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Sajal K. Das

Decision and Game Theory for Security

4th International Conference, GameSec 2013 Fort Worth, TX, USA, November 2013

Proceedings

LectureNotesinComputerScience8252

CommencedPublicationin1973

FoundingandFormerSeriesEditors: GerhardGoos,JurisHartmanis,andJanvanLeeuwen

EditorialBoard

DavidHutchison LancasterUniversity,UK

TakeoKanade

CarnegieMellonUniversity,Pittsburgh,PA,USA

JosefKittler UniversityofSurrey,Guildford,UK

JonM.Kleinberg

CornellUniversity,Ithaca,NY,USA

AlfredKobsa UniversityofCalifornia,Irvine,CA,USA

FriedemannMattern ETHZurich,Switzerland

JohnC.Mitchell StanfordUniversity,CA,USA

MoniNaor

WeizmannInstituteofScience,Rehovot,Israel

OscarNierstrasz UniversityofBern,Switzerland

C.PanduRangan IndianInstituteofTechnology,Madras,India

BernhardSteffen TUDortmundUniversity,Germany

MadhuSudan MicrosoftResearch,Cambridge,MA,USA

DemetriTerzopoulos UniversityofCalifornia,LosAngeles,CA,USA

DougTygar UniversityofCalifornia,Berkeley,CA,USA

GerhardWeikum

MaxPlanckInstituteforInformatics,Saarbruecken,Germany

SajalK.DasCristinaNita-Rotaru MuratKantarcioglu(Eds.)

4thInternationalConference,GameSec2013 FortWorth,TX,USA,November11-12,2013

Proceedings

VolumeEditors

SajalK.Das

MissouriUniversityofScienceandTechnology,DepartmentofComputerScience 500West15thStreet,325BComputerScienceBuilding,Rolla,MO65409,USA E-mail:sdas@mst.edu

CristinaNita-Rotaru

PurdueUniversity,DepartmentofComputerScience,LWSN2142J 305N.UniversityStreet,WestLafayette,IN47907,USA E-mail:cnitarot@purdue.edu

MuratKantarcioglu UniversityofTexasatDallas,DataSecurityandPrivacyLab 800W.CampbellRoad,MSEC31,Richardson,TX75080,USA E-mail:muratk@utdallas.edu

ISSN0302-9743e-ISSN1611-3349 ISBN978-3-319-02785-2 e-ISBN978-3-319-02786-9 DOI10.1007/978-3-319-02786-9

SpringerChamHeidelbergNewYorkDordrechtLondon

LibraryofCongressControlNumber:2013950386

CRSubjectClassification(1998):C.2.0,J.1,D.4.6,K.4.4,K.6.5,F.1-2

LNCSSublibrary:SL4–SecurityandCryptology

©SpringerInternationalPublishingSwitzerland2013

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Preface

Securityisamultifacetedproblemareathatrequiresacarefulappreciationof manycomplexitiesregardingtheunderlyingtechnicalinfrastructureaswellasof human,economic,andsocialfactors.Securingresourcesinvolvesdecisionmakingonmultiplelevelsofabstractionwhileconsideringvariableplanninghorizons. Atthesametime,theselectionofsecuritymeasuresneedstoaccountforlimitedresourcesavailabletobothmaliciousattackersandadministratorsdefending networkedsystems.Variousdegreesofuncertaintyandincompleteinformation abouttheintentionsandcapabilitiesofmiscreantsfurtherexacerbatethestruggletoselectappropriatemechanismsandpolicies.

TheGameSecconferenceaimstobringtogetherresearchersworkingonthe theoreticalfoundationsandbehavioralaspectsofenhancingsecuritycapabilitiesinaprincipledmanner.Thesuccessfuleditionsoftheconferenceseries inthepastthreeyearstookplaceinBerlin,Germany(2010),CollegePark, Maryland,USA(2011),andBudapest,Hungary(2012).Contributionstothese meetingsincludedanalyticmodelsbased ongame,information,communication, optimization,decision,andcontroltheoriesthatwereappliedtodiversesecurity topics.Inaddition,researcherscontributedpapersthathighlightedtheconnectionbetweeneconomicincentivesandreal-worldsecurity,reputation,trust,and privacyproblems.Webelievethatsuchcontributionswillplayanimportantrole indefininganddevelopingthescienceofcybersecurity.

The4thInternationalConferenceonDecisionandGameTheoryforSecurity (GameSec2013)tookplaceinFortWorth,Texas,USA,onNovember11–12, 2013.Inresponsetothegeneralcallforpapers,manypaperswerereceived coveringvariouseconomicaspectsofsecurityandprivacy.Theinternational ProgramCommitteeevaluatedthesubmittedpapersbasedontheirsignificance, originality,technicalquality,andexposition.

Thiseditedvolumeoftheconferenceproceedingscontainsfivefullpapers, threeshortpapers,andseveninvitedpapersthatconstitutedtheconferenceprogramdividedintoseveralsessionsheldovertwodays.Inaddition,theconference programhadtwoexcitingkeynotetalksdeliveredbyProf.AndrewOdlyzko(UniversityofMinnesota)andanotherfromDr.CliffWang(ArmyResearchOffice). Wesincerelythankalltheorganizingmembers(listedhere)fortheirhardwork.

November2013SajalK.Das

CristinaNita-Rotaru MuratKantarcioglu

Organization

ProgramCommittee

TansuAlpcanTheUniversityofMelbourne,Australia

RossAndersonCambridgeUniversity,UK

JohnBarasUniversityofMaryland,USA

TamerBasarUniversityofIllinoisatUrbana-Champaign, USA

AlvaroCardenasFujitsuLaboratoriesofAmerica

NicolasChristinCarnegieMellonUniversity,USA

JohnChuangUCBerkeley,USA

MarkFelegyhaziBudapestUniversityofTechnologyand Economics(BME),Hungary JensGrossklagsPennStateUniversity,USA

CelineHoeUniversityofTexasatDallas,USA

BenjaminJohnsonUCBerkeley,USA

EduardJorswieckTechnicalUniversityDresden,Germany

MuratKantarciogluUniversityofTexasatDallas,USA

Xiang-YangLiIllinoisInstituteofTechnology,USA

RefikMolvaEURECOM,France

TylerMooreSouthernMethodistUniversity,USA

JohnMusacchioUniversityofCalifornia,SantaCruz,USA

CristinaNita-RotaruPurdueUniversity,USA

MehrdadNojoumianSouthernIllinoisUniversity,USA

AndrewOdlyzkoUniversityofMinnesota,USA

AlinaOpreaRSALabs,USA

StefanSchmidTUBerlin&TelekomInnovationLaboratories, Germany

RaduStateUniversityofLuxembourg

NanZhangTheGeorgeWashingtonUniversity,USA

OrganizingCommittee

GeneralChair

SajalK.DasMissouriUniversityofScienceandTechnology, USA

TPCCo-chairs

MuratKantarciogluUniversityofTexasatDallas,USA

CristinaNita-RotaruPurdueUniversity,USA

LocalArrangementsChair

MatthewWrightUniversityofTexasatArlington,USA

FinanceandRegistrationChair

DonggangLiuUniversityofTexasatArlington,USA

PublicityChair

Yingying(Jennifer)ChenStevensInstituteofTechnology,USA

Webmaster

VaibhavKhadilkarUniversityofTexasatDallas,USA

OnCommunicationoverGaussianSensorNetworkswithAdversaries: FurtherResults .................................................. 1 EmrahAkyol,KennethRose,andTamerBa¸sar ATrueRandomGeneratorUsingHumanGameplay

MohsenAlimomeni,ReihanehSafavi-Naini,andSetarehSharifian AGameTheoreticAnalysisofCollaborationinWikipedia

S.Anand,OferArazy,NarayanB.Mandayam,andOdedNov ControllabilityofDynamicalSystems:ThreatModelsandReactive

CarlosBarreto,AlvaroA.C´ardenas,andNicanorQuijano AdaptiveRegretMinimizationinBounded-MemoryGames

JeremiahBlocki,NicolasChristin,AnupamDatta,and AruneshSinha

LanahEvers,AnaIsabelBarros,andHermanMonsuur

YiHan,TansuAlpcan,JeffreyChan,andChristopherLeckie

MonotonicMaximin:ARobustStackelbergSolutionagainstBoundedly RationalFollowers ...............................................

