A New Dynamic Trust Approach for Cloud Computing

Transcription

1 Internatonal Worshop on Cloud Computng and Informaton Securty (CCIS 203 A New Dynamc Trust Approach for Cloud Computng Zhongxue YANG College of Computer Scence and Technology Nanng Unversty of Aeronautcs and Astronautcs Nanng, Chna School of Mathematc and nformaton Technology Nanng Xaozhuang Unversty Nanng, Chna young-sr@vp.sna.com Xaoln QIN College of Computer Scence and Technology Nanng Unversty of Aeronautcs and Astronautcs Nanng, Chna qnxcs@nuaa.edu.cn Ynge YANG Centre for Computatonal Intellgence De Montfort Unversty Lecester, UK yyang@dmu.ac.u Wenru LI School of Mathematc and nformaton Technology Nanng Xaozhuang Unversty Nanng, Chna lwr2255@yahoo.com.cn Abstract-Due to hghly dstrbuted, non-transparency and dynamc natures of cloud computng, cloud users often confused whether cloud servce provders could be trusted. Therefore, a certan trust mechansm or approach should be establshed to secure a cloud user n selectng a relable cloud servce provder whle transactons are tang place. A new dynamc trust approach for cloud computng s proposed n ths paper, and mult-level Drchlet dstrbuton s ntroduced to compute the value of trust degree. At the same tme, confdence factor and tme decay factor are taen nto account n trust evoluton. Smulatons are to be set up to observe the performance of the approach, and the results show that the Drchlet dstrbuton approach for cloud computng s an effectve and an effcency soluton n computng the value of trust degree. Keywords-Trust; Cloud Computng; Drchlet Dstrbuton; Confdence Factor; Tme Decay factor I. INTRODUCTION Cloud computng ams to provde convenent, on-demand, networ access to a shared pool of confgurable computng resources (e.g. networs, servers, storage, applcatons, and servces, whch can be rapdly provsoned and released wth mnmal management effort or servce provder nteractons[]. As cloud computng s achevng ncreased popularty, concerns are beng voced about the securty ssues ntroduced through the adopton of ths new model, whch mae users anxous about safety and relablty. Users n cloud envronment need some mechansms or approaches to protect data from undesred dsclosure. However, tradtonal approaches or mechansms based on access control polcy are not adequate as they expect that the enttes can be statcally enumerated and assgned the approprate prvleges n advance. In cloud computng, actually, the enttes often do not now each other and need to trust other enttes before tang an acton. At the same tme, the users have to outsource ther senstve data to clouds and feel that they are falure to control over ther data, whch are generally beyond the same trusted doman as data owners. Moreover, due to hghly dstrbuted, non-transparency and dynamc natures of cloud computng, cloud users confuse that whether cloud servce provders can be trusted. Thus, t s a sgnfcant ssue that how to provde trustworthy computng n cloud envronment and how to establsh a certan mechansm or approach to help cloud users n selectng a relable servce provder. A new dynamc trust evaluaton approach for cloud computng s proposed n ths paper and Drchlet 203. The authors - Publshed by Atlants Press 379

