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ExploitingtheClientVulnerabilitiesinInternetE-votingSystems:
HackingHelios2.0asanExample
SagharEstehghari
UniversityCollegeLondon
s.estehghari@cs.ucl.ac.uk
YvoDesmedt
UCLandRCIS
Abstract
Heliosisaweb-basedopen-auditvotingsystemde-
signedusingstateoftheartwebtechnologiesandad-
vancedcryptographictechniquestoprovideintegrityof
ballots andvotersecrecyinaninsecureInternetenvi-
ronment. In n this s paper, , we e demonstrate a simple at-
tackagainstHelios2.0thattakesadvantageofthefact
thateverycandidateinHelioscanprovideaURLrefer-
ringtohis/hercandidacystatement. Amaliciouscan-
didate, whowishes towinaHelios-managedelection,
uploadsaspeciallycraftedPDFfilecontainingacandi-
dacystatementtohis/herwebsite.Theattackisthentrig-
geredagainsteachvoterwhoisusingavulnerablema-
chine. Thesecurityofthemachineisundermined,e.g.,
whenthevotervisitstheattacker’swebpage.Inessence,
weexploitAdobeAcrobat/Reader’svulnerabilitiestoin-
stallamaliciousbrowserextensiononthevoters’ma-
chines. Suchanextensionprovidesanopportunityfor
anattackerwhichmayfoolthevoter(usingSocialEngi-
neering)intoacceptingahackedballot.Duetoourattack
Helios2.0wasupgradedtoHelios3.0.Wediscussgen-
eralizationsandtheimpactofthelatestupgradeofHelios
onsecurity.Wealsodiscussdefencesagainstthisattack,
generalizationsandtheimpactofthelatestupgradeof
Heliosonsecurity.
1 Introduction
Votingsystemsarehighlysensitiveinnature,andassuch
theyareaprimetargetforopportunistsinterestedinbi-
asingtheresultsofanelection. Electronicvotingsys-
temshaveopenednewavenuestotheseadversariesthat
mightbetemptedtoexploitanysoftwarevulnerabilityin
thesesystemstobreaktheintegrityandsecrecyofballots
(alsocalledanonymityofthevoters). Someofthecur-
rente-votingsystemssufferfromexploitationofmany
vulnerabilities,e.g.[32],[30],[9],etc.Indeed,thesevot-
ingsystemsarehighlydependentonthesecurityofthe
softwarerunonthevotingterminal,andthereforethese
arevulnerabletoanyflawinthesecuritydesign.
Researchers have e proposed cryptographic voting
schemes(e.g. Mix-Nets,Homomorphicschemes,etc.),
toprovideseveraldesirablepropertiessuchassecurity,
efficiencyandaccuracy. Inthispaperweanalyzethe
impactofsoftwarevulnerabilitiesonsuchcryptographic
votingschemes.
Web-basedvoting,amongthesee-votingsystems,are
providingorganizationswithmoreflexibilitytoconduct
theirinternalelections.IndesigninganddeployingInter-
netvotingsystems,advancedcryptographictechniques
andstateoftheartwebtechnologiescanbeemployed
toprotectsuchsystems ontheserverside. . However,
suchtechnologiesarelimitedontheclientside which
maymakethesesystemsvulnerabletodifferenthacking
techniques. As s aresulttheimpactofbothserverand
clientsecurityontheprivacyandintegrityoftheInternet
votingsystemsmustbecarefullyconsidered.
Inthis paper, we focus onHelios2.0, , apromising
web-basedopen-auditvotingsystem[6],whichuseslots
ofcryptographytodemonstratehowanadversarymight
exploitclients machines vulnerabilities to o mislead the
voter.
AlthoughwedidnotbreakHelios2.0from acryp-
tographicviewpoint,ourattackwasabletodemonstrate
thatcompromisingHelios2.0ontheclientsideismore
feasiblethanpreviouslybelieved.Moreprecisely,inHe-
lios2.0, exceptifoneiscertainthatlargemajorityof
voterstakeextraordinaryprecautions,ourattackunder-
minestheintegrityofthevotingsystem(formoredetails,
seetherestofthispaper).
Weidentifypotentialapproachestoallowforasuc-
cessfulattackagainstanHelios-basedelectionandselect
oneofthemwhichismorefeasibleandlessdetectable
(Section4).WeremarkhowHelios2.0wasmodifiedto
weakentheimpactofourattack(Section5.1).Thenwe
discussthelegalissues,costs,impactoftheattack(Sec-
tion6). Alongwiththeflawsweexploittolaunchour
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successfulattack,weidentifyitslimitations,andhence
proposedefensesandtechniquesthatcouldpreventsuch
typeofattacks(Section7).
