Broadcast Authentication for Wireless Sensor Networks Using Nested Hashing and the Chinese Remainder Theorem

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1 Sensors 2010, 10, ; do: /s OPEN CCESS sensors ISSN rtcle roadcast uthentcaton for Wreless Sensor Networks Usng Nested Hashng and the Chnese Remander Theorem Mohamed Hamdy Eldefrawy 1, Muhammad Khurram Khan 1, *, Khaled lghathbar 1, 2 and Eun-Suk Cho Center of Excellence n Informaton ssurance (CoEI), Kng Saud Unversty, PO ox 92144, Ryadh 11653, Saud raba; E-Mals: meldefrawy@ksu.edu.sa (M.H.E.); kalghathbar@ksu.edu.sa (K..) Informaton Systems Department, College of Computer and Informaton Scences, Kng Saud Unversty, Saud raba Department of Multmeda, Hannam Unversty, 133 Oeong-dong, Daedeok-gu, Daeeon , Korea; E-Mal: eunsukk@empal.com (E.S.C.) * uthor to whom correspondence should be addressed; E-Mal: mkhurram@ksu.edu.sa; Tel.: Receved: 23 July 2010; n revsed form: 23 ugust 2010 / ccepted: 6 September 2010 / Publshed: 17 September 2010 bstract: Secure broadcastng s an essental feature for crtcal operatons n wreless sensor network (WSNs). However, due to the lmted resources of sensor networks, verfyng the authentcty for broadcasted messages s a very dffcult ssue. µtesl s a broadcast authentcaton protocol, whch uses network-wde loose tme synchronzaton wth one-way hashed keys to provde the authentcty verfcaton. However, t suffers from several flaws consderng the delay tolerance, and the chan length restrcton. In ths paper, we propose a protocol whch provdes broadcast authentcaton for wreless sensor networks. Ths protocol uses a nested hash chan of two dfferent hash functons and the Chnese Remander Theorem (CRT). The two dfferent nested hash functons are employed for the seed updatng and the key generaton. Each sensor node s challenged ndependently wth a common broadcastng message usng the CRT. Our algorthm provdes forward and non-restrcted key generaton, and n addton, no tme synchronzaton s requred. Furthermore, recevers can nstantly authentcate packets n real tme. Moreover, the comprehensve analyss shows that ths scheme s effcent and practcal, and can acheve better performance than the µtesl system.

2 Sensors 2010, Keywords: wreless sensor network; authentcated broadcast; nested hashng chans; Chnese Remander Theorem 1. Introducton chevng broadcast securty s a must for wreless sensor networks; hence t s necessary for the base staton to broadcast commands and data to sensor nodes. Wthout secure communcaton, sensors may be nvolved n ncorrect operatons and can t meet the network requrements. The current securty solutons for wred and wreless networks cannot be utlzed for a wreless sensor network because of the energy, memory and computaton restrctons of the latter. These lmtatons make the desgn and operaton completely dssmlar to those of regular wreless networks. roadcast authentcaton based on asymmetrc key cryptography cannot deal wth the lmted resource constrans. Symmetrc key cryptography and hash functons are cheaper n ther computatonal requrements and are more wdely utlzed n sensor networks [1,2]. WSNs broadcast authentcaton was frst covered by TESL [3], and µtesl [4] that provdes the asymmetrc cryptographc property of authentcated broadcast through delayed dsclosng (tme-varyng) of symmetrc keys. The base-staton nstalls a key chan by repeatedly applyng a one way hash functon (OWHF) to an ntal random value, called seed. The chan constructon allows nodes to verfy the authentcty of the dsclosed keys. Loosely tme synchronzed and MC (Message uthentcaton Code) generatons are requred. Revelaton of sesson keys by the base-staton s delayed, thus allowng nodes to verfy the key valdty. Multlevel µtesl [5] s proposed to reduce the need to rentalze the network by mplementng multple levels of key chans, n whch hgh-level keys are used to communcate root-keys (or commtments) for low-level chans, whch are used n turn for broadcast authentcaton as n standard µtesl. Network lfetme s extended. Sgnfcant computaton and storage are requred. Recevers can t deal wth the receved messages nstantly and have to store them wthn one or several tme ntervals. Consderng the broadcastng of urgent messages lke alerts and alarms; the TESL famly has great shortcomngs n dealng wth such matters. Furthermore, the delayed authentcaton can be subect to Denal-of-Servces (DoS) attacks. Merkle tree utlzaton [6] was ntroduced to overcome ths shortage n bandwdth and storage resources utlzaton. TIK [7] was proposed to acheve mmedate authentcaton based on senstve tme synchronzaton between the snk and the recevng nodes. However, ths technque s not sutable for WSNs, as mentoned by ts nventors. Sensor nodes have a lmted battery lfe, whch can make usng asymmetrc key technques mpractcal as they use much more energy for ther mathematcal calculatons. We propose a new algorthm that uses two dfferent types of hash functons, whch come wth a nested chan and the Chnese Remnder Theorem n order to get a common broadcastng message. The resultng chan provdes the forwardness and the nfnteness, and no process restartng s requred. The proposed protocol s compared wth others n terms of ts computatonal cost and securty attrbutes. The rest of ths paper s organzed as follows: Secton 2 dscusses the related work, Secton 3 dscuses the requred attrbutes, Secton 4 proposes our new algorthm, Secton 5 evaluates our

