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Nijmegen,March12,DismantlingcontactlessOnMarch7,2008researchersandstudentsoftheDigitalSecuritygroupoftheRadboudUniversityNijmegenhavediscoveredaserioussecurityflawinawidelyusedtypeofcontactlesssmartcard,alsocalledRFIDtag.Itconcernsthe"MifareClassic"RFIDcardproducedbyNXP(formerlyPhilipsSemiconductors).Earlier,GermanresearchersNohlandPlötzpointedoutsecurityweaknessesofthiscards.Worldwidearound1billionofthesecardshavebeensold.ThistypeofcardisusedfortheDutch`ov-chipkaart'[theRFIDcardforpublictransportthroughouttheNetherlands]andpublictransportsystemsinothercountries(forinstancethesubwayinLondonandHongKong).Mifarecardsarealsowidelyusedascardstocontrolaccesstobuildingsandfacilities.Allthismeansthattheflawhasabroadimpact.Becausesomecardscanbecloned,itisinpriplepossibletoaccessbuildingsandfacilitieswithastolenidentity.Thishasbeendemonstratedonanactualsystem.Inmanysituationswherethesecardsareusedtherewillbeadditionalsecuritymeasures;itisadvisabletostrengthenthesewherepossible.TheDigitalSecuritygroupfoundweaknessesintheauthenticationmechanismoftheMifareClassic.InTheworkingoftheCRYPTO-1encryptionalgorithmhasbeenreconstructedinThereisarelativelyeasymethodtoretrievecryptographickeys,whichdoesnotrelyonexpensiveCombiningtheseingredientswesucceededonmountinganactualattack,inwhichaMifareClassicaccesscontrolcardwassuccessfullycloned.Insituationwheretherearenoadditionalsecuritymeasures,thiswouldallowunauthorisedaccessbypeoplewithbadintentions.TheMifareClassicisacontactlesssmartcarddevelopedinthemid90s.Itisamemorycardthatofferssomememoryprotection.Thecardisnotprogr ble.Thecryptographicoperationsitcanperformareimplementedinhardware,usingaso-calledlinearshiftfeedbackregister(LSFR)anda"filterfunction".TheencryptionalgorithmthisimplementsisaproprietaryalgorithmCRYPTO-1whichisatradesecretofNXP.ThesecurityofthecardreliesinpartonthesecrecyofCRYPTO-1algorithm,whichisknownas"securitybyobscurity".MifareClassiccardsaretypicallyusedforauthentication.Herethegoalisthattwopartiesprovewhotheyare.Thisisdonebydemonstratingthattheyknowsomecommonsecretinformation,aso-calledsharedsecret(cryptographic)key.Bothparties,inthiscasetheMifarecardandthecardreader,carryoutcertainoperationsandthencheckeachother'sresultstobesureofwhomtheyaredealingwith.Authenticationisneededtocontrolaccesstofacilitiesandbuildings,andMifarecardsarecommonlyusedforthispurpose.SuccessfulAuthenticationisalsoaprerequisitetoreadingorwritingpartofthememoryoftheMifareClassic.Thecard'smemoryisdividedintosectors,eachprotectedbytwocryptographickeys.Properkeymanagementisasubjectinitsownright.Roughlyspeaking,therearetwoAllcardsandallcardreadersusedforasomeapplicationhave thesamekeysforauthentication.Thisiscommonwhencardsare usedforaccesscontrol.Eachcardhasitsowncryptographickeys.Tocheckthekeysof acard,thecardreadershouldthenfirstdeterminewhichcard itistalkingtoandthenlookuporcalculatetheassociated key(s).Thisiscalledkeydiversification.Itiedthat thisapproachisusedfortheDutchpublictransportcard.SecurityweaknessoftheMifareTheDigitalSecuritygroupfoundweaknessesintheauthenticationmechanismoftheMifareClassic.InTheworkingoftheCRYPTO-1encryptionalgorithmhasbeenreverseengineered,andwedevelopedourownimplementationofthealgorithm.Wefoundarelativelyeasymethodtoretrievecryptographickeys,whichdoesnotrelyonexpensiveToreverseengineertheCRYPTO-1encryptionalgorithmweusedflawedauthenticationattempts.Ifonedoesnotpreciselyfollowtherulesoftheprescribedprotocol,onecanobtainsomeinformationaboutofthewayitworks.Combiningsuchinformationiswaspossibletoreconstructthealgorithm.Oncethealgorithmisknown,onecanfindoutthekeysthatareusedbyaso-calledbruteattack,i.e.simplytryingallpossiblekeys.Inthiscasethekeysare48bitslong.Tryingallthekeysthenrequiresaroundninehoursonadvancedequipment,accordingtotherecentTNOreport34643"SecurityysisoftheDutchOV-chipkaart",