AlbertXinJiang,ThanhH.Nguyen,MilindTambe,and ArielD.Procaccia

DefeatingTyrannyoftheMassesinCrowdsourcing:Accounting forLow-SkilledandAdversarialWorkers

AdityaKurve,DavidJ.Miller,andGeorgeKesidis QuantifyingNetworkTopologyRobustnessunderBudgetConstraints:

AronLaszkaandAssaneGueye

AronLaszka,BenjaminJohnson,andJensGrossklags

AhtoBuldasandAleksandrLenin

WenlianLu,ShouhuaiXu,andXinleiYi

JohnRossWallrabensteinandChrisClifton

QuanyanZhuandTamerBa¸sar

EmrahAkyol1 ,KennethRose1 ,andTamerBa¸sar2

1 UniversityofCalifornia,SantaBarbara {eakyol,rose}@ece.ucsb.edu

2 UniversityofIllinois,Urbana-Champaign basar1@illinois.edu

Abstract. Thispaperpresentsnewresultsonthegametheoreticalanalysisofoptimalcommunicationsstrategiesoverasensornetworkmodel. OurmodelinvolvesonesingleGaussiansourceobservedbymanysensors,subjecttoadditiveindependentGaussianobservationnoise.Sensors communicatewiththereceiveroveranadditiveGaussianmultipleaccess channel.Theaimofthereceiveristoreconstructtheunderlyingsource withminimummeansquarederror.Thescenarioofinteresthereisone wheresomeofthesensorsactasadversary(jammer):theyaimtomaximizedistortion.Whileourrecentpriorworksolvedthecasewhereeither allornoneofthesensorscoordinate(userandomizedstrategies),the focusofthisworkisthesettingwhereonlyasubsetofthetransmitter and/orjammersensorscancoordinate.Weshowthatthesolutioncruciallydependsontheratioofthenumberoftransmittersensorsthatcan coordinatetotheonesthatcannot.Ifthisratioislargerthanafixed thresholddeterminedbythenetworksettings(transmitandjamming power,channelnoiseandsensorobservationnoise),thentheproblemis azero-sumgameandadmitsasaddlepointsolutionwheretransmitters withcoordinationcapabilitiesuserandomizedlinearencodingwhilethe restofthetransmittersensorsisnotusedatall.Adversarialsensorsthat cancoordinategenerateidenticalGaussiannoisewhileotheradversaries generateindependentGaussiannoise.Otherwise(ifthatratioissmaller thanthethreshold),theproblembecomesaStackelberggamewherethe leader(alltransmittersensors)usesfixed(non-randomized)linearencodingwhilethefollower(alladversarialsensors)usesfixedlinearencoding withtheoppositesign.

Keywords: Gametheory,sensornetworks,source-channelcoding, coordination.

1Introduction

Communicationsoversensorsnetworksisanactiveresearchareaofferingarich setofproblemsoftheoreticalandpracticalsignificance,seee.g.,[8]andthe

S.K.Das,C.Nita-Rotaru,andM.Kantarcioglu(Eds.):GameSec2013,LNCS8252,pp.1–9,2013. c SpringerInternationalPublishingSwitzerland2013

referencestherein.Gametheoreticconsiderations,i.e.,thepresenceofadversary anditsimpactonthedesignofoptimal communicationstrategieshavebeen studiedforalongtime[9,10].Inthispaper,weextendourpriorworkonthegame theoreticanalysisofGaussiansensornetworks,onaparticularmodelintroduced in[7],byutilizingtheresultsonthegametheoreticanalysisoftheGaussiantest channelin[3–6].

Inthispaper,weconsiderthesensornetworkmodelillustratedinFigure1and explainedindetailinSection2.Thefirst M sensors(i.e.,thetransmitters)and thereceiverconstitutePlayer1(minimizer)andtheremaining K sensors(i.e., theadversaries)constitutePlayer2(maximizer).Thiszero-sumgamedoesnot admitasaddle-pointinpurestrategies(fixedencodingfunctions),butadmits oneinmixedstrategies(randomizedfunctions).

Ourpriorworkconsideredtwoextremalsettings[2],dependingonthe“coordination”capabilitiesofthesensors.Coordinationherereferstotheabilityof usingrandomizedencoders,i.e.,alltransmittersensorsandthereceiver;andalso theadversariesamongthemselvesagreeonsome(pseudo)randomsequence,denotedas {γ } (fortransmittersandthereceiver)and {θ } (foradversaries)inthe paper.Themainmessageofourpriorworkisthat“coordination”playsapivotal roleintheanalysisandtheimplementationofoptimalstrategiesforboththe transmitterandadversarialsensors.Dependingonthecoordinationcapabilities ofthethetransmittersandtheadversaries,weconsideredtwoextremesettings. Inthefirstsetting,weconsideredthemoregeneralcaseofmixedstrategiesand presentthesaddle-pointsolutioninTheorem1.Inthesecondsetting,encoding functionsoftransmittersarelimitedtothefixedmappings.Thissettingcanbe viewedasaStackelberggamewherePlayer1istheleader,restrictedtopure strategies,andPlayer2isthefollower,whoobservesPlayer1’schoiceofpure strategiesandplaysaccordingly.

Inthispaper,weconsideramorepracticalsettingwhereonlyagivensubsetof thetransmittersandalsotheadversarialsensorscancoordinate.Ourmainresult is:ifthenumberoftransmittersensorsthatcancoordinateislargeenoughcomparedtoonesthatcannot,thentheproblembecomesazero-sumgamewitha saddlepoint,wherethecoordinationcapabletransmittersuserandomizedlinear strategyandincapabletransmittersarenotusedatall.Discardingthesetransmittersensorsisrathersurprisingbutthegainfromcoordinationcompansates forthisloss.Coordinationisalsoimportantfortheadversarialsensors.When transmitterscoordinate,adversariesmustalsocoordinatetogenerateidentical realizationsofGaussianjammingnoise.Incontrastwithtransmitters,theadversarialsensorswhichcannotcoordinateisofuse:theygenerateindependent copiesoftheidenticallydistributedGaussianjammingnoise.Otherwise,i.e., thenumberofcoordinatingtransmittersarenotlargeenough,transmittersuse deterministic(purestrategies)linearencoding,i.e., gT (X )= αT X andoptimal adversarialstrategyisalsouncodedcommunicationsintheoppositedirectionof thetransmitters,i.e., gA (X )= αA X forsome αT ,αA ∈ R+ .Forbothsettings, uncodedcommunicationisoptimalandseparatesourceandchannelcodingis strictlysuboptimal.

