Wireless Sensor Network Security: The State of the Art

Wireless Sensor Wireless Sensor Network Security:Network Security:The State of the ArtThe State of the Art

Credit: Yee Wei LawThe University of Melbourne

22

RoadmapRoadmap

Primer to cryptography andPrimer to cryptography andWSNsWSNs

Secure data aggregationSecure data aggregation Key managementKey management Other areas: Other areas:

secure remote reprogrammingsecure remote reprogramming secure localizationsecure localization energy-efficient jamming attacksenergy-efficient jamming attacks

Information Assurance

Protection Detection Reaction

33

Information assurance

Introduction to securityIntroduction to security Security threats: either somebody wants to steal Security threats: either somebody wants to steal

something from you or sabotage yousomething from you or sabotage you

Information assurance (IA) is a set of measures that Information assurance (IA) is a set of measures that protect and defend information and information protect and defend information and information systems by ensuring their systems by ensuring their availability, integrity, availability, integrity, authentication, confidentiality, and non-repudiationauthentication, confidentiality, and non-repudiation. . These measures include providing for restoration of These measures include providing for restoration of information systems by incorporating information systems by incorporating protection, protection, detection, and re-actiondetection, and re-action capabilities. capabilities.

Information security

Operationsecurity

44

PrimitivesPrimitives Security objectives:Security objectives:

ConfidentialityConfidentiality IntegrityIntegrity AuthenticationAuthentication Non-repudiationNon-repudiation

Encryption / decryptionEncryption / decryption Symmetric-key: Symmetric-key: EE((KK, , MM) / ) / DD((KK, , MM)) Asymmetric-key: Asymmetric-key: EE((PKPK, , MM) / ) / DD((SK, MSK, M))

Signature / verificationSignature / verification Symmetric-key: message authentication code (MAC), denotedSymmetric-key: message authentication code (MAC), denoted

MACMAC((KK, , MM)) Asymmetric-key: digital signature, denotedAsymmetric-key: digital signature, denoted

SignSign((SKSK, , MM), ), VerVer((PKPK, , MM))Notation:Public key = Public key = PKPKPrivate key = Private key = SKSK

55

Common usageCommon usage

EE((KK11, , MM) || ) || MACMAC((KK22, , EE((KK11, , MM))))

EE((KK11, , MM) || ) || SignSign((SKSK, , hh((EE((KK11, , MM))))))

Confidentiality

Confidentiality Integrity, authentication

Integrity, authentication,non-repudiation

Diff keys for encryption and authentication

Signing on hash is more efficient

66

Challenges in WSNsChallenges in WSNs

Sensor node hardware, resource constraints

Algos must be energy- and storage-efficient

Nodes operate unattended Adversary can compromise any node

Nodes not tamper-resistant Adversary can compromise any node’s keys

No fixed infrastructure Cannot assume any special-function node in vicinity

No pre-config’ed topology Nodes don’t know neighbours in advance

Communicate in an open medium

Communications are world-readable and world-writeable by default

Constraints Implications

77

Security design principlesSecurity design principles

Favour computation over communicationFavour computation over communication Communication 1000 times more energy-consuming Communication 1000 times more energy-consuming

than computationthan computation

Minimal public-key cryptoMinimal public-key crypto Tate pairing costs 5s (54mJ) on a Tmote Sky Tate pairing costs 5s (54mJ) on a Tmote Sky

(fastest recorded by [Szczechowiak et al. 08])(fastest recorded by [Szczechowiak et al. 08])

Favour resilience (tolerance) over absolute Favour resilience (tolerance) over absolute securitysecurity Strength in numberStrength in number

Part OnePart One

Secure data aggregationSecure data aggregation

99

Data aggregationData aggregation

aggregate

aggregate

aggregate

Purposes: (1)Save bandwidth (limited data rate)(2)Save energy (limited energy)

Reason why we put a processor on every node in the first place

1010

Phase 1: Query Phase 1: Query disseminationdissemination

Sample query: SELECT AVERAGE(temperature) FROM sensorsWHERE floor = 6EPOCH DURATION 30s

1111

Phase 2: Data Phase 2: Data aggregationaggregation

aggregate

aggregate

aggregate

Types of aggregation:(1) basic aggregation, (2) data compression, (3) parameter estimation

1212

Phase 3: Result Phase 3: Result verification (optional)verification (optional)

“Did you really report this?”