2 RelatedWork
2.1 E-votingSystems
Beforewesurveypriorattacksone-votingsystems,we
mentionthatwediscuss hereexploits thatcanbeper-
formedone-votingsystemsusingnon-verifiablevoting
schemes. From m a cryptographic viewpoint t there is a
fundamentaldifferencebetweenverifiablecryptographic
votingschemesandnon-verifiableones.Helios,theweb-
basede-votingsystemthatisanalyzedinthispaper,uti-
lizesverifiablevotingschemewhereeachvoterisable
toaudittheballotsandtoverifyitscorrectness(seeSec-
tion3.1).InSections4westudytheimpactofourattack
oninternetbasedverifiablevotingsystems,suchasHe-
lios.
Priorresearchhasfocusedonanalyzingandevaluat-
ingsecuritymeasuresconsideredinthecontextofbooth-
basede-votingsystems.Insuchsystems,votersgotothe
pollingstationsandsubmittheirballotelectronicallyus-
ingthevotingterminal,i.e. [32], , [30],[9], etc. . These
systemsarehighlydependentonthesecurityandcor-
rectnessofthesoftwarerunningonthevotingterminal.
Anyflawsorbugscontainedinthesoftwarecanpromote
amaliciousvoterorinsidertoexploitthesystem.
TheanalysisofDieboldAccuVote-TScarriedoutby
Kohnoetal. [32]wasonethefirstpapersinthisfield.
Theresearchershadaccesstothesourcecodeofthesys-
tem,sinceithadappearedontheInternet.Accordingto
[32],Dieboldviolatesconfidentialityandintegrityofthe
ballotsandanonymityofvoters.
The researchconducted byHursti [30] on Diebold
AccuVote-TS6andAccuVote-TSxexploitedpubliclyac-
cessible documentations, , “source e code e excerpts” ” [30]
andtheresultswereobtainedbytestingarealsystem.
Accordingto[30],exploitationofdesignflawsinthese
systemsmayhelpanattackertocompromisethesecurity
ofanelection.
Recently,researchwascarriedoutbyAppeletal. [9]
onSequoiaAVCAdvantagevotingmachine.Duetothe
Courtorder,theauthorscouldstudytheactualvotingma-
chine’shardwareandsourcecodeforamonth. In[9],
theyshowedthatfraudulentfirmwarecanbeinstalledon
themachine,allowingonetotamperwiththeintegrityof
anelection.
OtherresearchrelatedtoAVCAdvantagemachines
wascarriedoutbyCheckowayetal.[15].Intheirwork,
thescientistsdidnothaveaccesstothesourcecodeof
thevotingsoftwareortothedocumentation.Theyfound
securityflawsinthesystembyreverseengineeringthe
votingsoftwareandhardware. Theycouldsuccessfully
showthepossibilityofavote-stealingattackbyemploy-
ingReturn-OrientedProgramming[39]technologywith-
outcodeinjection(asthesystemrejectsanycodeinjec-
tion).
2.2 BrowserRootkits
Thebasicideaforourattackisinspiredbyanattackpro-
posedin[33]. Inthispaper,researchershavedeveloped
amaliciousextensionforFirefoxcalledBROWSERSPY.
Theyinstalledtheextension(withoutintroducingaspe-
cificapproachforinstallation)byinjectingittoGoogle
Toolbarextension. The e BrowserRootkit steals saved
passwordsinthebrowserandsendthemtogetherwith
theURLtotheremoteattacker(alsosee[42]).
HackershavealreadyexploitedInternetExplorerex-
tensions to develop and install l Browser Rootkits s on
client’smachines. Download.ject[37]isamalwarethat
installsamaliciousextensiononInternetExplorer. The
taskofsuchextensionwastolistentothebrowsinghis-
toryuntildetectinganSSL/TLSconnectionrelatedto
abanktransaction. Thenitcapturedtheusernameand
passwordofthevictimandsenttheconfidentialinfor-
mationtotheattacker.
3 Background
Inthissection,wesurveyHelios2.0andatypicalvot-
ingprocess,thenwebrieflypresentbothsoftwarevul-
nerabilitiesandcurrentbrowserscapabilitiesthatmight
beexploitedtoallowforremotecodeexecutionsoasto
compromiseanelectioninHelios2.0.
3.1 HeliosVotingSystemVersion2.0
Heliosisanopen-sourceweb-basedvotingapplication
whichcanbe deployedbyanyorganization, groupor
communitytosetupanelection. Awebbrowseristhe
onlytoolneededbybothvotersandsystemadministra-
torstointeractwiththesystem. Accordingto[6,7],it
isasecureby-designVotingsystem,asit“implements
advancedcryptographictechniquestomaintainballotse-
crecywhileprovidingamathematicalproofthattheelec-
tiontallywascorrectlycomputed”.
