LOKS: Low-Overhead Forward and Backward Key Secrecy in WSNs

Transcription

1 LOKS: Low-Overhead Forward and Backward Key Seecy n WSNs Malvka Ashok CSE Department M. M. Engneerng College M. M. Unversty Mullana, Ambala, Haryana, Inda-3307 Roht Vad CSE Department M. M. Engneerng College M. M. Unversty Mullana, Ambala, Haryana, Inda-3307 ABSTRACT Securty plays an mportant role n desgnng a wreless sensor networks (WSNs. As the medum s wreless n nature whch s more vulnerable to adversares attacks n the network. Key management s used to acheve securty n WSNs. If symmetrc key s used n the network then one key s more enough to secure the network but the ssue s that once the key s compromsed, entre network gets compromsed. On the other hand, f an ndvdual key s provded to every sensor n the network then sensor to sensor communcaton s not possble n the network. Therefore sensors are grouped together to form a cluster. Each cluster s assgns a cluster key shared by every cluster member of the group. If ths key s fxed then key compromsng affects the securty prncple n the entre group. So to avod ths type of attack, key s updated after a fx nterval of tme. But f key updatng s done by the base staton, communcaton overheads n updatng the keys are neased n the network. Therefore resource constraned wreless sensor networks; the concept of key generaton s used nstead of key dstrbuton. In key generaton process, the key s generated by applyng a one way hash functon on a gven seet. But the problem s that f ths seet s compromsed, all the keys whch are generated n past or to be generated n the future are mmedately compromsed. In ths research paper, we present a LOKS: Low-Overhead Forward and Backward Key Seecy scheme to secure WSNs. Ths scheme updates the keys of each cluster n the network wth reslency to attack. Smulaton results proves that presented scheme takes less number of communcaton overheads as compared to exstng schemes gven n lterature to update the group keys for every groups after every round. Keywords Backward Seecy, Cluster Head, Forward Seecy, Key Generaton, Key Seecy, Sensor nodes, WSNs.. INTRODUCTION Due to the advancements n the technology wreless networks are used n ntense and dffcult stuatons where humans can t reach easly. Due to ts self-confgured nature t s used n varous applcatons. But sensor networks are resource constraned, so the man challenge n these networks s to provde secure communcaton n an effcent manner. There are many protocols that have been developed to utlze these knds of networks n an effcent way so to nease the lfe tme of the network. One of such technque s also presented n ths paper to provde secure communcaton over the network wth low communcaton overhead. As the sensor network s more prone to nsde and outsde attacks because the medum s wreless. So Securty s one of the bg ssues n Wreless sensor network. As the data s communcated through the wreless medum so the attacker or ntruder can easly hack or tamper the data, make the nodes to be compromsed, ntroduce malcous nodes n the network. All these actvtes make the network less secure and also deease the lfetme of the network as they work on battery. So t becomes very mportant to make the network secure and generate the methods whch should nease the network lfetme. To provde securty, t s very mportant that the data that s beng communcated must be enypted and verfed. The data enypton means hdng the data or nformaton nto some code words so that no unauthorzed person could harm the data. The same data when reached securely to the recever sde t s deypted frst.e. changng the data nto ts orgnal form. But the problem s how to secure the nformaton between the sender and the recever and how to keep the network away from the reach of the attackers or ntruders. All ths process of enypton and deypton nvolves the keys. Keys are the seet number or code that s used to provde securty to the nformaton n the network.. Forward and Backward Key Seecy The process of key generaton s shown n Fg. The key whch s used n each round by dfferent sensor node n a partcular round s generated by applyng a one way hash functon on round key of prevous round,.e. th round key s used to generate the round key of (+ th round. Key generaton s used n replace of key dstrbuton to reduce complexty n the network. Fg : Key Generaton Process The drawback of key generaton scheme s that once a key of some round s compromsed, the key of any round s easly calculated by an attacker as the functon used to generate a key s statc n nature and s open to all sensors. The problem s shown n Fg where the key of th round s compromsed whch results to dsclose the round key of all the rounds. 36