3 Sensors 2010, scheme s performance, Secton 6 analyzes the securty attrbutes, and fnally Secton 7 concludes the paper. 2. Related Work The followng subsecton dscuses some of the schemes related to WSN authentcaton broadcastng. Ther effcency and shortcomngs accordng to the desrable securty attrbutes that wll be dscussed wll also be llustrated Lamport s Scheme Hash chans were frst proposed by Lamport [8]. They nvolve applyng a hash functon h ( ) N tmes to a seed ( s ) to form a hash chan of length N: The user calculates the -th key accordng to ths relaton: h ( s), h ( s),, h N ( s), h N ( s ) (1) k ( s) = h N ( s ) (2) The host authentcates the user by checkng that the followng equalty holds: 1 = N + h k ( s) h ( s ) (3) t where the value h N +1 (s) s already saved n the host system s fle from the prevous -th authentcaton. fter any successful authentcaton, the system password fle s updated wth the new key. Ths scheme has a lmtaton on the number of authentcatons, so that after reachng N authentcatons, a process restart s requred. In addton, t s vulnerable to an opponent who sends small challenge values to users that respond wth the chan ntal values [9]. Ths attack can be referred to as a small challenge attack. lso, the users are charged wth computatonal processes through the ntalzaton phase, whch makes the system unsutable for WSNs cakc et al. s Scheme The nfnte length hash chans (ILHC) proposed by [10] use a publc-key algorthm,, to produce a forward and nfnte one way functon (OWF). cakc et al. utlzed RS [11], where d s the prvate key and e s the publc key. The OTP orgnatng from ntal nput s usng the RS publc-key algorthm for the -th authentcaton s: k ( s) = s, d (4) and the verfcaton of the -th key s done by: k, 1 s = k e (5) ncreasng the number of cascaded exponentatons ncreases the computatonal complexty, makng ths algorthm very dffcult to mplement n lmted computaton devces [12].