Specifically,Heliosisimplementedutilizingadditive
homomorphictechniques fore-votingandExponential
ElGamal, avariantofElGamalwhereg
m
is encrypted
insteadofm,astheencryptionalgorithm. Heliossup-
portsthresholddecryption[20]withjointkeygeneration
[8].
ThewebbrowserplaysanimportantroleinHelios’
functionality.InparticularJavaScriptisextensivelyused
intheHeliosapplication. Theelectiondata,suchasthe
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encryptedballot,theballot’splaintextandtherandom-
nessare storedinbrowser’s memorybeforetheballot
submission. Inaddition, , ExponentialElGamalis im-
plementedusingtheJavaScriptlanguage. Becausethe
JavaScripttechnologyitselfisquiteslowwhenperform-
ingcomputationallycomplicatedtaskssuchasgenerat-
ingrandomnessandmodularexponentiationforElGa-
mal,Javaisutilizedforsuchcomputation.Thisrequires
thattheJavaVirtualMachine(JVM)needstobeinstalled
onthewebbrowser. Consequently,theencryptionpro-
cessesareprimarilyperformedinthewebbrowser.
Theintegrityandsecrecyofvotesareamongthemain
objectivesoftheHeliossecuritymodel. Fromthatper-
spective,Heliosdesignersclaimthat“... evenifHelios
isfullycorrupt,theintegrityoftheelectioncanbever-
ified”and“... assumingenoughauditors,evenafully
corruptedHelioscannotcheattheelectionresultwithout
thehighchanceofgettingcaught”. Asforanyothere-
votingsystem,differenttypesofattacksmaythreatenthe
securityofHelios. Byleveragingonthebenefitsofaso
calledopenauditelection,theHeliossystemclaimsto
defeatmost,ifnotall,oftheseattacks,suchaschanging
aballot,voterimpersonation,ballotcorruption,incorrect
decryption,etc.[6].
3.1.1 RoleofAuditing
Accordingto[6], Helios2.0usestwoverificationpro-
grams,theonewhichcanbeutilizedbythevoterduring
thevotingprocessandpriortovotesubmissiontoaudit
theencryptedballotgivenbytheBallotPreparationSys-
tem(BPS),andtheother,usedbytheadministrator,to
verifytheshuffling,decryptionandtallyingoftheelec-
tion.
Benaloh’sSimpleVerifiableVoting[11]isemployed
forthepurposeoftheformerverificationprogram. The
keyfeature ofthis protocolis theseparationofballot
preparationandthecastingofavoteinavotingsystem,
whereauthenticationofthevoterisnotrequiredpriorto
theballotcastingtime.Accordingto[6],thismakestest-
ingofBPSbyanyone(botheligible/non-eligiblevoters)
possibleandincreasesauditability.
Usingthesecondverificationprogram,anadministra-
tor(oranyoneelse)canauditthewholeelection, after
tallyinganelection.Inthisprocess,alistofvotersalong
withthehashoftheirballotsispublishedbythesystem.
Theadministratorshouldrepublishthislist, wherethe
voterscancomparetheirownhasheswiththepublished
onesandverifythecorrectnessoftheirsubmittedbal-
lot. Themainintuitionbehindtheauditingprocessis
thatifavastmajority[6]ofvotersverifythecorrectness
oftheirvotesthentheoutcomeisnotbiasedbyanytype
ofattack, andthevoteresultis deemedcorrect. . This
techniquegivesanopportunitytothevoterstocomplain,
inthecasethehashesdonotmatch. Auditingisthenof
primordialimportanceduringHelioselectionsprocesses.
ItssecurityofthispartisanalyzedfromSection4on.
3.1.2 VotinginHelios
Atypicalvotingprocessstartswhenavoterreceivesan
invitationemailfromtheHeliosadministrator.Oncevis-
itingtheelectionwebpage,avoterinteractswiththeBal-
lotPreparationSystem(BPS)bygoingthroughtheques-
tionsandselectinghis/heranswers. Theseanswersare
thenrecordedbytheBPS.Itisinterestingtonotethat
nexttoeachanswer,thevoterisprovidedwithalinkto
thecandidates’personalwebpagethattypicallycontain
thecandidacystatements.
Ontheconfirmationpage,theselectedvotes/answers
are displayedonthe screen. . Ifthe e voterisnotsatis-
fiedwiththechoices,he/shecangobackandupdatethe
votes/answers. Otherwisehe/sheconfirmsthem. . The
BPSencryptstheballotandshowsthehashoftheen-
cryptedballottothevoterinthenextstep.
Thevotermaydecidetoaudittheballot.TheBPSre-
turnsadatastructure,containingtheelectionID,thebal-
lot’splaintext,ciphertext,andtherandomnessusedfor
encryption,asanauditedballottothevoter. Moreover,
thevotermaydecidetoverifytheencryptedballot.The
BPSdisplaysthenatextareainsidewhichhe/sheshould
copyandpastetheauditedballot. TheBPSthenreturns
thehashoftheelection,thehashoftheencryptedballot,
andtheballot’splaintextasanoutputandstateswhether
theencryptionwasverifiedandwhethertheproofiscor-
rect.
Finally,he/shecanchoosetosubmittheencryptedbal-
lot. Howeveriftheballotwasaudited,itcannotbecast
andthe selectedchoice mustbeencryptedagain. . At
thissteptheBPSclearstherandomnessandtheballot’s
plaintextfromthebrowser’smemoryandbringsupthe
authenticationpage.Thevotershouldentertheusername
andpasswordspecifiedin theinvitationemail. . Ifthe
combinationofthe usernameandpasswordis correct,
thentheballotissignedbytheBPSandstoredinthe
database.Thevoterwillreceiveanemail,containingthe
hashoftheencryptedballotandthehashoftheelection,
confirmingthathis/herballothasbeenrecorded.Yet,for
insuringtheforwardsecrecyofthevote,theemaildoes
notcontainthenameofthechosencandidateintheplain-
text.
Note: TheBallotPreparationSystem(BPS)isimple-
mentedbyutilizingJavaScripttechnologyanditman-
agestheclientsidepartoftheHeliosapplication.
3
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3.2 Software e Vulnerabilities and d Remote
CodeExecution
Today bufferoverflow is s a well known n technique en-
ablingsoftwareexploits[17]. Typically, , itconsists in
providingalongargumentasaninputtotheprogram,
sothatitmayresultinthecorruptionofadjacentparts
oftheprogram’s state, i.e. . pointersandstoredmem-
oryaddresses. Thiserraticbehaviorgivesthepossibil-
itytoanattackertowritearbitrarycodeinthispartof
theprogram’sstate.Thisinjectedcodeiscalledthepay-
loadorshellcode[44]. Asaresult,theflowofthepro-
gramisdivertedtothehacker’sprogram.Thepayloadis
thenexecutedwithprivilegesofthevulnerablesoftware.
Consequently,theattackercangainfullorpartialcontrol
overthevictim’smachine. Inthefollowingwedescribe
thebufferoverflowvulnerabilitiesinAdobeAcrobatand
AdobeReader.Weexploitsuchvulnerabilitiesinourat-
tack,asdescribedinSection4.3.
AdobeandJavaScriptVulnerabilities. AdobeAcro-
batandAdobe Readerarepopulartools forviewing,
searching,printinganddigitallysigningPDFdocuments.
In its s recent versions, , Adobe e Acrobat allows creators
tocustomizePDFdocumentsusingJavaScript. Tothis
aim, AdobedevelopedaJavaScriptAPIwhichisonly
supportedbyAdobeAcrobatandReaderintheirv7.0.0
versionsorlater. UsingthisAPI,itispossibletobind
JavaScriptfunctionstoelementsofaPDFfileandevents,
suchasuseractions.
However,severalAdobeversions,suchas7.0.0,8.0.0,
8.1.0,9.0.0etc.arevulnerabletobufferoverflowattacks.
Theattackcanbelaunchedagainstmachinesonwhich
clientsopenaspeciallycraftedPDFfilewithavulner-
ableversionofAdobeAcrobat/Reader. Asmentioned
earlier,thismakestheexecutionofapayloadpossibleon
thesemachines. Thebufferoverflowvulnerabilitiesare
partiallyduetothewaytheJavaScriptfunctionsareim-
plementedinAdobeJavaScriptAPI.Someofthesefunc-
tionsinclude,
Collab.collectemailinfo()
[24],
util
.printf()
[25],
Collab.getIcon()
[23]etc. Later, , in
SectionA.1,weshowhowthevulnerabilitiesin
Collab
.getIcon()
functioncanbeexploitedandutilizedasa
tooltolaunchanattackagainstHelios2.0.
3.3 BrowserRootkitsandExtensions
Browserextensions(oradd-ons)arefacilitiesprovided
tocustomizeabrowsertoofferadditionalfeaturesthat
fitthepersonalneeds ofeachuser[33, 21]. . Browser
extensionsaresupportedbybothInternetExplorerand
MozillaFirefox.Forthepurposeofourstudy,inthefol-
lowingweonlyfocusonFirefoxextensions.However,it
isworthnoticingthatourresultsaregenericandourat-
tackiseasytosetupusingInternetExplorerextensions.
Theextensionsbecomepartofthebrowserafterinstal-
lationandareabletochangethebrowser’sbehavior,as
seenbytheclient.Theyhavethefollowingcapabilities:
 They y have e access to o Document Object Model
(DOM)ofwebpagesandareabletoadd,editor
removeDOMelementsofanHTMLdocument.
 TheyhaveaccesstoJavaScriptfunctionsofaweb
pageandareabletoeditJavaScriptcontents.