2 Current Round Seed value generated by sensor node S S S 3 S 3 Seed value generated by sensor node S Table. Key Updatng [5] Sensor node S send the enypted value to sensor node S S0 Sensor node S send the enypted value to sensor node S S Generaton of round Key S S S S S S Key ( S S S S, S S S S, S Key ( S, S S S S S S Key ( 3 3 S 3 S3 : : : : : : : : : : : : : : : : : : th S 3 3 S3 S S S S, S S S S, S Key ( S, S S and smlarly sensor node S generates ts own random seed value S. Now S enypts S wth the help of a S 0 and send t to S as shown n Eq.. Fg : All key of the System are compromsed due to one key compromsng So the process of key generaton s mplemented n such a way that the prncple of Forward and Backward Seecy s mantaned n the network. Accordng to the prncple of Forward Seecy, the compromse of a key does not dsclose the round key of any round that s used n future. Smlarly the prncple of Backward Seecy states that compromse of a key does not dsclose the round key of any round that s used n past. The prncple of Forward and Backward Seecy s shown n Fg 3. The system s hghly secured by ths technque. S0 S S ( Smlarly S enypts S wth the help of as shown n Eq.. S S and send t to S S S ( Now both sensor nodes generates a round key used n frst round by applyng a one way hash functon as shown n Eq. 3. S Key ( (3 So now n round sensor node S generates ts random seed value S and enypt t wth S and send t to sensor node S as shown n Eq. 4. S S S, S (4 Fg 3: Forward and Backward Key Seecy. EXISTING SCHEME TO ACHIEVE FORWARD AND BACKWARD KEY SECRECY IN WSNs [5] Let there are two sensor nodes S and S deployed n wreless sensor networks. These two sensor nodes want to share a round key K n round wth each other. Every sensor node generates a random seed value n each round. Ths seed value s used n generatng the round key of current round n whch t s generated. Let S s the ntal seed value generated by 0 sensor node S before the startng of frst round and smlarly S s the seed value generated by sensor node S n round. Let S0 s the ntal seed value generated by sensor node S before the startng of frst round and smlarly S s the seed value generated by sensor node S n round. It s also assumed that both sensor nodes already share ts own seed value wth other sensor before the startng of frst round. In frst round sensor node S generates ts random seed value Smlarly S enypts S wth the help of as shown n Eq. 5. S and send t to S S S S, S (5 Now both sensor nodes generates a round key used n round by applyng a one way hash functon as shown n Eq. 6. Key ( S, S (6 Proceedng n ths way the nodes generate new key n every round and use that key to communcate wth each-others. The key updatng method s shown n Table.. Issues n Exstng Scheme The exstng system works well wth two nodes only. As soon as the number of nodes n the network neases, the complexty of the system neases very hgh. The number of messages that communcates between all the nodes n the network to share and generates the key also neases. If the exstng scheme to generate the keys s used then key generaton n the system becomes the bottleneck of the network and becomes more dffcult to exchange key generaton parameters n the network. Each node generates ts 37

3 own random number and broadcast n the network. Fg 4 shows that the sensor node A transmttng ts random number Ra to each sensor n the network. Fg 4: Sensor node A broadcast ts random number n the Network Ths random number s enypted wth the random number of frst sensor node wth the help of a random number generated n prevous round by frst sensor node and transmtted to frst sensor node then sensor node A enypts Ra wth the help of a random number generated by second sensor node n prevous round and transmt to second sensor node and so on. Ths process s acheved by all the sensors n the network. Each sensor generates ts random number and sent t to each and every sensor one by one as shown n Fg 5. round. But the drawback of ths scheme s that once a key of any round s compromsed, the prncple of Forward and Backward Seecy s not mantaned n the network. So broadcastng s not used n ths system. If each sensor broadcast ts random number wth the round key of prevous round. The second method to broadcast the random number s that each sensor node enypts ts random number wth the help of ts own random number that s used n prevous round, as random number of all the sensor n prevous round are known to each sensor n the network.. Communcaton Overheads n Exstng Scheme Let there are n number of sensors n the network that wants and generate a key shared among all of them. To generate a key wth the gven scheme, each sensor node sent the random number by enyptng ths random number wth the prevous round random number of ndvdual sensor. Ths way each sensor transmts ts random number to all the other sensors n the network one by one. So each sensor sent expectedly n- number of messages n the network, thus total n*n- messages communcated n the network as shown n Fg 6. As the number of sensors neases n the network, the communcaton overheads nease at very hgh speed. So ths scheme s not effcent for more than two sensors. Fg 5: Each sensor nodes broadcastng ts random number n the network The drawback of ths scheme s that broadcastng s not possble n ths system, as the random numbers of each sensor n prevous round are dfferent. Only the round key of prevous round s common between all the sensors. So the only method to broadcast the ransom number of current round s to enypt t wth the help of a round key used n prevous Fg 6: Communcaton Overheads n Exstng Scheme Table. Lst of abbrevatons S. No Name Despton CH Cluster head for current round CH Cluster head n next round CH Cluster head n prevous round R Random number generated by cluster head of current round R Random number generated by cluster head of next round R Random number generated by cluster head of prevous round K Round key used n current round 8. E k Enypton key 38