4 Sensors 2010, Chnese Remander Theorem (CRT) If the ntegers n1, n2,, n k are par-wse relatvely prme, then the system of smultaneous congruence: x r mod n 1 1 x r mod n 2 2 x r mod n k k (6) has a unque soluton: 1 = k = 1 x r N N mod N where; 2.4. TESL Famly roadcast uthentcaton = k N n (7) N =1 N = n 1 1mod N N n (9) Tmed Effcent Stream Loss-tolerant uthentcaton (TESL) [3] s a multcast stream authentcaton protocol. Keys used to authentcate the -th message s dsclosed along wth ( + 1)-th message. µtesl [4] provdes authentcaton for data broadcasts, and requres that base staton and sensor nodes be loosely tme synchronzed. ccordng to Lamport s scheme, a base staton (S) randomly selects the last key k n, the chan seed, and apples a one-way publc functon h ( ) to generate the rest of keys: k0, k1,, k n 1 as k = h( k +1 ). Gven k, every sensor node can generate the sequence k 0, k 1,, k n 1. However, gven k, no one can generate k + 1. t -th tme slot, S sends an authentcated message MCk ( message ). Sensor nodes store the message tll the verfcaton key n the ( + 1)-th tme slot s dsclosed. Sensor nodes verfy dsclosed key k+ 1by usng key k as k = h( k +1 ). In µtesl, nodes are requred to store a message untl the authentcaton key s dsclosed. Ths operaton may create storage problems, and encourages DoS types of attacks. µtesl has been expanded to Mult-level µtesl [4] by smplfyng the key dstrbuton phase and ntroducng a new concept of a mult-level key chan generaton usng pseudo-random functons that mproves the protocol effcency. Mult-level µtesl reduces the need to rentalze the network (although re-ntalzaton s stll requred) by mplementng multple levels of key chans, n whch hgh-level keys are used to communcate root-keys (or commtments) for low-level chans whch are used n turn for broadcast authentcaton as n standard µtesl. The chans are further connected n that each root-key s derved from the correspondng hgh-level chan usng another pseudo-random functon. Network lfetme s extended many tmes over, but t s stll lmted. problem would result f a recever dropped a related commtment dstrbuton message ntalzng a new low-level chan; t would be unable to verfy any broadcast data receved durng ths entre lfetme of the chan tself. The data would stll be verfable eventually as the recever could use any later commtment dstrbuton message to reconstruct all the lost hgh-level keys and the correspondng chans. Ths would requre sgnfcant computaton and storage. (8)

5 Sensors 2010, CRT roadcast uthentcaton The scheme proposed n [13] s dvded nto three phases: Dstrbuton, Message Sgnng, and fnally Message uthentcaton phase. efore deployment all nodes are loaded wth the chan seed, k n, the OWHF h ( ), and two dfferent modules values, n and n for the CRT. When the S needs to broadcast a message m to sensor nodes for the -th sesson, S calculates the MC of the message m usng k to get M = MCk ( m ). fter that S cpher k and M usng the two secrets values n and n through the CRT to get: U k mod n and U M mod n, then t broadcast U. Upon the occurrence of U recepton by sensor nodes, they recover k from U, and then apply the OWHF ( ) k = h k where k s the last authentc key that sensor nodes have receved. Fnally, to verfy the message ntegrty, the sensor nodes compute the correspondng MC usng k of the receved message and then compare the result. Unfortunately, ths scheme also has a length restrcton consderng the use of a backward hashng chan to generate keys. 3. Requred ttrbutes h, to check Here we lst a number of desrable securty attrbutes for authentcated broadcast: 3.1. Data Integrty Data ntegrty ensures that data has not been altered by unauthorzed enttes Data Orgn uthentcaton Data Orgn uthentcaton guarantees the orgn of data. It s a fundamental step n achevng entty authentcaton n protocols as well as establshng keys. We may say that data orgn authentcaton mples data ntegrty. So t s not possble to acheve data ntegrty wthout data orgn authentcaton Freshness Packets that have been captured and replayed at a later tme should be gnored by the sensor nodes Delay Tolerance No tme synchronzaton should be requred n the system for data verfcaton. Each packet must be verfable wthout havng to wat for addtonal data Confdentalty Confdentalty ensures that data s only avalable to those authorzed to obtan t Denal-of-Servce ttack The denal of servce attack s an attempt to make a node resource unavalable to ts ntended users Small Challenge ttack Ths attack challenges the backward hashng wth small values to respond wth the chan ntal values.?