1313

Security goals of data Security goals of data aggregationaggregation

Robustness: Byzantine Robustness: Byzantine corruption of data would corruption of data would not make aggregation not make aggregation result totally result totally meaninglessmeaningless

Confidentiality: To Confidentiality: To ensure that other than ensure that other than the sink and the sources, the sink and the sources, no intermediate node no intermediate node should have knowledge should have knowledge of the raw data or the of the raw data or the aggregation resultaggregation result

perform averaging1

23

1000

So the average is 251.5… Oh wait a

minute

sources

sinkWhat the hell

am I aggregating

?

What the hell am I

forwarding?

1414

Securing data Securing data aggregation: aggregation: multipronged defencemultipronged defence

Sink

Sources

...

...Aggregators

Forwarders

'Witness nodes'vote on validityof aggregationresult

Sink verifies aggregationresult with sources

End-to-endkeying

Privacy homomorphismResilient aggregation

Privacy homomorphism

1

2

3

4

1515

Resilient aggregationResilient aggregation

Objective: To bound the effect of data Objective: To bound the effect of data corruptioncorruption

Corruption can be arbitrary – ByzantineCorruption can be arbitrary – Byzantine By convention, we denote the number of By convention, we denote the number of

corruptions as corruptions as kk Methods:Methods:

Robust statistics (1-hop networks)Robust statistics (1-hop networks) RANBAR (1-hop networks)RANBAR (1-hop networks) Quantiles aggregation (multi-hop networks)Quantiles aggregation (multi-hop networks)

1616

Robust statisticsRobust statisticsSay an aggregation function is actually an estimator

Say we are estimating a parameter Θ and there are k rouge nodes

An aggregation function is (k,)-resilient if

ˆ ˆrms*( , ) rms( )k

That is, the RMS error as a result of k-corruption, must be bounded by a constant factor of the original RMS error

We win if we can limit

The attacker wins if he manages to unbound

1717

Examples of (k,Examples of (k,)-)-resilient aggregation resilient aggregation functionsfunctions

AVG

x1 x2 x3 x4

y

AVG

x1 x2 x3 x4+4

y=y+Non-resilient, example: Average

Resilient, examples

rms(y)> rms(y)

Aggregation function Resilience Breakdown point ε*

Sample median wrt Gaussian distribution

21 2 ( / )k n , if k n 0.5

5%-trimmed average wrt Gaussian distribution

1 6.278 /k n , if k < 0.05n 0.05

[l, u]-truncated average wrt Gaussian distribution

1 ( ) / /u l k n Not applicable

Count wrt Bernoulli distribution with parameter p

21 / [ (1 )]k np p Not applicable

1818

RANBARRANBAR

Based on RANdom SAmple ConsensusBased on RANdom SAmple Consensus, which , which originates in computer vision (hence the name originates in computer vision (hence the name RANBAR = RANsac-Based AggRegation [ButtyRANBAR = RANsac-Based AggRegation [Buttyáán n et al. 06])et al. 06])

Step1: Use as few samples as possible to Step1: Use as few samples as possible to determine a preliminary modeldetermine a preliminary model

Step 2: Use the preliminary model to identify Step 2: Use the preliminary model to identify samples that are consistent with the modelsamples that are consistent with the model

Step 3: Refine the model with all the samples that Step 3: Refine the model with all the samples that are found to be consistentare found to be consistent

1919

Quantiles aggregation Quantiles aggregation (extending resilient (extending resilient aggregation to multihop)aggregation to multihop)

Median

1 2 3

6

Median

4 16

Median

Actual median = 3

Median

1 2 3 4 16

Median

4

This approach suggests that instead of taking a median every hop on the way, we should compress the data judiciously at each hop

2 10 2

2020

Quantiles aggregationQuantiles aggregation

Rules for deriving a q-digest: Rule (A): count(node) + count(parent) + count(siblings) ≥ n/k + 1 Rule (B): count(node) n/k

q-digest in this example: {<8,2>,<9,2>,<1,1>}

tree nodes are numbered

count

2121

Quantiles aggregationQuantiles aggregation

Derived median = data value represented by node 9 = 3.5Actual median = 3

tree nodes are numbered

count

2222

Resilient aggregation Resilient aggregation guidelinesguidelines

1-hop1-hop multihopmultihop

Data Data distribution distribution knownknown

Robust Robust statistics, statistics, RANBARRANBAR

Quantiles Quantiles aggregationaggregation

Data Data distribution distribution unknownunknown

Robust Robust statisticsstatistics

Quantiles Quantiles aggregationaggregation

Two approaches actually:(1)estimate by minimizing

effects of outliers(2)detect outliers and

estimate without outliers

Two approaches actually:(1)estimate by minimizing

effects of outliers(2)detect outliers and

estimate without outliers

2323

Progress so far…Progress so far…

Sink

Sources

...