 Theycanreadfromandlistentothesessionhistory
ofthewebbrowser. This s impliesthattheexten-
sionscanwait(listen)foraparticularuser’saction
(event),suchasclickingonthe“back”or“forward”
buttononthebrowser,etc.,andreturntheURLof
thepagethattheuseriscurrentlyvisiting.
Structure of f Installed Extensions : : Firefox x exten-
sions contain two important files: : install.rdf f and
chrome.manifest. Install.rdfcontainsmetadataaboutan
extensionanditscreator. Theexistenceorabsenceof
this file suggests whether r anextension is s installed d or
not.Chrome.manifestisusedbythebrowsertofindand
loadtheextension’sfilescorrespondingtoUserInterface
(UI),LocaleandSkin.
Exploiting Firefox Extensions s : : Extensions s allow
Firefox users s to o enhance the e functionalities s of the
browser.However,attackerscandevelopmaliciousadd-
onsthatareabletostealuser’ssensitivedataortamper
withtheintegrityofsuchdata(e.g.[33,37]).Thesema-
liciousextensionsarecalledBrowserRootkits. Clients
canbetrickedtoinstallsuchanextensionontheirma-
chines. Despitethesecuritymeasuresimplementedin
Firefox,itisstillpossibletoinstallanarbitraryextension
onclients’machineswithouttheirpermission. Indeed,
theFirefoxextensionmanagernevercheckstheintegrity
ofextensionsonthebrowserstartup,asitisunawareof
thepastandpresentstatusoftheseextensions.Thisgives
anopportunitytoanattackertoinjectamaliciousadd-on
toanalreadyinstalledextension.Thisisdonebycopying
UIfilesintothevictimextension’sfolderandchanging
thechrome.manifestfiletopointtothemaliciousfiles.
Thefilescanbecopiedbyamalwareorbyapayload,
whichisrunafterexploitingsoftwarevulnerabilitiesas
explainedinSection3.2.
4 AnalysisandDesign
Inthissection,westartbyprovidingourmotivationfor
focusingtheresearchontheHeliosVotingsystem.Then
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weidentifypotentialapproachestoallowforasuccess-
fulattackagainstaHelios-basedelection.Following,the
mostfeasibleapproachisselectedandabriefoverview
of the e attack k is given. . Finally, , we e describe the as-
sumptionsunderwhichoursuccessfulattackislaunched
againstvoters.
4.1 WhyAnalyzingHelios?
Therearelotsofweb-basedvotingsystemsavailableon
theInternet, suchasBigPulse[12],Civitas[16],True-
Ballot[41],Adder[36], HeliosVotingSystem[7]etc.
Someoftheseapplicationsareopensource,suchasCivi-
tas,HeliosandAdder,andsomeareproprietary,i.e.Big-
Pulse.Ouraimwastoanalyzeandevaluatethesecurity
measuresofanopensourcee-votingsystem,sinceany-
onecandownloadthesourcecodeanddeploytheappli-
cationaccordingtohis/herneed. Moreover,wewanted
toanalyzethevulnerabilitiesofaweb-basedvotingsys-
tememployingthestateoftheartcryptographictools.
The International Association for Cryptologic c Re-
search(IACR)hasdecidedtomovetowardse-votingsys-
temsforitsinternalelectionsandtoexploredifferentop-
tions.IntheIACRBoardMeetingonE-VotinginAugust
2008[5],8proposalswerepresented,includingHelios.
Moreover,thedeploymentofHeliosin[8]wonthe‘Best
Paper’awardatEVT/WOTE2009.
Forallabovereasons, webecameconfidentthatthe
HeliosVotingsystemwasperfectlysuitedfortheobjec-
tivesofthisresearch.
4.2 PotentialApproachestoLaunchanAt-
tack
Forabetterunderstandingofthe natureoftheattack,
weassumedaparticularcandidatewhowantstofraud-
ulentlyincreasethenumberofvotesthatareinfavorof
his/herinanelection.Topreventdetection,theattacker
wantstohidetheattack.Thisimpliesthattheattackmust
beimplementedinawaythatvoter’spermissionforthe
execution/installationofamalicioussoftwareisnotre-
quired. Moreover,thevotershouldnotnoticethemodi-
ficationoftheballot.
Inthe rest ofthis section, , wesurvey potential ap-
proachestowardsanattack.
Malware: As s mentionedearlier, each h candidate can
haveacandidacystatement. Suchadocumentcanbe
inPDF, DOCorHTMLformats. . Supposethattheat-
tackerhaspreparedaPDFfileinwhichavirusorworm
isembedded. Priortotheelection,thecandidatecane-
mailthisfile toalistofvoters. . The e malwareisacti-
vateduponopeningofthefilebythevoters,whotrust
thecandidate. Themalwareissetouttoinfecttheweb
browserand,consequently, changethebehaviorofthe
application.Therefore,theinfectedbrowsermaybeable
totamperwiththeintegrityoftheelectionandviolatethe
voter’sanonymity(exceptifthevoterisverydedicated
totakespecialmeasures,asexplainedin[18]andSec-
tion7.2). However,wormsandvirusesreplicatethem-
selvesandspreadovernetworksandtheInternet. The
aimoftheresearchwastomountanattackagainstthe
voterswhileavoidinganydamagetothesensitiveinfor-
mationontheirmachines.