6 Sensors 2010, Lmtaton for an N tmes uthentcatons Process re-ntalzaton after N of authentcatons s necessary. 4. Our pproach The basc dea of our scheme s to expand Lamport s scheme [8] wth some modfcatons that produce the desrable nfnteness and forwardness, avodng the use of publc key cryptography. The shortcomng of those two parameters, nfnteness and forwardness, causes the nsuffcency shown wth respect to the prevous work. Notaton h( ) h ( ) (, ) x ( ) Represents the frst hash functon Table 1. The Proposed Scheme Notaton. Represents the second hash functon Descrpton x y The nested hashng progress values for -th authentcaton y h h s Hashng the seed by h ( ) for x tmes followed by h ( ) hashng for y tmes for the -th sesson k Sesson key for the -th authentcaton x, y U P The encrypton of the concatenated message wth the sesson by the sesson key The podcasted packet for the -th authentcaton X The broadcasted chan ndexes, calculated by the CRT s The current seed crt s The next seed nxt Thus we need to ntegrate Lamport s scheme usng two dfferent one way hash functons, h ( ) and h ( ), one for the seed chan and the other for the sesson key s producton, as shown n Fgure 1. Fgure 1. Sesson key producton consderng a nested hash chan usng two dfferent hashes. 2 k2, = y y h h ( s ) ( ) k h h s 2 2 2,2 = h ( ) seed( s ) h 1 ( s ) h 2 ( s) h x ( s) 1 2 2,1 = k h h s h ( )

7 Sensors 2010, Key Pre-loadng Phase Each node n s loaded wth two unque CRT modules r and r. Those modules, regardng the all nodes, are relatvely prmes. lso all sensors are loaded wth key seed s and the two dfferent hash functons, h ( ) and h ( ). From the other way the base staton s loaded wth all ths nformaton consderng the all the CRT modules for all the network s nodes, the key seed s, and the two dfferent hash functons h ( ) and h ( ) Message uthentcaton. efore the broadcastng operaton, S has to do the followng: y x () Calculate the sesson key k h h ( s) x, y = for the -th authentcaton. () Encrypt the broadcasted message m concatenated wth the sesson key k, to get = E ( m ) U k k x, y x, y () Calculate the broadcasted chan ndexes, X, for the all N nodes consderng the CRT X x mod r n1 X y mod r X x mod r n1 n2 X y mod r X x mod r X y mod r X x mod r X y mod r The S constructs the broadcasted packet to be P { E ( m k ) X } all sensors uthentcaton Verfcaton. n2 nn nn k x, y x, y x y wth the sesson key (10) = and then broadcast t to Upon the recepton of P by the all sensors, they wll need to ensure that the broadcast packets come from the authentcated S. The verfcaton process s done as follows: () Each sensor node wll extract X to perform the module operaton to obtan the chan ndexes, e.g., n1 wll get x X mod rn and y X mod r n. 1 () fter gettng the chan ndexes, they wll perform the key generaton accordng to these ndexes y x k h h s. by usng the two dfferent hash functons to get ths x, = y () y decryptng kx, y 1 D U, sensors wll be able to get the message m and the sesson key k x, y.

8 Sensors 2010, (v) Then the sensor nodes need to compare the two sessons they have establshed and receved, f the comparson s postve, then sensor nodes wll recover the message. Otherwse the receved broadcast message has been altered. The message ntegrty also checked mplctly through the authentcaton verfcaton, that way tamperng wth U n a way of message modfcaton wll sequentally affect the receved sesson key. (v) fter the completon of one sesson, sensor nodes and S have to update the current seed to the next one: s = h x s (11) ( ) nxt crt Fgure 2. The Proposed roadcastng uthentcaton Scheme n Wreless Sensor Network. 5. Performance nalyss In ths secton, we are gong to analyze the performance of our algorthm wth respect to the storage and computatonal cost [14] Storage nalyss The storage complexty s the amount of memory (RM sze) requred to store securty credentals. The storage complexty affects the hardware prce of sensor nodes. Our proposal requres the base staton to save two keys for each sensor nodes to buld the conference X, two dfferent hash functons h ( ) and h ( ), and one seed s. Ths storage overhead s neglected to the base staton, snce the base staton regarded as resource-rch node. In the other way, sensor node n has to store two prvet keys r and r, and one seed s, each one of them s 160-bt. Ths tells us that the memory requred for credentals per module (RM) s bt = 480-bt = 60-bytes. Hash functons h ( ) and