4 3. PRESENTED KEY GENERATION SCHEME WITH FORWARD AND BACKWARD KEY SECRECY The problem n the exstng system was the complexty. As the number of nodes n the system neases the system becomes more and more complex. The presented scheme for key generaton s very effcent n terms of communcaton overheads by mantanng the prncple of Forward and Backward Key Seecy n the network. Varous notatons used n understandng the presented scheme are gven n Table. In presented system, communcaton overheads are reduced by lmt the number of nodes parcpatng n key generaton process to three only. In each round all the nodes n the group does not partcpate n generatng the random number that s used n key generaton process, only three nodes wll generate the random numbers. These three nodes are cluster head nodes n three dfferent consecutve rounds,.e. cluster head of prevous round ( CH, cluster head of current round ( CH and cluster head n next round ( CH. CH becomes CH n round number (+. So n every round a new cluster head s elected that s known as cluster head of next round ( CH. Ths cluster head becomes the current cluster head n round number (+. But n frst round only two cluster heads are choosen whch partcpate n key generaton process by generatng ther random numbers. From these two cluster heads, one cluster head plays the role of current cluster head and other play ther role of next round cluster head. In ths round there s no prevous round cluster head. In ths round, CH sends the generated random number ( RCH to CH as shown n Fg 7. Fg 8: Prevous and next round CH sendng random numbers to th round CH Fg 8 shows the concept of three cluster heads. In ths fgure, cluster head CH and CH sent seed values to cluster head CH. Now after recevng the random numbers from CH and CH, the cluster head of current round CH generate the key K used n current round by applyng a one way hash functon on the values RCH, RCH and RCH. After generatng the round key, CH broadcast ths round key ( K n the network as shown n Fg 9. Fg 7: Next round CH sendng ts random number to current CH Fg 9: CH broadcastng key to all sensors n the network Ths round key s broadcasted n the network n enypted form. Ths key s enypted wth enypton key. Ths enypton key ( KE s generated by applyng a one way hash functon on round key of prevous round and seed value of current round as shown n Eq. 8. Where seed value n each round s generated by apply a one way hash functon on seed 39

5 value of prevous round and round key of prevous round as shown n Eq. 9. K Hf ( RCH, RCH, RCH (7 KE Hf ( K, sd (8 sd Hf ( K, sd (9 3 After recevng the enypted key, ndvdual sensor deypt t wth the deypton key to get the round key of current round. Deypton key s generated wth same technque that s choosen by the current round cluster head as shown n Eq. 7. As enypted key s broadcasted n the network so there s no chance that ths key s hacked by the adversary because the enypton key s generated wth the help of a seed value whch s never communcated over the network. So the presented system provdes a scalable forward and backward key seecy n WSNs. 3. Key Generaton Process The followng algorthm shows the steps nvolved n presented key generaton scheme A. B. C. CH CH : RCH Cluster head n prevous round sendng ts random number to current round cluster head. CH CH : RCH Cluster head for next round sendng ts random number to current round cluster head. CH : RCH Cluster head n current round generatng ts random number n current round. D. CH : K Hf ( RCH, RCH, RCH Cluster head n current round generatng the current round key by applyng one wat hash functon on the random numbers of prevous, current and next round cluster heads. E. CH KE Hf ( K, sd : Cluster head n current round generatng key Enypton key for current round by applyng one way hash functon on prevous round key and the current round seed value. F. CH KE ( K : Current round cluster head broadcastng current round key to all ts cluster members by enyptng the key wth the enypton key G. sd Hf 3( K, sd Every sensor updates ts seed value for next round by applyng a one way hash functon on current round key and the current round seed value. In ths way n each round new cluster head s selected and new key s generated for the new round. The seed value s never communcated on the network, t always reman hdden from the adversary. So f an adversary somehow hack the medum and gets the random number of any sensor node, t may never be able to generate the key. Due to the seed value that s nvolved n the enypton key the adversary may not be able to get the keys of any round nor past nether future keys. So the network s secure and the concept of forward and backward seecy s mantaned n the network. 4. COMMUNICATION OVERHEADS Communcaton overheads n the proposed scheme are reduced due to number of sensors nvolved n key generaton process s lmted to three only. So n presented scheme, communcaton overheads are gven below n Eq. 0. CO=+ (n- (0 Where CO s the communcaton overhead and n s the total number of sensor nodes n the network. Fg 0: Communcaton overheads n presented system Fg 0 shows the communcaton overheads n presented system wth varable number of sensors n the network. Comparson between communcaton overheads n presented v/s exstng scheme s shown n Fg. Fg : Communcaton overhead n presented v/s exstng scheme 5. CONCLUSION In ths paper, we present a model to acheve the prncple technque of forward and backward key seecy n the WSNs. The presented model s scalable for large network wth hgh number of sensor nodes. Presented scheme reduces the number of communcaton overheads as compared to exstng scheme wth reducng number of communcaton overheads. The scheme s very benefcal for cluster based approach where a group of nodes share a key wth cluster head node. In ths scheme, cluster heads are selected n each round where a current head of current round s responsble for generatng and 40

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