Cross-SiteScripting(XSS)Attacks: Thesekindsof
attacksareapplicationspecific. Theexploitlargelyde-
pendsonthesecuritymeasuresconsideredintheapplica-
tion.HeliosdefeatedpersistentXSSattacksbyavoiding
insertionofanyuser’scontributedcontentsintheappli-
cation. Inotherwords,Heliosdoesnotprovideediting
facilitiesandcandidatescannotcomposetheircandidacy
statementinsidetheapplication.Moreover,thedesigner
wascarefulaboutnon-persistentXSSattacksbyvalidat-
inginputdataandURLsenteredbytheenduser.
Browser Rootkits: : As s explained d in Section 3.3,
browserextensionsareabletomonitortheuser’snavi-
gationhistoryandtomanipulatetheDOMtreeofaweb-
page.Theattackercanexploitthesecapabilitiesofexten-
sionstomodifyoroverridetheJavaScriptfunctionsthat
controltheclientsideoftheapplication. Consequently,
he/shecandistortthebehaviorofthevotingsystem.The
candidatecanmakeuseofsoftwarevulnerabilitiestoin-
stallaBrowserRootkitonvoters’machinewhenvisiting
thecandidate’swebpage.Throughthisapproach,there-
quireduserpermissionforinstallationofanextension,
iscircumvented. BrowserRootkitsgobeyondXSSat-
tacks. Likemalware, , theycanchangethebehaviorof
awebbrowserwithoutreplicatingandspreadingthem-
selves.
4.3 TheSelectedApproach
Theattackproposedinthispaperanddevelopedinsoft-
wareisacombinationoftwoapproaches. First,theat-
tackerexploitsvulnerabilitiesinAdobeAcrobat/Reader
toinstallaBrowserRootkitonavoter’smachine. For
thistowork, theattackerpreparesaPDFfilecontain-
ingnotonlythecandidacystatement, butalsobindsa
malicious JavaScriptfunctiontothe“open”eventofa
file. Second, , thepayloadofthefirsttypeofattackis
a BrowserRootkit, , whichis s may y beabletofool the
voter(usingSocialEngineering)intoacceptinganincor-
rectballotinHelios2.0(fordefensessee[18]andSec-
tion7.2).
Wenowbrieflydescribehowtheattackworksonthe
client’smachine. UponopeningthePDFfile,abuffer
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overflowiscreatedandashellcodewillbeexecuted.The
payloadinstallsthemaliciousextensiononthevoter’s
machine.Moreprecisely,theBrowserRootkitisinjected
intoanalreadyinstalledextensiontosuppressthenotifi-
cationgivenbytheFirefoxextensionmanager. Indeed,
asexplainedinSection3.1,HeliosrequiresJavaRuntime
Environment(JRE)foritsfunctionality.Thisimpliesthat
eachvotermustinstallJREonhis/hermachine.Thesoft-
wareinstallsanextension,calledJavaConsole,onFire-
fox.ThisinsuresthatJavaConsoleisinstalledonvoters’
machinesandmakesthisextensionanidealvictiminthe
attack.
AftertheBrowserRootkithasbeeninstalled,thepay-
load closes s the browser, , as s Firefox x needs s a restart to
reloadthechangesthathavebeenmadetoJavaConsole.
WhentheclientrestartsFirefox,theBrowserRootkitlis-
tenstothebrowsinghistoryofthevoter. Thebrowsing
rootkitcommences its maliciousactionswheneverthe
votervisitsthevotingwebsite.Theextensiontheninjects
someJavaScriptcodesintotheDOMtreetooverridethe
existingfunctionsandmodifiestheoriginalfunctionsto
altertheballotsthatareinfavorofotherstobecomein
favorofthemaliciouscandidate.
Throughthisattack,themaliciousextensionhasfull
controlovertheclientsideofthevotingsystem.Itisable
tochangethebehavioroftheapplicationandtodeceive
thevoterstobelievethattheyhavevotedforthedesired
candidate.Formoretechnicaldetails,seeAppendixA.1.
Formoregeneralscenario,seeSection8.
4.4 Assumptions
We presented d a a certain n number of conditions under
whichasuccessfulattackcanbelaunchedagainstvot-
ers. Consequently,ifavoter’smachinedoesnotsatisfy
theserequirements,thentheattackwillnotbetriggered.