GaussianSensorNetworkswithAdversaries3

Thispaperisorganizedasfollows.InSection2,wepresenttheproblemdefinition.Wereviewpriorwork,particularly[2]inSection3.InSection4,wepresent ourmainresultandfinallyweprovideconclusionsinSection5.

2ProblemDefinition

Ingeneral,lowercaseletters(e.g., x)denotescalars,boldfacelowercase(e.g., x)vectors,uppercase(e.g., U,X )matricesandrandomvariables,andboldface uppercase(e.g., X )randomvectors. E( ), P( )and R denotetheexpectationand probabilityoperators,andthesetofrealnumbersrespectively. Bern(p)denotes theBernoullirandomvariable,takingvalues1withprobability p and 1with 1 p.Gaussiandistributionwithmean μ andcovariancematrix R isdenotedas N (μ,R ).

ThesensornetworkmodelisillustratedinFigure1.Theunderlyingsource {S (i)} isasequenceofi.i.d.realvaluedGaussianrandomvariableswithzero meanandvariance σ 2 S .Sensor m ∈ [1: M + K ]observesasequence {Um (i)} definedas Um (i)= S (i)+ Wm (i), (1)

where {Wm (i)} isasequenceofi.i.d.Gaussianrandomvariableswithzeromean andvariance σ 2 Wm ,independentof {S (i)}.Sensor m ∈ [1: M + K ]canapply arbitraryBorelmeasurablefunction g N m : RN → R totheobservationsequence oflength N , U m soastogeneratesequenceofchannelinputs Xm (i)= g N m (U m ) underpowerconstraint:

Thechanneloutputisthengivenas

where {Z (i)} isasequenceofi.i.d.Gaussianrandomvariablesofzeromean andvariance σ 2 Z ,independentof {S (i)} and {Wm (i)}.Thereceiverappliesa Borelmeasurablefunction hN : RN → R tothereceivedsequence {Y (i)} to minimizethecost,whichismeasuredasmeansquarederror(MSE)betweenthe underlyingsource S andtheestimateatthereceiver ˆ S as

))2 } (4)

for m =1, 2,...,M + K .

Thetransmitters g N m ( )for m ∈ [1: M ]andthereceiver hN ( )seektominimize thecost(4)whiletheadversariesaimtomaximize(4)byproperlychoosing g N k ( )

4E.Akyol,K.Rose,andT.Ba¸sar for k ∈ [M +1: M + K ].Wefocusonthesymmetricsensorandsymmetricsource where Pm = PT and σ 2 W

and P

= PA , ∀k ∈ [M +1: M + K ].

Atransmitter-receiver-adversarialpolicy(g N ∗ m ,g N

k ,hN ∗ )constitutesasaddlepointsolutionifitsatisfiesthepairofinequalities

Thesensornetworkmodel

3ReviewofPriorWork

3.1FullCoordination

Firstscenarioisconcernedwiththesettingwhere”all”transmittersensorshave theabilityto coordinate,i.e.,alltransmittersandthereceivercanagreeon ani.i.d.sequenceofrandomvariables {γ (i)} generated,forexample,byaside channel,theoutputofwhichis,however,notavailabletotheadversarialsensors1 .Theabilityofcoordinationallowstransmittersandthereceivertoagree onrandomizedencodingmappings.Surprisingly,inthissetting,theadversarial sensorsalsoneedtocoordinate,i.e.,agreeonani.i.d.randomsequence,denoted as {θ (i)},togeneratetheoptimaljammingstrategy.Thesaddlepointsolution ofthisproblemispresentedinthefollowingtheorem.

1 Analternativepracticalmethodtocoordinateistogeneratetheidenticalpseudorandomnumbersateachsensor,basedonpre-determinedseed.

Fig.1.

Theorem1([2]). Theoptimalencodingfunctionforthetransmittersisrandomizeduncodedtransmission:

where γ (i) isi.i.d.Bernoulli( 1 2 )overthealphabet {−1, 1}

Theoptimaljammingfunction(foradversarialsensors)istogeneratei.i.d. Gaussianoutput

where θ (i) ∼N (0,PA ), (9) andisindependentoftheadversarialsensorinput Uk (i).Theoptimalreceiver istheBayesianestimatorof S given Y ,i.e.,

Costatthissaddlepointasafunctionofthenumberoftransmitterandadversarialsensorsis:

where

Theprooffollowsfromverificationofthefactthatthemappingsinthistheoremsatisfythesaddlepointcriteriagivenin(5).

Remark1. Coordinationisessentialforadversarialsensorsinthecaseofcoordinatingtransmittersandreceiver,inthesensethatlackofadversarialcoordinationstrictlydecreasestheoverallcost.

3.2NoCoordination

Inthissection,wefocusontheproblem,wherethetransmittersdonothavethe abilitytosecretlyagreeonarandomvariable,i.e.,“coordination”togenerate theirtransmissionfunction Xk .Inthiscase,ouranalysisyieldsthattheoptimal transmitterstrategy,whichisalmostsurelyunique,isuncodedtransmission withlinearmappings,whiletheadversarialoptimalstrategyforthe(jamming) sensorsisuncoded,linearmappingswiththeoppositesignofthetransmitter functions.Thefollowingtheorempresentsourmailresultsassociatedwith“no coordination”setting.Arathersurprisingobservationisthattheadversarial coordinationisuselessforthissetting, i.e.,eveniftheadversarialsensorscan

cooperate,theoptimalmappingsandhencetheresultingcostatthesaddle pointdoesnotchange.Notehoweverthat,aswewillshowlater,coordination capabilityofadversarialsensorsisessentialinthesecondextremalsettingwhere transmittersareallowedtocoordinatetheirchoices.

Theorem2([2]). Theoptimalencodingfunctionforthetransmittersisuncodedtransmission,i.e.,

Theoptimaljammingfunction(foradversarialsensors)isuncodedtransmissionwiththeoppositesignofthetransmitters,i.e.,

TheoptimaldecodingfunctionistheBayesianestimatorof S given Y ,i.e.,

Costasafunctionof M and K is

Theproofoftheorem,canbefoundin[2],involvesdetailedinformationtheoreticanalysisandisomittedhereforbrevity.Thisproblemsettingimpliesa StackelberggamewheretransmittersandthereceiverplayfirstasthePlayer1, astheyselecttheirencodingfunctions.Then,Player2(theadversarialsensors), knowingthechoiceofPlayer1,choosesitsstrategy.

Remark2. Notethatinthissetting,thecoordinationcapabilityfortheadversariesdonothelp,insharpcontrasttotheprevioussettingwhere,bothtransmittersandadversariescoordinate.

4MainResult

Thefocusofthispaperisthesetting betweenthetwoextremescenariosof coordination,namelyfullornocoordination.Weassumethat M transmitter sensorscancoordinatewiththereceiverwhile M (1 )ofthemcannotcoordinate.Similartotransmitters,only Kη oftheadversarialsensorscancoordinate while K (1 η )adversarialsensorscannotcoordinate.Letusassume,without lossofgenerality,thatfirst M transmittersand Kη adversariescancoordinate. Letusalsodefinethequantity 0 asthesolutionto:

C (M 0 , K 2 η 2 + K (1 η ))= JNC (M,K )(16)

Thefollowingtheoremcapturesourmainresult.