...Aggregators

Forwarders



End-to-endkeying



1

2

3

4

2424

VotingVoting

Resource-intensive, only good for mission-critical, small-scale networks

1

1

2

3 300

malicious

malicious

No

No

No

No Yes

“is mean = 61.4 reasonable?”

malicious

Alright, 61.4 is not

reasonable!

2525


Sink

Sources

...

...Aggregators

Forwarders



End-to-endkeying



1

2

3

4

2626


Sink

Sources

...

...Aggregators

Forwarders



End-to-endkeying



1

2

3

4

2727

Privacy homomorphism Privacy homomorphism (PH)(PH)

First proposed by Rivest et al. in 1978 to process encrypted data First proposed by Rivest et al. in 1978 to process encrypted data without decrypting the data firstwithout decrypting the data first

A function is (A function is (,,)-homomorphic)-homomorphic ifif

ff((xx) ) ff ((yy) = ) = ff ((xx yy))

where ‘where ‘’ is an operator in the range and ‘’ is an operator in the range and ‘’ is an operator in the ’ is an operator in the domain. domain.

If If ff is an encryption function and the inverse function is an encryption function and the inverse function ff--11 is the is the corresponding decryption function, then corresponding decryption function, then ff is a PH.is a PH.

2828

Types of PHsTypes of PHs

There are three main approaches to PHs in WSNs so There are three main approaches to PHs in WSNs so far:far: PHs that are based on PHs that are based on polynomial ringspolynomial rings, e.g., , e.g.,

Domingo-Ferrer’s schemeDomingo-Ferrer’s scheme PHs that are based on PHs that are based on one-time padsone-time pads homomorphic homomorphic public-keypublic-key cryptosystems cryptosystems

Insecure under known-plaintext attacksAttacks involve only computation of gcd and linear algebra [Wagner 03]

Insecure under known-plaintext attacksAttacks involve only computation of gcd and linear algebra [Wagner 03]

2929

PHs based on one-time PHs based on one-time padspads

Encryption: Encryption:

Decryption by sink:Decryption by sink:

Drawbacks:Drawbacks: Use of the addition operator in place of the XOR operator in the plaintext Use of the addition operator in place of the XOR operator in the plaintext

space is unproven in terms of securityspace is unproven in terms of security Synchronization of keys causes scalability problemSynchronization of keys causes scalability problem

1 1 1

( , ) ( ) modn n n

i i i ii i i

C E k m m k p

1 1

mod modn n

i ii i

m p C k p

sinkm1 + k1

One-time pad

One-time pad

m2 + k2

m1 + m2+ k1 + k2

m3 + k3

m4 + k4

m1+m2+m3+k1+ k2+k3

m1+m2+m3+m4+k1+ k2+k3+k4

3030

aggregate

aggregate

aggregate

In Secure Data Aggregation, we secure one-way traffic.

In Key Management, we secure generic traffic.

generalized

PartPart Two TwoKey managementKey management

3131

ComponentsComponents

Protocolverification

Key managementKey establishment

Key refreshment

Key revocation

1

2

3

4

3232

Protocol verificationProtocol verification

Verification gives us indication and confidence Verification gives us indication and confidence of securityof security

If we simulate unbounded sessions, verification If we simulate unbounded sessions, verification of secrecy and authentication is of secrecy and authentication is undecidableundecidable

If we limit number of parallel sessions, we can If we limit number of parallel sessions, we can use use constraint solvingconstraint solving for verification for verification

Model: strand space modelModel: strand space model Tool: CoProVe implements the strand space Tool: CoProVe implements the strand space

model using constraint solving (Prolog)model using constraint solving (Prolog)