2 dstrbuton s ntroduced to calculate the value of trust degree. The rest of the paper s organzed as follows: In secton 2, related wors of trust are ntroduced, and trust problem defntons are descrbed n secton 3. A dynamc trust approach for cloud computng s proposed and mult-level Drchlet dstrbuton are ntroduced to compute the value of trust degree n 4th secton, the confdence factor and the tme-decay factor are also represented n ths secton. In the 5th secton the smulatons and comparatves are made for effectveness verfcaton of approach. The paper s concluded n the last secton. II. RELATED WORK Even though trust s a human noton, whch s ntroduced to the area of computer scence as the semnal wor by Marsh[2], many researches on trust evaluaton, however, have been extensvely performed for a wde range of applcatons, such as publc ey authentcaton[3],[4], electroncs commerce[5], peer-to-peer networs[6], [7], ad hoc and sensor networs [8] []. Trust establshed n these servce envronments are successfully used to support users to select the dependable servce provders. In recent years, smlar ssues of establshng trust are already nown from cloud computng, and related deas are needed to support cloud users n selectng trustworthy cloud servce provders. Khan KM et al. [2] gve an outlne of solutons usng emergng technologes for establshng trust n cloud computng. Taab H et al. [3] dscuss several securty and prvacy challenges n cloud computng envronments and suggest consderng a trust-based framewor for supportng adaptve polcy ntegraton. Rong Hu et al.[4] propose a trustworthness fuson model for servce cloud platform based on D-S evdence theory and three partes namely servce provder, servce requestor and servce publsher are taen nto account to establsh a trustworthy servce. A trust computng mechansm for cloud computng s proposed by Mohamed Frdhous[5]. The mechansm formulates trust scores for dfferent servce level requrements. The mechansm also taes the dynamcs of performance varaton nto computatons. Mono et al.[6] present a trust management archtecture whch conssts of a Cloud Servce Regstry and Dscovery, a Trust Calculator and a Dynamc Trust Montor whch dd not tae nto account the thrd party provders. Xaonan Wu[7] descrbes a trust evaluaton model for cloud computng based on the nteracton evdence by D-S evdence theory also. However, few of these wors focus on provdng effectve and effcent methods to calculate the value of trust degree and smulatons on the well-nown securty attacs. The same trust applcatons based on Drchlet dstrbuton approach are ntroduced n the prevous researches. K. Thrunarayan et al[8]. provde the method of Drchlet dstrbuton for drect trust computaton and evoluton n response to dfferent behavors n some reputaton systems, but not menton cloud computng system. Yang and Cemerlc[9] dscuss the approach of Drchlet dstrbuton to compute reputaton to mnmze rs n usage control n a collaboratve envronment. Josang et al[20]. analyze the mult-level reputaton system of e-commence by usng Drchlet dstrbuton. Fung et al.[2] use Drchlet dstrbuton for trust management to collaboratve host-based ntruson detecton networs to mprove securty. However, all of wors mentoned above do not focus on usng the approach of Drchlet dstrbuton to compute the trust value n cloud computng. III. PROBLEM DEFINITION Dego Gambetta[22] defnes trust as follows: Trust (or, symmetrcally, dstrust s a partcular level of the subectve probablty wth whch an agent assesses that another agent or group of agents wll perform a partcular acton, both before he can montor such acton (or ndependently of hs capacty ever to be able to montor t and n a context n whch t affects hs own acton. The concept of trust, adusted to the case of two partes nvolved n a nteracton, can be descrbed as follows: An entty A s consdered to trust another entty B when entty A beleves that entty B wll behave exactly as expected and requred. Two enttes nvolved n a nteracton can be called Trustor and Trustee, respectvely. In a trustng acton, Trustor must decde whether and how much to trust a Trustee based on rather an assessment of ts hstorcal experence and the 380