Theorem3. If > 0 , M capabletransmittersuserandomizedlinearencoding, whileremaining M (1 ) transmittersarenotused.

where γ (i) isi.i.d.Bernoulli( 1 2 )overthealphabet {−1, 1}

Theoptimaljammingpolicy(forthecapableadversarialsensors)istogenerate theidenticalGaussiannoise

whileremainingadversarieswillgenerateindependentGaussiannoise

areindependentoftheadversarialsensorinput Uk (i). If < 0 ,thentheoptimalencodingfunctionforalltransmittersisdeterministiclinearencoding,i.e.,

Theoptimaljammingfunction(foradversarialsensors)isuncodedtransmissionwiththeoppositesignofthetransmitters,i.e.,

Proof. Thetransmittershavetwochoices:i)Alltransmitterswillchoosenot touserandomization.Then,theadversarialsensorsdonotneedtouserandomizationsincetheoptimalstrategyisdeterministic,linearcodingwiththe oppositesign,asillustratedinTheorem2.Hence,costassociatewiththisoptionis JNC (M,K ).ii)Capabletransmitterswilluserandomizedencoding.This choiceimpliesthatremainingtransmittersdonotsendinformationasthey donothaveaccesstorandomizationsequence {γ },hencetheyarenotused. Theadversarialsensorswhichcancoordinategenerateidenticalrealizationof theGaussiannoisewhile,remainingadversariesgenerateindependentrealizations.Thetotaleffectivenoiseadversarialpowerwillbe((Kη )2 +(1 η )K )PA , andthecostassociatedwiththissettingis JC (M , K 2 η 2 + K (1 η )).Hence, thetransmitterwillchoosebetweentwooptionsdependingontheircosts,

JC (M , K 2 η 2 + K (1 η ))and JNC (M,K ).Since, JC isadecreasingfunctionin M andhencein ,whenever > 0 ,transmittersuserandomization(and hencesodotheadversaries),otherwiseproblemsettingbecomesidenticalto”no coordination”.TherestoftheproofsimplyfollowsfromtheproofsofTheorem 1and2andisomittedhereforbrevity.

Remark3. Notethatinthefirstregime( > 0 ),wehaveazero-sumgamewith saddlepoint.Inthesecondregime( < 0 ),wehaveaStackelberggamewhere alltransmittersandreceiverconstitutethe leaderandadversariesconstitutethe follower.

5Conclusion

Inthispaper,wepresentednewresultsonthegametheoreticalanalysisofoptimalcommunicationstrategiesoverasensornetwork.Ourrecentprior[2]work hadsolvedtwoextremecoordinationcaseswhereeitherallornoneofthesensors coordinate.Inthiswork,wefocusedonthesettingwhereonlyasubsetofthe transmitterand/orjammersensorscancoordinate.Weshowedthatthesolution cruciallydependsonthenumberoftransmittersandadversariesthatcancoordinate.Inoneregime,thentheproblemisazero-sumgameandadmitsasaddle pointsolutionwheretransmitterswithcoordinationcapabilitiesuserandomized linearencodingwhiletheremainingthetransmittersensorsarenotusedatall. AdversarialsensorsthatcancoordinategenerateidenticalGaussiannoisewhile otheradversariesgenerateindependentGaussiannoise.Intheotherregime,the problembecomesaStackelberggamewheretheleader(alltransmittersensors) usesfixed(non-randomized)linearencodingwhilethefollower(alladversarial sensors)usesfixedlinearencodingwiththeoppositesign.

Ouranalysishasuncoveredaninterestingresultregardingthemixedsetting consideredinthispaper.Theoptimalstrategyfortransmitterssensorscanbe todiscardtheonesthatcannotcoordinate.Notethatthecoordinationaspect oftheproblemisentirelyduetogame-theoreticconsiderations,whicharealso highlightedinthissurprisingresult.

Severalquestionsarecurrentlyunderinvestigation,includingextensionsofthe analysistovectorsourcesandchannels,theasymptotic(inthenumberofsensors M and K )analysisoftheresultspresentedhere;andextensionofouranalysis toasymmetricand/ornon-Gaussiansettings.Aninitialattempttoextendthe resultsassociatedwiththeGaussiantestchanneltonon-Gaussiansettingcan befondin[1].

Acknowledgments. ThisworkwassupportedinpartbytheNSFundergrants CCF-1016861,CCF-1118075andCCF-1111342.

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1.Akyol,E.,Rose,K.,Ba¸sar,T.:Onoptimaljammingoveranadditivenoisechannel. draft, http://arxiv.org/abs/1303.3049

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ATrueRandomGenerator UsingHumanGameplay

MohsenAlimomeni,ReihanehSafavi-Naini,andSetarehSharifian

UniversityofCalgary,DepartmentofComputerScience,Canada {malimome,rei,ssharifi}@ucalgary.ca

Abstract. TrueRandomnessGenerators(TRG)usetheoutputofan entropysourcetogenerateasequenceofsymbolsthatissufficientlyclose toauniformlyrandomsequence,andsocanbesecurelyusedinapplicationsthatrequireunpredictabilityofeachsymbol.ATRGalgorithm generallyconsistsof(i)anentropysourceand(ii)anextractoralgorithm thatusesarandom seed toextracttherandomnessoftheentropysource. WeproposeaTRGthatusestheuserinputinagameplayedbetween theuserandthecomputerbothastheoutputofanentropysource,and therandomseedrequiredfortheextractor.Animportantpropertyof thisTRGisthatthe(randomness)qualityofitsoutputcanbeflexibly adjusted.Wedescribethetheoreticalfoundationoftheapproachand designandimplementagamethatinstantiatestheapproach.Wegive theresultsofourexperimentswithusersplayingthegame,andanalysis oftheresultingoutputstrings.Ourresultssupporteffectivenessofthe approachingeneratinghighqualityrandomness.Wediscussourresults andproposedirectionsforfuturework.

1Introduction

Manysecuritysystemsandinparticularcrypto-algorithmsuserandomvalues asaninputtothesystem.Cryptosystemsneedrandomnessforpurposessuch askeygeneration,datapaddingorchallengeinchallenge-responseprotocols. Innearlyallcasesunpredictabilityofrandomvaluesiscriticaltothesecurity ofthewholesystems.Generatingtruerandomnesshoweverisnotaneasytask andneedsaphysicalentropysource.OperatingsystemssuchasWindowsand Linuxusespecialsub-systemsthatcombinerandomnessfromdifferentpartsof thehardwareandsoftwaresystemtocollectentropy[Mic,GPR06].Poorchoices ofrandomnesshasleadtobreakdownofnumeroussecurity systems.Importantexamplesofsuchfailuresare,attackonNetscapeimplementationofthe SSLprotocol[GD]andweaknessofentropycollectioninLinuxandWindows Pseudo-RandomGenerator[GPR06,DGP09].Arecentexampleoftheneedfor carefultreatmentofrandomnessincryptographicsystemswashighlightedin [HDWH12,LHA+ 12]wherethesamepublicandprivatekeysweregeneratedby keygenerationmodulesandthiswaspartlyattributedtorandomnessthatwas poorlygeneratedinLinuxkernelrandomnessgenerationsubsystem.

