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The directedChinesePostman Problem
&&The Chinese Postman Problem has man y applications,includingrobot exploration,and analysinginteractive system and w eb siteusability. This paper reviewsthe wide range ofapplicationsofthe problem and presentscomplete,executablecode to solve itforthe caseofd
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你可能喜欢SolvingtheByzantinePostm；LenSassaman1andBartPrene；KatholiekeUniversiteitLe；KasteelparkArenberg10,B-；{len.sassaman,bart.prene；Abstract.Overthelastseve；Inthispaper,weexaminethe；Wesho
SolvingtheByzantinePostmanProblemLenSassaman1andBartPreneel1KatholiekeUniversiteitLeuvenKasteelparkArenberg10,B-3001Leuven-Heverlee,Belgium{len.sassaman,bart.preneel}@esat.kuleuven.beAbstract.Overthelastseveraldecades,therehavebeennumeroussys-temsproposedwhichaimtopreservetheanonymityoftherecipientofsomedata.Somehaveinvolvedtrustedthird-partothershavebeenconstructedontopoflink-layeranonymitysystemsormixnetworks.Inthispaper,weexaminethePynchonGate[34],apseudonymousmes-sagesystemwhichtakesanalternateapproachtothisproblembyusingPrivateInformationRetrieval(PIR)asthebasisforitspseudonymityproperties.Werestrictourexaminationtoa?awinthePynchonGatesystem?rstdescribedinourtechnicalreport[35];asitwasoriginallypre-sented,thePynchonGatedetectsthepresenceof(andprotectsagainstcertainattacksby)Byzantineserversoperatinginthesystem,butitfailstoidentifywhichserverorsetofserversisByzantine,thusopeningthedoorfordenialofserviceattacksaswellasotherpotentialanonymitycompromisesbyByzantineservers.Weshowatrivialmodi?cationtotheoriginalPynGPwhichallowsfordetectionandidenti?cationofByzantinenodes,withnoweakeningofthesecuritymodelnecessary,attherelativelya?ordablecostofgreaterbandwidthrequirementsduringcertaincommunicationoperations.Wedemonstratethatthisadequatelysolvestheproblemsraisedby[35],andarguethatitisthemostsuitablemethodofaddressingtheattackinquestionyetproposed.Wethenevaluateanalternateapproachtosolvingtotheproblemde-scribedin[35],proposedbyGoldberginhisrecentpaper[21].Wecom-parethesecurityandperformancetrade-o?smadeinthatproposal,and?nditlesssecureagainstanonymityattacksascomparedtotheorigi-nal(but?awed)PynchonGateProtocol(PynGP)[24]presentedinthe?rstPynchonGatepaper.Weshowthatthisproposalissigni?cantlyweakerthanthesolutiono?eredinthispaper,whichretainsthesecuritypropertiesoftheoriginalPynchonGateProtocol.1IntroductionSeveralproposalshavebeenmadefortheuseofprivateinformationretrieval(PIR)[8]primitivestobuildsecure,fault-tolerantpseudonymousmailretrievalsystems[10,3,23,34].PIR-basedpseudonym(ornym)servershaveseveralsigni?cantadvantagesovernymserversbasedonothertechnologies.PIRprotocolscanbedesignedtoo?erinformation-theoreticsecurity,i.e.,assumingthatthesystemiscorrect,anattackerwithunlimitedcomputationalpowercannotdefeatthesystemmerelybyvirtueofbeingabletoperformcalculationswhichrevealtheprivateinformation.OtherPIRprotocolsmerelyo?ercomputationalsecurity:inComputationalPIRsystems[7],theprivacyofthePIRqueryisprotectedonlyagainstanadversaryrestrictedtopolynomial-timecomputationalcapability.CPIR-basedsolutionshavethesigni?cantadvantagethattheycanbeperformedusingasingleserver,anddonotrequiredistributionoftrusttoensurethattheinformationretrievalrequestsremainprivate.However,suchsystemspresentlyhaveprohibitivecom-putationalcostoncommodityhardware.1.1DistributionofTrustinPublicAnonymitySystemsIndistributed-trustinformation-theoreticPIRsystems,theprivacyofthesystemispredicateduponnosingleentitybeingabletogainaccesstosensitivedata,beittheprivateinformationitself,orsecond-orderinformationwhichcanbeusedtoobtaininformationabouttheprivateinformation.Ofparticularconcernarethepossibilitiesthatasingleadversarymayoperatemultiplenodesunderdi?er-entidentities,e?ectivelyensuringnodecollusion[19],orthatsigni?cantamountsoftheanonymityinfrastructuremaylackgoodlocationindependence[20].Thus,systemswhichencourageparticipationbymanyuna?liatedoperatorsofdiversebackgroundsacrossawiderangeofnetworkproviderscanprovidestrongerser-vicesthanthoseinwhichinfrastructureoperationismoretightlyrestricted.Ratherthanattemptingtoensurethattheadversaryisunabletogaincontrolofanypartoftheinfrastructure,thislaissez-faireapproachtoanonymityser-viceoperationtakenbysomeofthemoresuccessfulanonymityservices[17,28]simplyacceptsthatsomenodeswillbecontrolledbyanadversary,andaccountsforthisfactinthedesignofthesesystems.1.2BackgroundonNymServersPseudonymousmessagingservicesallowuserstosendmessagesthatoriginateatapseudonymousaddress(or“nym”)unlinkedtotheuser,andtoreceivemessagessenttothataddress,withoutallowinganattackertodeducewhichusersareassociatedwithwhichpseudonyms.Thesesystemscanbeusedforpartiestocommunicatewithoutrevealingtheiridentities,orasabuilding-blockforothersystemsthatneedabi-directionalanonymouscommunicationchannel,suchasFreeHaven[15].1.3ThePynchonGateandTheByzantinePostmanProblemThemostrecentproposalforanymserverbasedonPIRwithinformation-theoreticsecurity,thePynchonGate[34],o?ersgreaterrobustness,strongeranonymityassurances,andbettertra?canalysisresistancethanpreviouslypro-posedpseudonymsystems.However,aswehavepreviouslynoted[35],itcon-tainsa?awinitsprotocolwhichcanbeusedtolaunchadenialofserviceattackagainstthesystem,renderingitunusable.1Furthermore,theattackisnotmerelylimitedtodecreasduetothenetwork-e?ectspropertiesofanonymitysystems,denyingservicetoonesetofuserscane?ectivelyweakentheanonymityprovidedtoadi?erentsetofusers[1].2BackgroundonThePynchonGateToaddressthereliabilityproblemsofsilentnodefailure,aswellastheseri-oussecurityproblemsofstatisticaldisclosureattacks[33,11,13]andend-to-endtra?canalysis[26],Sassaman,Cohen,andMathewsonproposeacompletearchi-tecturaldesignofaPIR-basedpseudonymserviceo?eringinformation-theoreticprotection,calledthePynchonGate[34].2.1ArchitectureOverviewThearchitectureofthePynchonGateconsistsofanInternet-facingcomponentreferredtoasthe“nymserver”,whichreceivesmessagesaddressedtousersofthesystemandactsasagatewaybetweenthepseudonymserviceandotherInternetservicessuchasemail.Behindthenymserverisacomponentknownasthe“collator”,whichstructurestheincomingmessagesintheformofathree-levelhashtree,whichisthenreplicatedtoaseriesofmutuallyuntrusteddistributiondatabasesreferredtoas“distributors”.Emailaddressedtoaspeci?cpseudonymisstoredinaspeci?clocationinthedatabase,suchthattheownerofthepseudonymknowswhatinformationtorequesttoobtainhismessage.UsingthePIRprotocoldescribedinSection2.2,theusersubmitsaPIRqueryto??distributors,andhismessageisreturnedwithnoneofthedistributorsabletodeduceanyinformationabouttheuser’squeryunlessall??distributorscollude.ThisformofPIRisreferredtoasaninformation-theoretic(???1)-private??