publishedFebruary26th2008.However,heretoocertainflawsintheauthenticationprotocolcouldbeexploited,aswediscovered.Thisleadsustothesecondpoint:thereisawaytorelativelyeasilyretrievethekeywithoutcarryingoutalengthybruteattack.Thiscanbedonebyfirstcarryingoutmanyfailedauthenticationattempts,whichdoprovidesomeinformation.Storingtheresultsofthisinabigtable,onecanlookforamatchandretrievethekey.Thetableonlyhastobeconstructedonce,andcanbepreparedinadvancebyrepeatedlyrunningtheCRYPTO-1algorithmonafixedinput.Ourproof-of-conceptdemonstrationofthisattackstillrequiredmanyauthenticationattemptsoncethistablehadbeenconstructed.Recordingtheseattemptstookseveralhours,butcouldbecarriedoutbyahiddenantennatoeavesdroponacardreader.Itseemsthatthecomplexitycanbefurtherreduced,possiblydramaticallyso,makingtheattackmuchsimpler.ExploitingtheseOncethesecretcryptographickeyisretrieved,therewillbepossibilitiesforabuse.Howseverethesepossibilitiesarewilldependonthesituation.Ifallcardssharethesamekey,thenthesystemwillbeextremelyvulnerable.Thismaybethecaseifcardsareusedforaccesscontroltobuildingsandfacilities,bothintheprivateandpublicsector.Thereishowevernoinformationonhowcommonthisis.Forsuchasettingwedemonstratedanactualattack,whereacardof,say,anemployeecanbeclonedbybumintothatwithaportablecardreader.Thewhoseidentityisbeingstolenmaythenbecompleyunawarethatanythinghashappened.Inasituationwherediversifiedkeysareused,abusewillbemoredifficult,butnotimpossible.Noactualattackshavebeendemonstratedforsuchascenario.Atthetechnicalleveltherearecurrentlynoknowncountermeasures.Shieldingcardswhentheyarenotinuse,e.g.inametalcontainer,reducestheriskofanattackersecretlyreadingoutacard.However,whenthecardisbeingused,itisstillpossiblytoeavesdroponthecommunication,withahiddenantennaneartheaccesspoint.Strengtheningoftraditionalaccesscontrolmeasuresisthereforeadvisable.Accesstosensitivefacilitieswill(orshould)beprotectedbyseveralprotectionmechanismsanyway,ofwhichtheRFIDtagisonlyone.GermanInDecember2007,KartenNohlandHenrykPlötzannouncedthattheyhadreconstructedCRYPTO-1atahackers'conferenceinBerlin.Wehavebeenintouchwiththem,andourworkbuildsontheirresults.However,NohlandPlötzkeptsomeinformationaboutCRYPTO-1tothemselves.ToreverseengineerCRYPTO-1,theycarriedoutaphysicalattack,wheretheystudiedthelayoutofthehardwareimplementingthealgorithmonanactualMifareClassicchip.Theirapproachiscompleydifferentfromours,asweonlyexploitedweaknessesoftheprotocolanddidnotlooklookingatthehardwareWhendiscoveringasecurityflawthereisadilemmaonhowtohandlethisinformation.Immediatepublicationofthedetailscanencourageattacksanddoseriousdamage.Keetheflawsecretforalongperiodmaymeanthatnecessarystepstocounterthevulnerabilityarenottaken.Itiscommonpracticeinthesecuritycommunitytotrytostrikeabalancweentheseconcerns,andrevealflawsaftersomedelay.Thisistheapproachwehavetaken.OnFriday,March72008,theernmentwasinformed,becausenationalsecurityissuesmightbeatstake.OnSaturday,March8,expertsoftheDutchSignalsSecurityBureau(NBV)oftheGeneralInligenceandSecurityService(AIVD)visitedNijmegentoassessthesituation,wheretheyconcludedthattheapproachwedemonstratedwasaneffectiveattack.OnSunday,March9,NXPwasinformed,andonMonday,March10,TransLinkSystems(thedevelotheDutchpublictransportcard).Wespoketorepresentativesofbothcompaniesaboutthetechnicaldetails,andarecollaboratingwiththemtoysetheimpactandthinkofpossiblecountermeasures.OnWednesday,March12,ministerTerHorsthasinformedAbouttheDigitalSecurityTheDigitalSecurityGroupattheRadboudUniversityNijmegenconsistsofabout25researchers.Theresearchfocusesontwothemes:softwaresecurityandidentity-centricsecurity.Overtime,thegrouphasdevelopedaconsiderableexpertiseinthefieldofsmartcards.Thegrouphasforinstanceadvisedontechnicalaspectsoftheelectronicpassportthatwasintroducedlastyear.Thegroupisalsoactiveintheareasofelectron
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