An entropysource usesphysicalprocessessuchasnoiseinelectroniccircuits, orsoftwareprocessesthatare“unpredictable”,tooutputasequenceoveran

S.K.Das,C.Nita-Rotaru,andM.Kantarcioglu(Eds.):GameSec2013,LNCS8252,pp.10–28,2013. c SpringerInternationalPublishingSwitzerland2013

alphabetthatishighly“unpredictable”,whereunpredictabilityismeasuredby min-entropy (SeeDefinition3).Althoughtheoutputofanentropysequence canhavehighlevelofrandomness,buttheunderlyingdistributionmaybefar fromuniform.Tomaketheoutputofanentropysourcetofollowauniform distribution,apostprocessingstepisusuallyused. Randomnessextractors are deterministicorprobabilisticfunctionsthattransformtheoutputofanentropy sourcetouniformdistributionusingamappingfrom n to m bits(usually m ≤ n), extractingtheentropyofthesource.

Toguaranteerandomnessoftheiroutput,randomnessextractorsneedguaranteeontherandomnesspropertyof(e.g.themin-entropy)theirinputentropy source.Extractorsthatcanextractrandomnessfromsourcesthatsatisfyalower boundontheirmin-entropy,areprobabilistic[Sha11].Aprobabilisticextractor hastwoinputs:anentropysourcetogetherwithasecondinputthatiscalled seed.Goodprobabilisticextractorsuseashortseed(logarithmicintheinputsize) toextractalltherandomness(closetothemin-entropy)oftheinputentropy source.A TrueRandomnessGenerator(TRG) thusconsistsoftwomodules:(i) anentropysourcethatgeneratesasequenceofsymbolswithalowerboundon itsmin-entropy,followedby,(ii)arandomnessextractor.Inpracticeoneneeds toestimatethemin-entropyoftheentropysourcetobeabletochooseappropriateparametersfortheextractor.Thedistributionoftheentropysourceand itsmin-entropymayfluctuateovertimeandsoaTRGneedstouseanextractor thatprovidessufficienttoleranceforthesefluctuations.

HumanasEntropySource: In[HN09],Halprinetal.proposedaninnovative approachtoconstructanentropysourceusinghumangameplay.Theirwork builtontheresultsinexperimentalpsychology.Itisknownthathumans,if askedtochoosenumbersrandomly,willdoapoorjobandtheirchoiceswillbe biased.Wagenaar[Wag72]usedexperimentsinwhichparticipantswereasked toproducerandomsequencesandnoted thatinallexperimentshumanchoices deviatedfromuniformdistribution.In[RB92],Rapportetal.throughaseries ofexperimentsshowedthatifhumanplaysacompetitivezero-sumgamewith uniformchoicesasthebeststrategy,theirchoiceswillbeclosetouniform.In theirexperimenttheyusedmatchingpenniesgameinwhicheachplayermakes achoicebetweenheadortailusinganunbiasedcoin,andthefirstplayerwinsif bothplayerschoosethesamesideandthesecond,iftheychoosedifferentside. Inthisgametheoptimalstrategyofusersisrandomselectionbetweenheadand tail.Theirresultshowedthatusersalmostfolloweduniformrandomstrategy confirmingthathumancanbeagoodsourceofentropyiftheyareengagedina strategicgameandentropygenerationisanindirectresultoftheiractions.

HumanGamePlayforGeneratingRandomness. Halprinetal.usedthesestudies toproposeanentropysourceusinghumangameplayagainstacomputer.In theirworkhumanplaysazero-sumgamewithuniformoptimalstrategyagainst thecomputer.Thegameisanextendedmatchingpenniesgame(userhasmore thantwochoices)andisplayedmanytimes.Thesequenceresultingfromhuman choicesisconsideredastheoutputofanentropysource,andisusedastheinput toarandomnessextractor.TheresultisaTRGwithanoutputthatisarandom

sequence“close”touniform.Inadditiontothehumaninputsequence,theTRG usesasecondsourceofperfectrandomnesstoprovideseedfortherandomness extractor.

Inthispaperweproposeanintegratedapproachwherethegameplaybetween ahumanandthecomputerisusedtoimplementthetwophasesofaTRG includingrandomnesssourceandrandomnessextractionphase.Thatistheuser’s inputprovidestherequiredrandomnessfortheentropysourceandtheextractor both.

1.1OurContribution

WeproposeaTRGthatuseshumangameplayagainstacomputer,asthe only sourceofrandomness.Thegameconsistsofasequenceofsub-games.Eachsubgameisasimpletwoplayerzero-sumgamebetweentheuserandthecomputer whichcanbeseenasanextendedmatchingpenniesgame.Ineachsub-game thehumanmakesachoiceamonganumberofalternatives,eachwiththesame probabilityofwinning,resultingintheuser’sbeststrategytoberandomselectionamongtheirpossiblechoices.Thefirstgamecorrespondstotheentropy generationstepinTRGandsubsequentsub-gamescorrespondtostepsofan extractoralgorithm.

TheTRGalgorithmisbasedonaseeded extractorthatisconstructedusingan expandergraph.Expandergraphsarehighlyconnected d-regulargraphs whereeachvertexisconnectedto d neighbours.Thisnotioniscapturedbya measurecalled spectralexpansion.Ithasbeenprovedthatrandomwalksonthese graphscanbeusedtoextractrandomness[AB09].Assuminganinitialprobabilitydistribution p onthesetofverticesofthegraph,itisproved[AB09]that bytakingarandomwalkof stepsfromanyvertexinthegraphthatischosen accordingto p,oneendsupatavertexthatrepresentsadistributionoverthe verticesthatis -closetotheuniformdistribution.Inotherwordsstartingfrom anydistribution,eachstepoftherandomwalkresultsinanewdistributionover theverticesthatisclosertouniformandsobytakingsufficientlylongwalk,one canobtainadistributionthatis -closetotheuniformdistribution.

Weusehumaninputtoprovidetherequiredrandomnessintheframework above:thatisfortheinitialdistribution p aswellastherandomnessforeach stepofthewalk.Toobtainrandomnessfromhuman,asequenceofgamesis presentedtotheuserandthehumaninputinthegameisusedintheTRG algorithm.Inthefirstsub-game,thegraphispresentedtotheuserwhowillbe askedtorandomlychooseavertex.Thischoicerepresentsasourcesymbolthat isgeneratedaccordingtosomeunknowndistribution p;thatishumanchoiceis effectivelyasymbolofanentropysource.Humanchoiceshowever,althoughhave someentropybutcannotbeassumedtobeuniformlydistributed.Asubsequent randomwalkoflength overthegraphwillbeusedtoobtainanoutputsymbol forTRGwithclosetouniformrandomnessguarantee.

Tousehumaninputforthegenerationoftherandomwalk,oneachvertexofthegraphtheuserispresentedwithasimplegamewhicheffectively requiresthemtochooseamongthesetoftheneighbouringvertices.Thegameis

zero-sumwithuniformoptimalstrategyandsothehumaninputwouldcorrespondtouniformselection,andconsequentlyone randomstep onthegraph.For agiven andanestimateforthemin-entropyoftheinitialvertexselection,one candeterminethenumberofrequiredrandomstepssothattheoutputofthe TRGhastherequiredrandomness.

Intheaboveweeffectivelyassume humaninputinauniformoptimalstrategy zero-sumgameisclosetouniform.Thisassumptionisreasonablewhenhuman ispresentedwithafewchoices,(basedontheexperimentsin[RB92]).Inpractice howeverthehumaninputwillbeclosetouniformandsotheproposedextraction processcanbeseenasapproximatingtherandomwalkbyahighmin-entropy walk.Obtainingtheoreticalresultsonthequalityofoutputinanexpandergraph extractorwhentherandomwalkisreplacedwithawalkwithhighmin-entropy isaninterestingtheoreticalquestion.Wehoweverdemonstratefeasibilityofthis approachexperimentally.