9 Sensors 2010, h ( ) are mplemented, wrtten n nesc code for TnyOS, n approxmately 20 Kbyte of memory (ROM.) 5.2. Computaton nalyss Consderng the computatonal complexty, base staton has to buld the congruent equaton (10) to reach the chan ndexes for all sensors, X, also t has to perform two dfferent hash operatons to buld the sesson key k x, y ths computaton s affordable n the base staton. lternatvely sensor nodes have to do two dfferent modulo operaton and to perform the same two dfferent hash operatons accordng to h ( ) and h ( ). Ths also s very easy to the sensor nodes. Rather than the prevous technques whch use backward hash functons. Those prevous technques cost the sensor nodes to perform hashng operatons for many tmes, especally through the chan ntal values. Example: Consderng the chan length to be N = 1,000 the number of requred hash operaton consderng Lamport scheme wll be. (N + 1) (N/2) = 500,500. On the contrary the usage of nested hashng wll requre the sensors to perform 2 N hash operatons whch are equal to 2,000, accordng to our llustraton. Ths could show how the nested hashng usng two dfferent hash chans s very cheap, n a very smple way. Now, we consder the requred executon tme for a sensor node to calculate the sesson key y x k h h s. The utlzaton of the mcroprocessor Sparc(400) as the sensor nodes platform, wll x, = y ( ) gve us the followng: the requred tme to dgtze a plan text of sze 80 bytes usng MD5 wll cost us a = 39 µs and also, the requred tme to dgtze a plan text of sze 64 bytes usng SH-1 wll cost us b = 56 µs as shown n Table 2 [15], such that the total tme requred to calculate the sesson key y x k h h ( s ) s t exec = a x + b y. Consderng that the maxmum values for x and y are w = 10, x, = y hence t exec = 10( ) = 0.95 ms. Note we have consdered the worst case, hence we have consdered the largest nput plantext for the both two hash algorthms, but n fact the plan text sze wll be no more than 160-bts = 20-bytes, rather than the 80 bytes or 64 bytes. Table 2. Executon tmes [µs] for two dfferent hash algorthms, platforms and plantext szes [bytes]. lgorthm Sze tmega103 tmega128 M16C/10 StrongRM Xscale(400) Xscale(200) Sparc(440) MD5 0 5,863 1,466 1, :26 5,890 1,473 1, :80 10,888 2,722 2, SH ,249 3,812 2, ,781 3,945 5, ,107 7,777 10, However, the tme requred for ndvdual modulo operatons mod r and mod r for node n s tny compared to the calculaton of the two dfferent hash operatons.