Assumptions about t the voter’s s machine : : The e at-
tackworksontheclientmachinewhereWindowsXP
is the operatingsystem, , the e voter uses Firefoxas the
webbrowserandAdobeAcrobat/Readerversion7.0.0
to8.1.2or9.0.0,isinstalled. Moreover,weassumethe
voterhasthe“write”access totheFirefoxinstallation
folder.
Assumptions about the election : : A A mock IACR
Electionwaschosentodemonstratethevulnerabilities
oftheHelios2.0.Itisusedasthevotingsystemtargeted
intherestofthispaper. Theelectionconsists s ofone
questionandonlyonecandidatemustbechosen. There
areonlytwocandidates,oneoftheauthorsofthispaper,
wecall‘Alice’andtheother‘BartPreneel’(thecurrent
presidentofIACR).Thenamesofthesecandidatesare
sortedalphabeticallyinthequestion. Theadministrator
hasgivenextrainformationabouteachcandidatebypro-
vidingalinktotheirpersonalwebpage.Theattackeris
‘Alice’whowishestowintheelection.
4.5 Design
Twoweekswerespentonthedevelopmentoftheactual
attacksoftware. Around950linesofcodewerewritten
forthisattack.Ofthese,roughly50%isdedicatedtothe
developmentofthemaliciousextension.Theother50%
isrelatedtoembedJavaScriptforAdobeAcrobatandthe
executableprogram.Only10%ofthecodesisuniqueto
Helios. Thesoftwaredoesnotslowtheclientmachine
down. Theonlynoticeableeventduringtheattackrun-
timeisthesuddenclosureofthebrowser,astheFirefox
needsa restartforloadingthechangesthathavebeen
madetothevictim’sextension.
ComparingthedifficultyofhackingHelios2.0with
non-cryptographic Internetvoting systems, , Helios s 2.0
hasanauditingfeatureontheclient-side. Asaresult
ofthis,weaddextracodewhichmayfoolthevoterto
believethathis/hervotewasencryptedcorrectlyandthe
desiredcandidatewasselected. So,weinfactuseSo-
cialEngineeringtomisleadthevoter. AlthoughHelios
isaverifiablevotingscheme, nothingpreventsthisat-
tackagainstHelios2.0, exceptvoterstakingextraordi-
naryprecautions(see[18]andSection7.2).
5 RecentDevelopmentsandTheirImpact
5.1 Heliosversion3.0
In the earlier versions s (1.0and2.0)ofHelios anad-
versary was s able to o make the e auditing g process s much
morecumbersome,asexplainedinSection4. Afterour
Crypto2009Rump-Sessionpresentation[19]Helios2.0
hasbeenmodifiedtoaddresstheproblemwepointedout,
asfollowing.
Accordingto[29], inHelios3.0thevotersarenow
abletoposttheauditedballottotheHeliosserver. This
impliesthatnotonlythevoterisabletocheckwhether
thehashwasproperlycomputed,butalsotheballotdata,
i.e.therandomness,thevoteandthehash,canbeposted
onsomepublicwebpage. Thiscanbedonebeforecast-
ing. The e votercanwritedownthe vote andthe hash
valueonsomepieceofpaper(orcantakeapictureusing
acamera). Thatallowsthevoterorotherstousea2nd,
3rd,etc.,computertocheckthepublicWWW.Assuming
theservertobetrusted(whichwedo),thepublicWWW
willcomputethecorrecthash.(Incasenot,auditorscan
checkwhetherthecorrecthashispostedonthatWWW.)
Wenowdiscusswhetherthesemodificationsaddressour
securityconcerns.
6
5.2 ImpactofAlterationsonAnonymity
InSection8weexplainhowtheBrowserRootkitcan
be further extended to mount an attack k against the
anonymity(privacy)ofthevotersinHelios. Theafore-
mentioned upgradeofHelios s does not protectagainst
suchanattack.
6 Discussion
Inthissectionwewonderhowrealisticitisforsomeone
toactuallyusetheaboveattack.Toanswerthisquestion
severalissuesneedtobetakenintoaccount,whichin-
clude:themotivationoftheattacker(financial,political,
etc.),legalones,thecosttotheattacker(e.g.,settingit
up,possiblelegalfees,etc.),thelikelihoodofbeingable
tochangetheoutcome, whetheralternativeattacksare
moreeffective. Wenowdiscusstheseandconcludeby
reflectingbackonouroriginalscenario.