WedesignedandimplementedaTRGthatisbasedonagameona3-regular expandergraphwith10vertices.Thegameconsistsofasequenceofsub-games. AnumberofscreenshotsofthegameareshowninFigure1.Ineachsubgamethehumanandthecomputermakeachoicefromasetofvertices.Ifthe choicescoincide,computerwinsandiftheydonotcoincide,humanwins.Inour implementationthehumanfirstmakeachoiceandthenthecomputer’schoice isshown.Inthefirstsub-gameusermakesachoiceamongthe10verticesofthe graph,andinallsubsequentsub-games,amongthe3neighboursofthecurrent vertex.Weperformanumberofexperimentstovalidateourassumptions.

Experiments. Weimplementedtheabovegameandexperimentedwithninehumanusersplayingthegames.Wemeasuredmin-entropyofhumaninputinthe firstsub-game,thatiswhenhumanisanentropysource,andalsosubsequent sub-gameswhenhumaninputisusedtoemulatetherandomwalk.Fortheformer,thatistoestimatetheinitialdistribution p,wedesignedaoneroundgame whichrequirestheusertochooseavertexofthegraphandtheywiniftheir choiceisnotpredictablebythecomputer.WeusedNIST[BK12]testsforestimatingthemin-entropyofbothdistributions.Thedetailsofexperimentsare giveninSection4.Ourresultsonceagainshowsthathumans,onceengagedin atwo-partyzerosumgamewithuniformoptimalstrategy,aregoodsourcesof randomness.Themin-entropyofhumanchoicesinthefirstsub-gameis2 1bits persymbol(10symbol)inaverageandinthesubsequentsub-gamesis1.38bits persymbol(3symbols)inaverage.Thesecomparedtothemaximumavailable entropyofthesourceoncorrespondingnumberofsymbols,i.e.log 2 10=3.32 andlog 2 3=1.58,indicatethatindeedthehumanchoicesareclosetouniformly randomandthefinaloutputofTRGisexpectedtobeclosetorandom. Applications. TRGsareanessentialcomponentofcomputingsystems.User basedTRGaddanextralevelofassuranceabouttherandomnesssource:users knowthattheirinputhasbeenusedtogeneraterandomness.Anexampleapplicationofthisapproachisgeneratinggoodrandomkeysusinguser’sinput. Askingausertogeneratea64bitkeythatisrandomwillcertainlyresultina biasedstring.Usingtheapproachpresentedinthispaper,theusercanselectan

14M.Alimomeni,R.Safavi-Naini,andS.Sharifian

elementofthespace(sayapasswordwith13characters)randomly.Theuser choicewillbeusedastheinitialentropysource,andthensubsequentgames willensurethatthefinal64bitsisclosetouniform.Assuminga3-regularexpandergraphwith10vertices,oneneeds3stepsintheexpandergraphtoreach a1/4-closetouniform.Section4.1furtherdiscusseshowlongersequencescan begenerated.

1.2RelatedWork

Theideaofusingagametomotivatehumantogenerateunbiasedrandomnesswasproposedin[HN09].Authorsusedtheexperimentalresultsinpsychology[Wag72,RB92]alongwithgametheoreticapproachtoshowhumansplaying matchingpenniesgamegenerateasequencewhichisclosetouniform.Halprinet al.arguedthatthisgamewhenplayedbetweenacomputerandahumancanbe usedasanentropysource.Toincrease theamountofrandomnessgeneratedby humanwitheachchoice,Halprinetal.usedanextensionofthisgamethatuses n choicestotheplayer:theuserispresentedbyan n × n matrixdisplayedonthe computerscreenandisaskedtochooseamatrixlocation.Theuserwinsiftheir choiceisthesameasthesquarechosenbythecomputer.Theynotedthatthe visualrepresentationofthegameresultedintheuserinputtobebiasedasusers avoidedcornerpointsandlimitingsquares.Thesequencegeneratedbyhuman wasusedastheinputtoaseededextractor(Definition5)togenerateasequence thatis -closetouniform(Definition1).Theyprovidedvisualrepresentations ofhumanchoicesthatindicatesagoodspreadofpoints.Howeverstaticaland min-entropyevaluationofthesystemisrestrictedtousingstatisticaltestson theoutputoftheseededextractor.

Extractioncanuseageneralseededextractorthatwillguaranteerandomness oftheoutputfor anydistribution withmin-entropy(Definition3) k ,oranapproachproposedin[BST03]inwhichthesetofpossibleinputsourcesislimited toasetof2t possibleonesallwithmin-entropy k .Theformerapproachrequires afreshrandomseedforeachextractionbuthastheadvantagethattheinput sourcecanhaveanydistributionwiththerequiredmin-entropy.Thislatterapproachhoweverrequirestheinputsequencetobeoneofthesetof2t possible sources,buthastheadvantagethatonecanchooseafunctionfromaclassof availableextractorsandhardcodethatinthesystems.Thismeansinpractice norandomnessisrequired.Howevernoguaranteecanbegivenabouttheoutput iftheinputsequenceisnotoneofthe2t thathasbeenusedforthedesignof thesystemandthispropertycannotbetestedforaninputsequence.Halprinet al.usedthelatterapproach,usinga t-universalhashfunctionastheextractor. Therandomnessguaranteeofthefinalresultrequirestheassumptionthatthe humaninputisoneofthe2t sources.Inpractice, t cannotbearbitrarilylarge andmustbesmalltoguaranteeaminimumoutputrateforrandomness.This canposeasecurityriskthattheactualdistributionisnotoneofthe2t distributions.Halprinetal.didnotperformquantitativeanalysisofusersequencesand usedvisualrepresentationofthehumanchoicestoconcludethechoiceswere random.

WenotethatsimplerextractorssuchastheVonNeumannextractor[vN51] putstrongrequirementsontheirinputsequence.ForexampleVonNeumann extractorrequirestheinputstringtobeaBernoullisequencewithparameter p whichisnotsatisfiedbythesequenceofhumanchoiceswheresuccessivechoices maybedependentwithunknownandchangingdistribution.Theexpandergraph extractorworksforalldistributionswhosemin-entropyislowerboundedbya givenvalue,anddoesnotputextrarequirementsontheinputsequence.

Usinghumaninputasentropysourcehasalsobeenusedinadifferentform. Incomputersystems,users’usageofinputdevicessuchasmouseandkeyboard canbeusedforbackgroundentropycollection.Thisprocessisusedforexample inLinuxbasedsystems[GPR06].[ZLwW+ 09]usesmousemovementandapplies hashfunctions.

PaperOrganization. Section2providesthebackground.Section3outlinesour approachandSection4givestheresultsofourexperiments.Section5provides concludingremarks.

2Preliminaries

Wewillusethefollowingnotations.Randomvariablesaredenotedbycapital letters,suchas X .Arandomvariable X isdefinedoveraset X withaprobabilitydistributionPrX ,meaningthat X takesthevalue x ∈X withprobability PrX (x)=Pr[X = x].Uniformdistributionoveraset X isdenotedby UX or Un if X = {0, 1}n .Thelogarithmswillbeinbase2throughoutthepaper.