10 Sensors 2010, Securty nalyss ccordng to the securty attrbutes we have mentoned above, we are gong to evaluate our approach: 6.1. Data Integrty n mplct check for data ntegrty has been provded. ny data modfcatons that could be done wll consequently affect the receved vector E ( m ) U k k x, y = whch wll be dscovered through the key checkng, by comparng the two sessons they have establshed and receved Data Orgn uthentcaton x, y Sendng an orgnal copy of the sesson key concatenated wth the message and then encryptng them wth the same key provdes the orgnalty authentcaton n a straghtforward way. No one has the ablty to P = E m k X except for the base-staton or an ntruder that has buld the broadcasted packet { } captured the entre congruence keys k x, y postvty authentcaton check consderng the all sensor nodes Freshness x, y r and r for all nodes. Ths broadcast message has to provde the Our proposal allows the base staton to challenge the sensor nodes wth unpredctable unformly dstrbuted values of (x, y ). ccordng to these values, and accordng to the seed updatng every sesson, new refreshed keys have been establshed every sesson, so the communcaton system has a new and refreshed sesson key, and prevous messages cannot be replayed. If we suppose that x and y can take one value of forward m values, the probablty of successfully guessng a challenge wll be the ont probablty of x and y, whch s equal to 1/m 2. We can refer to ths property as the ablty to resst predctable attacks Delay Tolerance Our proposed scheme provdes an nstant authentcaton. Every broadcasted packet contans the authentcaton nformaton for tself, ndependently of prevous and followng messages. The authentcaton process s done n the same sesson Confdentalty Confdentalty cannot be guaranteed f one or more nodes have been compromsed. If an ntruder acqures the ablty to capture one node or more he wll be able to solve the congruent equaton usng the captured node n congruent keys r and r. The CRT [13] algorthm also dd not cover ths property, furthermore the broadcasted messages are sent n the plan form wthout encrypton. ctually, regardng certan applcatons lke the broadcastng of urgent alert notfcatons and warnng systems need nstant message authentcaton rather than confdentalty.

11 Sensors 2010, Denal of Servce ttacks In µtesl scheme, the sensor nodes can t authentcate the receved message mmedately after recepton. The ntruder can send a large amount of forged messages to consume the sensor nodes buffer. The nstant authentcaton provded n our scheme, overcomes ths weakness. The authentcaton process s done n the same sesson ndependently of the prevous or the next sessons. Ths vulnerablty s overcome wthout resources an extra bandwdth or an extra storage memory lke [5] and [6] Lmtaton for an N tmes uthentcatons ll TESL famles and also CRT, use backward hash chan. The backward chan has a restrcton of an N tme for authentcatons; a process restart s requred after reachng ths number of authentcatons. Our algorthm utlzes a new technque of employng two nested and dfferent hash functons for the key producton. Ths technque uses forward hashng and has no need for process restartng after reachng any number of authentcatons Small Challenge ttack Utlzng a one way hash functon to construct a hashng chan n the backward fashon encourages a new knd of attack called small challenge attack. Ths type of attack dscloses the hash chan ntal values. These ntal values help the ntruder to extract the remanng chan values by hashng those ntal values. Our algorthm covers ths vulnerablty by the utlzaton of two dfferent and nested hash functons n the forward fashon, whch prevents ths knd of attack rute Force ttack The ablty of generatng a truly random sequence of key bts can defeat a brute force attack, as a brute force attack would have no way of dstngushng one key from the other. Relyng on the generaton of random number can mpede the brute force. The nested hashng progress random values for -th authentcaton (x, y ). play a great role n preventng ths type of attacks accordng to the entropy of ther random generaton. 7. Conclusons new wreless sensor network broadcast authentcaton scheme based on forward hashng usng two dfferent nested hashes and the Chnese Remnder Theorem (CRT) has been presented. The broadcastng messages are bult usng the congruence of the CRT. The two dfferent hashng systems are utlzed n the sesson key generaton n a forward and unlmted way. Ths scheme acheves better characterstcs than the other schemes, we dscussed. Our proposal s not lmted to a certan number of authentcatons, and also does not nvolve computatonally expensve technques (PKC) to provde nfnteness. detaled securty analyss has been performed that covers many types of attacks that could nfluence our scheme. Our scheme satsfes all the securty attrbutes, we have dscussed, except for the confdentalty n case of one node or more has been captured. Ths scheme s applcable for alertng and warnng systems that need nstant broadcast authentcaton rather than message confdentalty.

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13 Sensors 2010, Khan, M.K.; Zhang, J. Improvng the Securty of Flexble ometrcs Remote User uthentcaton Scheme. In Computer Standards and Interfaces; Elsever Scence: North Holland, The Netherlands, 2007; Volume 29, pp by the authors; lcensee MDPI, asel, Swtzerland. Ths artcle s an open access artcle dstrbuted under the terms and condtons of the Creatve Commons ttrbuton lcense (