3333

Strand space modelStrand space model

Protocol Strand space model Example

Role: What a principal does in the protocol

Strand: A sequence of events Initiator, responder, server

Complete run: A complete iteration of the protocol

Bundle: A set of strands legitimate or otherwise hooked together where one strand sends a message and another receives that same message, that represents a full protocol exchange

1. Initiator Attacker: …

2. Attacker Responder: …

3. Responder Attacker: …

4. Attacker Initiator: …

3434

Node-to-node key Node-to-node key establishmentestablishmentA wants to establish a secure channel with B via a

common trusted node S:

A B: NA || AB S: NA || NB || A || B || MAC(KBS, NA || NB || A || B)S A: E(KAS, KAB) || MAC(KAS, NA || B || E(KAS, KAB))S B: E(KBS, KAB) || MAC(KBS, NB || A || E(KBS, KAB))A B: Ack || MAC(KAB, Ack)

3535

Node-to-node key Node-to-node key establishmentestablishment

NA || A

NA || N

B || A || B || MAC(K

BS , …)E(K AS

, KAB) ||

MAC(K AS

, NA ||

B || …

) E(KBS , K

AB ) || MAC(K

BS , NB || A || …

)

Ack || MAC(KAB, Ack)

3636




Key refreshment

Key revocation

1

2

3

4

3737

Key establishmentKey establishment

Definition: a process or protocol whereby Definition: a process or protocol whereby a shared secret key becomes available to a shared secret key becomes available to two or more parties, for subsequent two or more parties, for subsequent cryptographic usecryptographic use

Types:Types:Key establishment

Key transport Key agreement

Key pre-distribution

A key agreement protocol whereby the resultingestablished keys are completely determined a priori by initial keying material

3838

Protocol design by Protocol design by communication modescommunication modes

Global broadcasts: Global broadcasts: Authenticated broadcast using Authenticated broadcast using μμTESLATESLA

Local broadcasts: Local broadcasts: Passive participationPassive participation

Unicast:Unicast: Only consider neighboOnly consider neighbouur-to-neighbor-to-neighbouurr Multihop can be secured hop by hopMultihop can be secured hop by hop Random key pre-distribution schemesRandom key pre-distribution schemes LEAP+LEAP+ EBSEBS

3939

Global broadcast: Global broadcast: μμTESLATESLA

““Micro” version of the Timed, Efficient, Streaming, Micro” version of the Timed, Efficient, Streaming, Loss-tolerant Authentication Protocol Authenticated Loss-tolerant Authentication Protocol Authenticated broadcastbroadcast

i i+1 i+δ...Mi+δ || Ki || MAC(Ki+δ, Mi+δ || Ki)Mi || MAC(Ki, Mi)

Time interval:Message:

authentication succeeds if(1) Ki generates MAC

(2) and there exists a past key Kj = Hi-j(K i)

K1 K2 K3 K4 Kn……

keys are generated in reverse order

keys are released in forward order

Ki-1 = h(Ki)

4040

μμTESLA example (1)TESLA example (1)

K1 K2 K3 K4

h()

(1) Generate one-way reverse key chain on the base station

K1

(2) Give K1 to everybody

K1

(3) Generate one-way reverse key chain on the base station

K1

K1

M K2 MAC(K3, …)

4141

μμTESLA example (2)TESLA example (2)(4) K2 is genuine because h(K2) = K1 butpacket tagged with MAC(K3, M||K2) still needs to be authenticated

K2

(5) Base station later sends K3 that can be used to authenticate message M

M MAC(K3, …)

K2

M2 K3 MAC(K4, …)

M MAC(K3, …)

Authentication steps:(a) K3 is genuine because K2 = h(K3)(b) M is genuine because K3 is genuine and K3 authenticates M

4242

Local broadcast: Passive Local broadcast: Passive participationparticipation

A

B

C

D

E

Passive participation: nodes B, C, D, E suppress their transmissions when they find A transmitting about the same data

To secure passive participation, A uses a cluster key and a one-way key chain to achieve encrypted and authenticated local broadcast

A is just transmitting a similar data to I have, so I shall not transmit.