-serverPIRprotocol.2.2ThePynchonGatePIRProtocolTheprotocolrunsasfollows:afterchoosingdistributors,theclientestablishesanencryptedconnectiontoeach(e.g.,usingTLS[14]).Theseconnectionsmustbeunidirectionallyauthenticatedtopreventman-in-the-middleattacks,andcanbemadesequentiallyorinparallel.Theclientsendsadi?erent“random-looking”bitvector??νsβtoeachdistrib-utorsforeachmessageblockβtoberetrieved.Eachbitvectorhasalength1IfoneormoreoftheserversinthesystemisByzantine,theprotocolwilldetectthattheresultsofthePIRrequestarenotcorrect,andwilltreattheresultsaspoisoned,thuseliminatingpotentialdirectattacksbyByzantineserversagainsttheprivacypropertiesofthesystem.However,thespeci?cPIRprotocolusedinthePynchonGateisdesignedinsuchawaythatitisimpossibletoknowwhichserverwasByzantine,andthereforeByzantineserverscanactwithimpunity,inevitablylockingeveryuserofthesystemintoastateofconstantdenialofservice.equaltothenumberofmessageblocksinthedatabase.EachdistributorsthencomputesR(??νsβ)astheexclusive-orofallmessageblockswhosepositionsaresetto1in??νsβ.Theresultingvalueisthenreturnedtotheclient.Thus,inordertoretrievetheβ’thmessageblock,theclientneedonlychoosethevaluesof??νsβsuchthatwhenall??νsβarexoredtogether,allvaluesare0ateverypositionexceptβ.(Forsecurity,???1ofthevectorsshouldbegeneratedrandomly,andthebitvectorsshouldbesentinarandomordersothatthe??’th,specially-craftedvectorcannotbedistinguished.)WhentheclientreceivesthecorrespondingR(??νsβ)values,shecanxorthemtocomputethemessageblock’scontents.2.3ByzantineServerProtectionInadistributed-trustanonymitysystemsuchasthePynchonGate,thereex-iststhepossibilitythatsomeserversmaybeByzantine,i.e.,theymaybehaveincorrectly,eitherduetointentionalmaliceorsimpleerror.2InthecaseofthePynchonGate,theByzantinebehaviorweareconcernedwithisanincorrectresponsetoaPIRqueryofadistributor’sdatabase.Allndistributorsinthesystemhavetheexactsamecopyofthedatabase,andthesystemisdesignedsuchthatanyattemptbyaByzantineservertomodifyitsresponsetothePIRquerywillbedetectedbytheuserwhenheveri?estherootofthehashtree.Thisiscrucialtopreservingtheanonymitypropertiesofthesystem,forifanattackercanalteramessageorobservethecleartextofamessage,hemaypotentiallybeabletolaterlinkaninputmessagewithagivenoutputretrievedbythenymholder.ThePynchonGate’smessageandlinkencryptionpreventsanattackerfromobservingthecleartextofamessage.Activeattacksthataredependentupontheattacker’sabilitytoaltersomeofthedatabeingtransmittedtotheusersuchthattheattackermaylaterlinktheusertohispseudonymbasedeitheronavarianceintheuser’sresponsetoalteredversusunaltereddata,orbysimplyrecognizingtheproductofthealtereddataasitisprocessedbythesystem(collectivelyknownastaggingattacks[18])areine?ective,asTLSprotectsdataintegrityonthewire.Thus,anytaggingattacksanattackerwishedtoattemptagainstauserwouldhavetooccurthroughtheuseofacorruptdistributor.Toprotectagainstthecasewhereadistributorprovides(intentionallyorotherwise)anincorrectresponsetothePIRquery,theclientveri?esthatthehashofthemessageblockithasreceivedcanbeauthenticatedthroughthehashtreewiththeveri?edhashroot.3ARemainingByzantineServerAttackWepresentthefollowingattacknotpreventedbythehashtreeveri?cationsys-tem:acorruptdistributorcan,throughmaliceorerror,createadenialofservice2Thisconcernispresentinmanyotheranonymitysystems,includingChaumianmix-nets[6,28,12]andsystemsbuiltontopofthem[27,25].attackonthesystembyrespondingwithincorrectdatatoaclient’squery.Whiletheclientwilldetectthatthemessageblockisinvalidafterperformingthe?nalstepofthePIRprotocolinSubsection2.2,andthuscanconcludethatsomeserverwasByzantine,theclientcannotdeterminewhichserverorserversre-turnedtheincorrectresponse.Theclientcannotsafelypassthemessageblockcontents(assumingtheyconsistofanythingotherthangarbage)totheuser,lesttheuser’sanonymitybepotentiallycompromised.Furthermore,ifattacksonportionsofthepseudonymityinfrastructurea?ectsomeusersdi?erentlythanothers,anattackermayexploitsuchattacksoncomponentsofthesystemtofacilitateanintersectionattackagainstauserofthesystemasawhole[16].InthePynchonGate,ifaByzantinedistributorselectivelyperformeddenialofserviceattacksagainstcertainusersbyreturninggarbageresultstotheirqueries,butcorrectlyrespondedtootherusers’queries,theattackerwouldincreasehischancesoflearningtheidentityofcertainusers,basedonwhichusersrespondedtomessagesthatweresuccessfullydelivered.3Inothercases,apassiveadversarycouldobservetheactionsofByzantineserversnotunderhiscontrol(andperhapsnotevenbehavingmaliciously,butsimplyincorrectly)tohelpfacilitateintersectionattacks[38].Additionally,ifausercannotknowwithcon?dencewhichserverisbehavinginaByzantinefashion,sheismorelikelytochangethenodessheusesonaregularbasis,bothincreasingherexposuretolong-termintersectionattacksandincreasingtheprobabilityofselectingaserver-setthatconsistsofnodesoperatedentirelybyasingleadversary.4ByzantineServerDetectionIdeally,therewouldexistawaytoidentifyanindividualByzantineserverwith-outmodifyingtheexistingthreatmodelorpositivesecuritypropertiesofthePynchonGate.Thisisachallengingproblemtosolvewiththeexistingxor-basedPIRprotocol,whichmakesverifyingtheresultsofaPIRqueryreturnedbyaspeci?cdistributorimpossible.(Theclientdoesnotknowwhata“correct”responseR(??νsβ)fromanygivendistronlythatR(??νs1β)R(??νs2β)???R(??νs??β)=β??thmessageblockandthuscannotidentifywhichoftheresponseswereinvalid.)5ANovelSolutiontotheByzantinePostmanProblemWeproposeasolutiontotheByzantinePostmanProblemwhichpreservesthesamethreatmodelestablishedfortheoriginalPynchonGateProtocol(hence-3Thistypeofattackispresent(inaslightlydi?erentform)innon-PIR-basednymserversystemsaswell.Forinstance,inareply-blocksystem,anattackercoulddisablecertainmixesandobservewhichnymsceasedreceivingtra?c.Ifthenymholderhasa?xed-routereply-block,thiswouldenabletheattackertoidentifythemixesusedinthenymholder’sreply-blockpath,andincreasehischancesofsuccessfullylinkingthenymwiththenymholder’struename[36].三亿文库包含各类专业文献、行业资料、生活休闲娱乐、中学教育、各类资格考试、文学作品欣赏、外语学习资料、应用写作文书、幼儿教育、小学教育、专业论文、Solving the Byzantine Postman Problem05等内容。　
　The postman then placed the stone in his bag carefully. Because of this ...(思维方式) and deal with it well, jealousy can bring out many problems.... 　the Bullet (有源汇最大流) HDU 3157 Crazy Circuits (有源汇最小流) 最小费用流 HOJ 2715 Matrix3 HOJ 2739 The Chinese Postman Problem POJ 2175 ... 　 the And the traditional China the postman problem,established the corresponding equivalent model that can and can use odd degree of nodes to solving ... 　 four color problem,arrangement problem,Chinese postman problem.It also ... the method of most neighboring, the method of solving the minimum spanning... 　A solve B solved C solving D solution 29____...61 The postman A attacked B was attacked C ...A to B on C at D in 10 The problem is ... 　(deliver) 答案:.The postman delivered the express mail to the old lady ...It’s a problem that doesn’t need ( ) right now. A、to solve C、... 上传我的文档
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