheng Y. , A Secure Time SynchronizationProtocol for Sensor Network, Emerging Technologies in Knowled gDiscovery and Data Mining Lecture Notes in Computer Science Volume 4819, 2007, pp 515-526.

Authors Biography

J. S. Lather ( : [email protected]) received B.E. in Electrical En-gineering from REC Surat and M.Tech. and PhD in Control Sys-tems from REC Kurukshetra (presently NIT Kurukshetra). Hisresearch interests are Robust Control, Wireless Sensor Networks,Control of time delay systems etc.
http://link.springer.com/bookseries/558http://link.springer.com/bookseries/558

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