4343

Local broadcast: Passive Local broadcast: Passive participationparticipation

If only the key chain is used, the keys If only the key chain is used, the keys in the key chain would have to be in the key chain would have to be broadcast in the clear, and in the broadcast in the clear, and in the absence of time interval differentiationabsence of time interval differentiation, , a cluster-outsider would be able to a cluster-outsider would be able to forge messages using these keysforge messages using these keys

If only the cluster key is used, If only the cluster key is used, authentication of the sender cannot be authentication of the sender cannot be achievedachieved

But if used together, the cluster key But if used together, the cluster key can be used to encrypt messages as can be used to encrypt messages as well as to hide the key chain keys from well as to hide the key chain keys from cluster-outsiders; and at the same cluster-outsiders; and at the same time, the key chain keys can be used time, the key chain keys can be used for authenticationfor authentication

A

B

C

D

4444

Securing unicastSecuring unicast

Random key pre-distribution schemesRandom key pre-distribution schemes LEAP+LEAP+ EBSEBS

4545

Random key pre-Random key pre-distribution (RKP)distribution (RKP)

Pool

at random

at random

Able to establish session key?

‘Keying material’

P = pool size (4 in this example)K = key ring size (1 in this example)

4646

Random key pre-Random key pre-distribution (RKP)distribution (RKP)

Different types:Different types:

Type 1 Type 2 Type 3

Symmetric key[Eschenauer & Gligor 02]

Symmetric bivariate polynomial[Liu et al. 05]

Part of a matrix[Du et al. 05]

, 0

( , )t

i ji j

i j

f x y a x y

4747

Symmetric-key-based Symmetric-key-based RKPRKP

3

4

1

2

6

7

1

5

I’ve got keys 1, 2, 3, 4

I’ve got keys 1, 5, 6, 7

OK, so our session key can be derived

from key 1


from key 1

Although not all neighbouring pairs of nodes can establish a session key (aka pairwise key), the network will remain connected, with a suitable choice of K and P.K = key ring size (4 in this example)P = key pool size (7 in this example)

4848

Symmetric-key-based Symmetric-key-based RKPRKP

K = 4, P = 15, RMSE = 0.0427

Pr{connectivity ≥ k} vs k

K = 4, P = 30, RMSE = 0.0436

Pr{connectivity ≥ k} Expected connectivity

Derived from results of random geometric graphs [Law et al. 07]

4949

In this example, t = 2, K = 2, P = 3The pairwise key is f2(1,2) = f2(2,1) = 10 + 24 + 56 = 28 + 35 + 27 = 90*In reality, the value must of course be as large as normal crypto keysStorage requirement: K(t + 1) coefficients, where t is the threshold

Node 1

Polynomial-basedPolynomial-basedRKPRKP

I’ve got f2(), f3()


from f2()

f1(x, y) = 1+2y+3y2+2x+xy+4xy2

+3x2+4x2y+x2y2

Pool

f1(1, y) = 6+7y+8y2

f2(x, y) = 2+3y+5y2+3x+2xy+7xy2

+5x2+7x2y+2x2y2

f3(x, y) = 3+4y+5y2+4x+3xy+6xy2

+5x2+6x2y+3x2y2

f2(1, y) = 10+12y+14y2

Node 2

f2(2, y) = 28+35y+27y2

f3(2, y) = 31 + 34y + 29y2OK, so our session key can be derived

fromf2()

I’ve got f1(), f2()

, 0

( , )t

i ji j

i j

f x y a x y

5050

Matrix-basedMatrix-basedRKPRKP

2 3

2 2 2 3 2 2

2 3

1 1 1 ... 1

...

( ) ( ) ... ( )

( ) ( ) ... ( )

N

N

t t t N t

s s s s

G s s s s

s s s s

D1 D2 D3 D4

Randomsymmetricmatrices

M1=(D1G)T M2 M3 M4

N = number of nodes = number of columns

Vandemonde-likegenerator matrix

this seed can be used as an ID

5151

Matrix-basedMatrix-basedRKPRKP

Pairwise key = Pairwise key = MM22(1)(1)GG(2) = (2) = MM22(2)(2)GG(1)(1)

Storage requirement: Storage requirement: KK((tt+1)+1+1)+1 coefficients, where coefficients, where tt is the threshold is the threshold

Node 1

I’ve got M1, M2

I’ve got M2, M3

OK, so our session key can

be derived from M2


fromM2

Pool

Node 2

M1

M2

M3

M4

M1(1)

M2(1)M2(2)

M3(2)G(1)

G(2)

Here’s G(1)

Here’s G(2)