Definition1. Considertworandomvariables X and Y takingvaluesin X . Statisticaldistance ofthetwovariablesisgivenby,

Wesaythat X and Y are -close if Δ(X ; Y ) ≤ .

A source on {0, 1}n isarandomvariable X thattakesvaluesin {0, 1}n with agivendistributionPrX .

Definition2. Let C beaclassofsourceson {0, 1}n .A deterministic -extractor for C isafunction ext : {0, 1}n →{0, 1}m suchthatforevery X ∈ C , ext(X ) is “ -close”to Um

Deterministicextractorsexistforalimitedclassesofsources[Sha11].Asmall randomseedisusedinextractorstoallowformoregeneralclassesofsourcesto beusedasinputtotheextractor.Themoregeneralclassesofsourcesarethe sourceswithaguaranteedamountofrandomness.Toquantifytheamountof randomnessinarandomvariablemin-entropyisused.

Definition3. The minentropy ofarandomvariable X is: H∞ (X )= minx log 1 PrX (x) .

16M.Alimomeni,R.Safavi-Naini,andS.Sharifian

Definition4. Arandomvariable X isa k sourceif H∞ (X ) ≥ k ,i.e.,if PrX (x) ≤ 2 k .Themin-entropyrateisdenotedby δ andisdefinedas k = δn, where n isthenumberofsourceoutputbits.

Usingprobabilisticmethod,itisproved[Sha11]thatthereexistsprobabilistic extractorsthatcanextractatleast k bitsofrandomnessfroma k -source.

Definition5. Afunction ext : {0, 1}n ×{0, 1}d →{0, 1}m isa seeded(k, )extractor ifforevery k -source X on {0, 1}n ,Ext(X,Ud )is -closeto Um ,where Um istherandomvariableassociatedwiththeuniformdistributionon m bits.

Arelevantresultonseededextractorsisthefollowing.

Theorem1. [Sha11]Forevery n ∈ N, k ∈ [0,n] and > 0,thereexistsa (k, )extractor ext : {0, 1}n ×{0, 1}d →{0, 1}m with m = k + d 2log( 1 )+ O (1)

2.1ExpanderGraphs

Expandergraphsarewellconnectedgraphsinthesensethattomakethegraph disconnectedoneneedstoremoverelativelylargenumberofedges.Connectivity ofagraphcanbequantifiedusingmeasuressuchastheminimumnumberof neighbouringverticesforallsub-graphsorminimumnumberofedgesthatleave allsub-graphs(minimumsaretakenoverallsubgraphsofcertainsize)[HLW06]. Fora d-regulargraphthesecondeigenvaluesoftheadjacency matrixcaptures theconnectivityofthegraph.Thismeasureisreferredtoas spectralexpansion. Normalizedadjacencymatrix ofa d-regulargraphwith n verticesisan n × n binarymatrixwith Ai,j = 1 d ifvertex i and j areconnectedbyanedge,andzero otherwise.

ExpanderGraphsasExtractors. Givenagraphandastartingvertex X ,one canmakearandomwalkoflength ,byrandomlychoosingoneoftheneighbours of X ,say X1 ,moveto X1 ,thenrandomlychooseoneoftheneighboursof X1 , say X2 ,andrepeatthis times.

Let G denoteanexpandergraphwithnormalizedadjacencymatrix A,and let p denoteaninitialdistributionontheverticesof G.Afteronerandomstep fromeachvertex,thedistributionontheverticesisgivenby Ap andbecomes closertouniform.Thatis,thestatisticaldistancebetweenthedistributionon thegraphverticesandtheuniformdistributionreduces.Continuingtherandom walkonthegraphfor steps,thedistributionontheverticesbecomes Al p andgetsclosertotheuniformdistribution.Therateofconvergencetouniform distributionfor d-regularexpandergraphsisdeterminedbythesecondeigenvalue ofthenormalizedadjacencymatrix ofthegraphwhichisdenotedby λ fromnow on.

Lemma1. [AB09,lemma21.3]Let G bearegulargraphover n verticesand p beaninitialdistributionover G’svertices.Thenwehave:

where . 2 isthel2normdefinedas X 2 = n i=1 x2 i ,considering X tobethe vector (x1 ,x2 ,...,xn ).Notethat Un and Al p areconsideredasvectorsinabove.

Therandomwalkonanexpandergraphexplainedabovegivesthefollowing extractorconstruction.

Lemma2. [AB09,lemma21.27]Let > 0.Forevery n and k ≤ n,there existsanexplicit (k, )-extractor ext : {0, 1}n ×{0, 1}t →{0, 1}n where t = O (n k +log1/ ).

Theabovelemmaassumesanexpandergraphwith λ =1/2,butingeneralfor anarbitrary λ andmin-entropy k ,wecanderivethefollowingtheoremfromthe abovelemmas:

Theorem2. Let Un betheuniformdistributionand X bea k -sourcewithprobabilitydistribution p over {0, 1}n .Let G bea d-regularexpandergraphover 2n verticeswithnormalizedadjacencymatrix A.Forarandom-walkoflength l over thegraphstartingfromavertexselectedaccordingtodistribution p,wehave

TheproofoftheabovetheoremfollowsfromtheproofofLemmas1and2.

whereequation(1)followsfromthedefinitionofstatisticaldistance,equation (2)isfollowedfromtherelationbetweenl2andl1norms,i.e. |V |≤ √n V 2 , equation(3)comesfromtheproofoflemmas1and2,andequation(4)follows fromlinearalgebrafacts(

)andthatmin-entropy of p is k ,whichgives

Foranexpandergraph G,given and k asthemin-entropyoftheinitial distributiononvertices,wecancomputethenumberofrequiredstepsofthe random-walkontheexpandergraphsothatthedistributiononthegraphverticesbecomes -closetouniformdistribution.Notethatmin-entropyoftheinitial vertexdistributionresultsinclosenesstouniformdistribution,buttherandom walkwillamplifythiscloseness.

18M.Alimomeni,R.Safavi-Naini,andS.Sharifian

Let λ =2 α and =2 β .Tobe -closetouniform,wemusthave 1 2 √nλl (2 k/2 +2 n/2 ) ≤ .Thisgivesusthefollowinglowerboundon l : l ≥ 1 α [β +log(√n)+log(2 k/2 +2 n/2 )](5)

Theaboveboundrequiresthevalue λ forthegraph.Equation5showsthat foragiven andmin-entropy,smaller λ correspondtoshorterrandomwalk.So oneneedstofindgraphswithsmaller λ. Thefollowingtheoremshowsthatregulargraphshavesmall λ.

Theorem3. [AB09,section21.2.1]Foraconstant d ∈ N,any d-regular, Nvertexgraph G satisfies λ ≥ 2√d 1/d(1 o(1)) wherethe o(1) termvanishes as N →∞.

Ramanujangraphsare d-regulargraphsthatachieve λ ≥ 2√d 1/d and soareexcellentasspectralexpanders.Forafixed d andlarge N ,the d-regular N -vertexRamanujangraphminimizesthe λ.ThereareseveralexplicitconstructionsofRamanujangraphs.Hereweexplainoneofthesimplerconstructions.