3 trustee's reputaton than a blnd guess. The Trust Degree ndcates the trustworthy level of a certan entty whch s also classfed as two basc trust types, namely, Drect Trust Degree and Indrect Trust Degree. Drect trust degree s to be updated accordng to each hstory-based behavor and experence of the entty n a gven context. However, ndrect trust degree called Reputaton also can be assocated wth many sources of naccuracy not present n a trustor's drect experence, whch s decded by recommendatons from other ever gven evaluaton enttes. Overall trust degree, n general, s used to evaluate the trustworthy level as well, whch s computed accordng to weghted average value of drect trust degree and ndrect trust degree. In general, trust relatonshp s varable owng to the dynamc nature of nteractons and behavor evoluton n cloud computng envronment. Cloud users trust the ablty of cloud servce provders to provde servces wth the ncrease of ther successful nteractons. A cloud user wll gradually change and adust ts trustworthness to a cloud servce provder as tme goes on. Therefore, Trust s a contnuous process rather than an solated acton, n whch one entty's trust to another s to a certan extent, from low to medum, and then to hgh, or from hgh to medum, and then to low. Trust model n cloud computng has to face many new challenges n contrast wth the tradtonal model due to the dversty of trust relatonshps n cloud envronment, whch nvolves so many dfferent relatonshps between and among end users, data owners, cloud servce provders, and even among other cloud components. That a cloud user trusts a cloud server, n fact, s not equal to that the cloud server does trust the cloud user as well. Here, cloud users mght be defned as trustor and Cloud servers or servce provders as trustee, and the trustor tres to trust n the ablty of trustee to provde some servce n accordance wth an agreed polcy. On the other hand, trustee as a trustworthy party should assure that ts resources can be accessed effectvely and effcently by trustor. Wthout a trustworthy computng envronment, servce provders wll have lttle ncentve to contrbute ther computng or servce resources and cloud users may hestate to nteract wth servce provders because of the possblty of recevng corrupted or posoned fles or beng exploted by malware. IV. DIRICHLET DISTRIBUTION APPROACH A. Drchlet Dstrbuton for Cloud Computng The Drchlet dstrbuton[23] often denoted Dr(, s a famly of contnuous multvarate probablty dstrbutons parameterzed by vector a of postve reals. It s the multvarate generalzaton of beta dstrbuton. Drchlet dstrbutons are very often used as pror dstrbutons n Bayesan statstcs, and n fact the Drchlet dstrbuton s the conugate pror of categorcal dstrbuton and multnomal dstrbuton. That s, ts probablty densty functon (PDF returns the belef that the probabltes of rval events are x gven that each event has been observed tmes. Let x ( x,..., x be a random probablty functon, that s x 0 for,2,..., and x. In addton, suppose that [,..., ], wth 0 for each, and let 0. Then, x s sad to have Drchlet dstrbuton wth parameter, whch we denote by x ~ Dr(. The Drchlet PDF can express as below: ( f x x x, where (,, ;,, ( ( ( s denotes s the gamma functon, whch s a generalzaton of the factoral functon and ( n n ( n ( n! for postve nteger n. The mean of a Drchlet dstrbuton s denoted as Mean( x / where 0. Drchlet dstrbuton 0 serves as a conugate pror for the probablty parameter x of the multnomal dstrbuton and permts an effcent way to update the estmated dstrbuton as a result of a new experence by ust ncrementng the correspondng parameter, whch provdes a satsfactory mathematcal foundaton for cloud trust systems that use multlevel trust metrc. Ths mportant property means the posteror trust degree can be computed by combnng the pror trust 38

4 degree wth a new nteracton experence. Whle n cloud computng envronment, a user s allowed to evaluate a servce provder, wth any level from a set of predefned dfferent dscrete levels. Ths translates nto havng a state space of cardnalty for the Drchlet dstrbuton. Each trust level can be ndexed by and each x s the probablty at level, for a -level trust space. Note that, for parameters (,..., where each corresponds to the count of experences at the level. The Drchlet posteror dstrbuton of x after occurrences of level experence wth probablty x can be represented as f ( x,..., x ;,..., where the pror dstrbuton of x s unform. Snce the Drchlet dstrbuton s a conugate pror of the multnomal dstrbuton, the posteror dstrbuton does also sever the Drchlet dstrbuton, and stands for addton operaton. whch usually s also descrbed as shown below: ( ( c f x c x ( c (, ( ( ( c base ratng value and, where s a c s a pror constant whch s equal to the cardnalty of the state space. Then, the dstrbuton of a cloud user's dynamc trustworthness can be computed usng Drchlet PDF (, 2,, wth the count of experence at level. The best estmate for overall trust degree value s the expectaton of probablty functon,.e. the mean ( / 0,, / 0, and the best estmate for confdence n ndvdual mean s ts varance as shown follows: Varance ( x ( ( / ( ( The approach based on Drchlet dstrbuton PDF allows cloud users to update the trust degree value after transactons between cloud users and servce provders are taen place. The trust degree level can be characterzed by 5-star ratng model as sgnfyng (very untrustworthy, untrustworthy, neutral, trustworthy, very trustworthy whch s collected from cloud users' feedbac. A default base experence value can be assumed to set equal to /5 for all levels before any experence score values have been receved. B. Overall trust degree value In our approach, T, ( t denotes the overall trust degree value that cloud user has on cloud servce provder at tme t, whch s wthn the range of [0, ] and wth "0" ndcatng dstrust and "" ndcatng trust fully. The hgher value of T, ( t ndcates the stronger belef that cloud user has on servce provder. Let D, ( t and R, ( t denote the drect trust degree value and ndrect degree (reputaton value that -user trusts -provder at tme t, respectvely. Then, T, ( t can be defned as T, ( t, D, ( t (, R, ( t, where, [0,] s a parameter reflectng the confdence that the user trusts servce provder drectly. C. Confdence Factor and Tme Factor Obvously, the more the number of the nteractons among cloud users and servce provders, the hgher weght the trust degree, and the hgher confdence factor whch s ntroduced to estmate the trust degree. It s worth mentonng that the confdence factor level should be maxmzed f the trust degree s greater than a trust threshold value. For the sae of smplcty and ease of calculaton, we defne the functon as:, r, c, where r, s a postve nteger or zero whch counts the number of transactons between the -user wth the -provder and c s a constant value whch can be preset (usually c 2 by organzer accordng to the ntal value whle the frst transacton s tang place. Because of the dynamc characterstc of cloud computng, the ndrect trust degree (reputaton between cloud users and servce provders wll gradually reduce along tme ncreasng. Partcularly, f transacton record exceeds a gven tme nterval, reputaton trust degree has less weght 382