5252

Node-to-node key Node-to-node key establishmentestablishmentRKP schemes only good for keying two neighbouring nodes

with common key(s); what about neighbours without any common key? Use common trusted node

A wants to establish a secure channel with B via a common trusted node S:

A B: NA || AB S: NA || NB || A || B || MAC(KBS, NA || NB || A || B)S A: E(KAS, KAB) || MAC(KAS, NA || B || E(KAS, KAB))S B: E(KBS, KAB) || MAC(KBS, NB || A || E(KBS, KAB))A B: Ack || MAC(KAB, Ack)

5353

Node Ainitial key Kin

LEAP+LEAP+ LEAP+ is a key pre-distribution scheme but not randomLEAP+ is a key pre-distribution scheme but not random Every node is pre-distributed with Every node is pre-distributed with KKinin

Node Bnode key KB = PRF(Kin, B)Kin already deletedHello, I’m A

I’m B

A and B compute pairwise key = PRF(PRF(Kin, B), A)

KB

1

2

3

4 Timer fires, A deletes Kin

0 A sets timer

5454

EBS (Exclusion Basis EBS (Exclusion Basis System)System)

Nodes

Key

s

Pro: Two nodes always share at least 2K-P keys.

Con: When a node is compromised, more than half of the keys in the key pool are compromised.

615 key combinations

4

P

K

5555




Key refreshment

Key revocation

1

2

3

4

5656

Key refreshmentKey refreshment

Parallel re-keying:

Lose the key Lose the key KK, then , then allall past and future keys are past and future keys are exposedexposed

Not suitable for WSNsNot suitable for WSNs

Why? The more a key is used, the more it is open to Why? The more a key is used, the more it is open to cryptanalytic attacks, birthday attacks etc.cryptanalytic attacks, birthday attacks etc.

5757

Key refreshmentKey refreshment

Serial re-keying: preferable because of forward security

Only need to store this:Only need to store this:

Lose this, then all future keys are compromisedLose this, then all future keys are compromised But past keys are intactBut past keys are intact

0

1 times -1 times

(... ( ,0)...,0)i i

PRF PRF K

5858




Key refreshment

Key revocation

1

2

3

4

5959

Which keys to revoke?Which keys to revoke?

When When AA is compromised is compromised Global broadcast keys: Global broadcast keys: BB, , CC, , DD, , EE need to have their copies of need to have their copies of KKSS

globalglobal

replacedreplaced Local broadcast keys: Local broadcast keys: BB,, C C,, D D,, E E need to purge need to purge KKAA

clustercluster and and KKAAchainchain; ; BB

needs to re-gen and re-distribute needs to re-gen and re-distribute KKBBclustercluster and and KKBB

chainchain; similarly for ; similarly for CC,, D D,, E E

A

B

CD

EKD

cluster

KD

chain

KCcluster

KC

chain

KBcluster

KB

chain

KEcluster

KE

chain

KSglobal

KSchain

Compromisednode

KSglobal

KSchain

KSglobal

KSchain

Base stationS

Big picture:

6060

StrategyStrategyGateway

6161

Re-keying unicast keysRe-keying unicast keys

If using polynomial-based or matrix-based RKP or If using polynomial-based or matrix-based RKP or LEAP+, do nothingLEAP+, do nothing

If using symmetric key-based RKP, re-keying is If using symmetric key-based RKP, re-keying is desirable but can be done withoutdesirable but can be done without

If using EBS, re-keying is a mustIf using EBS, re-keying is a must

A

B

CD

EKD

cluster

KD

chain

KCcluster

KC

chain

KBcluster

KB

chain

KEcluster

KE

chain

KSglobal

KSchain

Compromisednode

KSglobal

KSchain

KSglobal

KSchain

Base stationS

Big picture:

6262

Re-keying local broadcast Re-keying local broadcast keyskeys

6363

Re-keying global Re-keying global broadcast keysbroadcast keys

New global key is propagated from the base station in two stages:

(1) The hash of the key is propagated(2) Then the key itselfOver each hop, the key is protected by a cluster key and a

cluster key chain

6464

Thank yThank y’’allallDank uDank uDankeDankeGrazieGrazieMult'umescMult'umescDziekujeDziekujeKöszönömKöszönömTesekkurlerTesekkurlerShukran Shukran

धन्यवा�दधन्यवा�द 谢谢谢谢

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Wireless Sensor Network Security: The State of the Art