2.2ASimpleExplicitConstructionforExpanderGraphs

Thereareexplicitconstructionsofexpandergraphsthatcanbeefficientlygenerated.Thatisverticesareindexedby i ∈ I andthereisanalgorithmthat forany i,generatestheindexofitsneighbours.Forexamplethe p-cyclewith inversechords constructiongivesusa3-regularexpandergraphwith p vertices, p isprime,inwhichavertex X islabelledby x ∈{0,p 1} andtheneighbour verticeshaveindexes x 1, x +1and x 1 .Hereallarithmeticaremod p and 0 1 isdefinedtobe0.Thespectralexpansionofthisgraphisveryclosetothe above-mentionedbound.TheconstructionisduetoLubotzky-Phillips-Sarnak [LPS86]andtheproofthattheconstructionisaRamanujangraph,usesdeep mathematicalresults.The λ forthisgraphisupperboundedby0.94.Other explicitconstructionsofexpandergraphsusegraphproducttechniquessuchas Zig-Zagproductandreplacementproduct[RVW00].

2.3GameTheoreticDefinitions

Agameconsistsofasetof players,eachwithasetofavailable actions and aspecificationofpayoffsforeachpairofactions.An actionprofile isatuple ofallplayers’actions.Anassumptioningametheoryisthatplayersplayrationally;thatistheyaimtomaximizetheirpayoff,givensomebeliefaboutthe otherplayers’actions.Thus,eachplayerhas preferences abouttheactionprofile. Fordescribingplayers’preferencesweuse utilityfunction.Anstrategicgameis definedasfollow.

Definition6. Astrategicgamewithordinalpreferencesconsistsof,

Afinitesetofplayers, N = {1,...,N }; – Asetofactionsforplayer i denotedby Ai ;

Actionprofile: a =(a1 ,...,an ) ∈ A = A1 × ... × An ; – Preferencesoverthesetofactionprofilesbasedonthe

UtilityFunction that capturespayoffs.

Utilityfunction forplayer i: ui : A → R ,which R isthesetofrealnumbers. Wesaythatplayer i prefers a to b iff ui (a) >ui (b)

A purestrategy specifiestheactionsofaplayerinallpossiblesituationsthat hewillbein.

Mixedstrategy ofaplayerinastrategicgamemeansmorethanoneactionis playedwithapositiveprobabilitybytheplayer.Thesetofactionswithnon-zero probabilityformthe supportofmixedstrategy. Mixedstrategyisspecifiedbya setofprobabilitydistributions.Thefollowingtheoremshowstheimportanceof mixedstrategies[Osb04].

Theorem4. EverystrategicgameinwhicheachplayerhasfinitelymanyactionshasamixedstrategyNashequilibrium.

Afinitetwo-playerstrategicgamecanberepresentedbyatable.Insucha representation“Row”playerisplayer1and“column”playerisplayer2.Thatis, rowscorrespondtoactions a1 ∈ A1 andcolumnscorrespondtoactions a2 ∈ A2 Eachcellinthetableincludesapairofpayoffs,startingwiththerowplayer followedbythecolumnplayer.Foratwo-partygame,eachplayeractionisa purestrategyandamixedstrategyisaprobabilitydistributiononthesetof actionsavailabletothem.

Halprinetal.usedanextendedformof “matchingpennies” usingatwodimensionalarraydefiningthechoicesoftheplayers.Inbasicmatchingpennies gamewithHeadandTailaspossibleactionsforeachplayerthepayofftableis asfollow:

- Head

Matchingpenniesgameisazero-sumgameandforanyactionprofile a wehave u1 (a)+ u2 (a)=0.Fromthetableitcanbeseenthatthereisnopurestrategy Nashequilibriumformatchingpennies.Itiseasyhowevertoshowthat the beststrategyforbothplayersinthegameisuniformdistributiononthesetof possibleactions. Thisiseasytoseeintuitivelyandalsoshowformallyusinga distribution p and1 p todenotetheprobabilitythatthefirstplayerchooses HeadorTail,respectively,andthenrequiring p tobechosensuchthatthe player2remainscompletelyindifferenceaboutthechoiceofplayer1.Thatis, u2 (Head)= u2 (Tail )whichgives p +1(1 p)= p (1 p)and p =1/2.A similarargumentshowsthatthebeststrategyofplayer2isuniformalso.

Tail
Head (1,-1) (-1,1) Tail (-1,1) (1,-1)

Inourworkweuse extendedmatchingpennies gamewhereeachplayerhas asetof m possibleactions.Usingasimilarapproachonecanshowthatthe beststrategyforplayersisuniformrandomselectionamongthe n alternatives. Extendedmatchingpenniesgamewasalsousedin[HN09]toincreasetheamount ofrandomnessfromeachuserinput.Traditionalmatchingpenniesgameprovides atmost1bitrandomnessforeachuser’sinput.Usingextendedmatchingpennies, thiscanbeincreasedtolog 2 m bitsperaction.Thishoweverisanupperbound anddependingonthesetupandtheactualvalueof m,themin-entropyof userinputcanbedifferent.Ourexperimentalresultsfor m =10and m =3 correspondingtothesizeanddegreeoftheexpandergraphusedinourwork, showed2 1and1 32bitsperinput,respectively.

3TRGUsingHumanInputinGames

WeproposeanintegratedapproachtoconstructTRGsusinghumaninputin games.Importantpropertiesofthisapproachare,(i)theonlysourceofrandomnessfortheTRGisthehumaninput,and(ii)thefinalresultshaveguaranteed andadjustablelevelofrandomness.Thislatterpropertymeansthattheusercan improvethequalityoftheoutputrandomnessatanytimeandtoanylevelof closenesstouniformrandomness,simplybyadjustingthelengthoftherandom walk.Incomparison,intheconstructionofHalprinetal.(i)theentropysource isbasedonhumaninputandasecondexternalsourceofperfectrandomnessis requiredtoprovideseedfortheextractor,and(ii)thesizeandqualityofthe finaloutputdependsontheextractorthatisusedafterobtainingtheoutput oftheentropysource.Herechangingthequalityofthefinaloutput,requires replacingtheextractorandperformingthecalculationsfromscratch.

TheoutlineoftheapproachisgiveninSection1.1andincludes,(i)choosingan expandergraphwithappropriateparameters,and(ii)designinganappropriate gamethatis“attractive”toplayandhastherequiredproperty(uniformoptimal strategyforeachsub-game).

ChoosingtheExpanderGraph. Thestartingpointischoosinga d-regularexpandergraphwithanappropriatenumberofvertices2n ,eachvertexlabelled withabinarystringoflength n.Thetwoparameters n and d willbedirectly relatedtothecomputationalefficiencyofthesystemingeneratingrandombits: larger n meanslongeroutputstringandmorerandombits,andlarger d means fasterconvergencetotheuniformdistributionandsoshorterwalktoreachthe samelevelofclosenesstotheuniformdistribution(SeeTheorem3).Inpractice however,becausethegraphisthebasisofthegamevisualpresentationtothe user,oneneedstoconsiderusabilityofthesystemandsothechoiceof n and d willtakethisfactorintoaccount.Anotherimportantrequirementisthatstepsof therandomwalkmustcorrespondtoanindependentanduniformlydistributed randomvariable.Experimentsinhumanpsychology[RB92]showsthatbiaswill increaseinhumanchoicesforlargersetsofpossiblechoices.andthustherandomwalkgeneratedbyhumaninputwillbefartherfromuniformlyrandom.In Section4.1wediscussissuesthatariseinchoosingthegraphandextendingthe approachwhenlongersequencesofrandombitsarerequired.

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