5 than trust degree that calculated fnally. Tme decay factor, to estmate reputaton s defned as, where,, denotes the tme stamp of transacton between the -user and the, -provder and s an agng factor (0. The value of s calculated usng the decson functon[24], as shown below. where ( s the comparson functon that returns the average ranng dstance of all the tems. Obvously, the smaller the value of, the more emphass puttng on the tme decay factor. Meanwhle, the longer the tme pasts, the more the trust degree reduces. Algorthm: The algorthm for determnng the value of the agng factor : Functon Agng_Factor 2: ; 0 ; ( 3: whle true do 4: 2 0 ; 2 ( 2 5: f 2 / 0. then 6: Return 7: End f ; 2; 8: 2 9: end whle V. SIMULATIONS In our experments, the performance and the effectveness of mult-level Drchlet dstrbuton approach for cloud computng system are traced and smulated on some well-nown attacs. Bad mouthng attac, ballot-stuffng attac, on-off attac and conflctng behavor attac are used to observed as long as recommendatons are taen nto consderaton. Table I. Comparatve Analyss of Robustness to Attacs Approach Beta-PDF D-S Evdence Our Approach Bad Mouthng Robust Robust Robust Ballot-Stuffng Wea Wea Robust On-off Wea Robust Robust Conflctng Behavor Wea Wea Robust As shown n table above, Drchlet dstrbuton approach for cloud computng s effectve and effcency on varous well-nown attacs whle comparng wth approaches of Beta-PDF and D-S Evdence theory. VI. CONCLUSION It s a crtcal ssue to establsh a certan trustworthy computng mechansm or approach to help a cloud user n selectng a relable servce provder n cloud envronment. The approach of mult-level Drchlet dstrbuton to compute the value of trust degree for cloud computng s proposed n ths paper and the confdence factor and tme decay factor are both tang nto account for trust evoluton. The dstrbuton of a cloud user's dynamc trustworthness can be computed by usng Drchlet dstrbuton PDF n accordance wth the hstory-based experences or recommendatons. Smulatons are to be set up to observe the performance of the approach, and t s shown n the smulatons that Drchlet dstrbuton approach for cloud computng s effectve and effcent for a cloud user to compute the value of trust degree. ACKNOWLEDGEMENT The authors would le to than the support by the Natonal Natural Scence Foundaton of Chna under Grant (Nos and the Natural Scence Foundaton of Jangsu Educaton Department under grant of the year 2009 and 202, respectvely. The wor s supported by Jangsu Provnce Scholarshps of Overseas Studes under grant of the year 200. REFERENCES [] Mell P, GranceT, "The NIST defnton of cloud computng (draft," NIST, ud-defnton. pdf ; 20. [2] S. P. Marsh, "Formalsn Trust as a Computatonal Concept," Ph.D. Dssertaton, Unversty of Strlng, 994. [3] A. Jøang, "An algebra for assessng trust n certfcaton chans," Proceedngs of the Networ and Dstrbuted Systems Securty (NDSS 99 Symposum,

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