6.1 MotivationoftheAttacker
Althoughwefocusedonhavingthecandidatehackthe
elections, in reality the attackmight come e from m out-
siders. Althoughhackingwasoriginallydonefor“fun,”
ortoshowoff,intoday’sInternetmanyattackshavefi-
nancialmotivations. So,obviouslybothmotivationsap-
plytothescenarioofelectronicelections. Tobetterun-
derstandthesemotivations, itisevidentlyimportantto
wonderforwhatpositiontheelectionisfor. Examples
include: private e clubs, , associations s (e.g., , professional
ones),insideaboardof(large)corporation,orpolitical
ones.Inthelastcaseanelectionformayorofsomesmall
villagewillbeverydifferentfromonefortheelectionof
apresidentofacountrywheretheattackermayalsohave
politicalmotivation.
Todayseveral countries, , suchas Estonia [34], , Fin-
land[1],Switzerland[4]havealreadymovedtowardsIn-
ternetvotingandothersasNorway[2]andtheUK[3]are
consideringitorhavedonepilottests.Moreover,dueto
severalpeoplebeingunabletovoteintherecentUKelec-
tion,therehasbeenacallfore-votingasapossibleso-
lutiontopreventfutureproblemsatelectionpollingsta-
tions[10].Sincethereareseveralproblemswiththisap-
proach,theNetherlands[22,27]didnotmakethismove.
Insuchpoliticalelections,hackerscouldbelobbyists(or
workingforthem), orpoliticians(e.g.,thesealreadyin
power),orbyforeigncountries.
Ontheotherhand,aspointoutbyananonymousre-
free, web-basedvoting systems s like Helios s are e likely
tobe usedincontests with somewhatsmallerstakes,
andthatcandidates(attackers)inthesescenariossome-
timesstillhavelargeincentivestocheatbutfacesmaller
chancesofbeingcaught.
6.2 Legalissues
Incertaincountrieslaunchingourattackmightbeille-
gal. However,duetotheopennessoftheInternet, , at-
tackscouldbelaunchedfromabroadwheresuchattacks
arenotillegalorthelikelihoodofbeingprosecutedare
small. Furthermore, , incasethe attackis launchedby
awellfinancedorganization,asalargelobbyinggroup,
legalfeesmaybesecondaryandwaystobypassprosecu-
tionmightbeanalyzedindetailbylegalexperts.
6.3 Costs
Inthecaseoftheproposedattack,thesizeoftheelec-
tiondecidesthenumberofalternativeserversrequired
tomanageconcurrentrequeststodownloadthefiles(the
PDFandtheexecutable). Thismayincludeasmallfee
forhostingsubscriptions.Theattackermayusefreehost-
ingservicestominimizethecosts.Ontheotherhand,as
explainedinSection4.5,theamountoftimeandeffort
spentbytheattackertodevelopsuchanattackisquite
low.
6.4 Impact
In Section 6.1 we listed some countries interested in
movingtowardsinternetvoting. Insuchcircumstances
theremightbeaclearincentivetohacktheelection,and
thousandsofcomputerswouldbeusedforvoting. The
impactmightbethatcomputersthatwereunlikelytobe
thetargetofanattackbecomecompromised.
Adobe Acrobat and Readers s were e likely the most-
hackedsoftwareproductsoftheyear2009[28].Accord-
ingto[31], 83.5% ofthe2.5millionusersruna vul-
nerableversionofAdobeAcrobat/Readerontheirma-
chines. Asremarkedbythereport,thisismainlydueto
theobstaclesinthesoftwareupdatemechanism,which
resultsinineffectivedistributionofsecuritypatches.On
theotherhand,nearly30%[35]ofInternetusersutilize
MozillaFirefoxasthebrowsingtool,andmorethan80%
[26]oftheseusershaveAdobeAcrobat/Readerplugin
installedontheirbrowsers. Inaddition, , WindowsXP
iscurrentlyoneofthemost popularoperatingsystem
amongusers[43][40].
Consequently, using Internet elections s ata national
scalemightmakemillionsofcomputersvulnerableand
theproposedattackeffectivewithprofoundimpact(also
seeSections7and8).
6.5 TheIACRElections
OurattackwasmotivatedbyIACR’s(InternationalAs-
sociation for Cryptologic Research)decision to move
towards electronic c voting. . One e can wonder whether
ahackerwouldreallytargettheirelections. However,
7
sinceIACRisanassociationofresearchersworkingon
aspectsofinformationsecurity,asuccessfulattackcould
embarrasstheirassociation. Aninterestingobservation
madeby[38]is:
A truly “successful” ” attack against IACR
wouldn’tbediscovered,andsowouldn’tem-
barrass them. . IACR R members are probably
better equipped d to pull l off such h an attack
than members s oftypical organizations, , so I
wouldn’tdismissthethreatsoreadily.
TheimpactofhavinganIACRelectionhackedarebe-
sidesapotentialembarrassment,ratherminimalincom-
parisontoscenariosdiscussedinSection6.1.
7 LimitationsandDefenses
Inthissection,wediscussthelimitationsoftheproposed
attack.Thenwepresentanumberofdefensesagainstthe
attackandhighlighttheirshortcomings.
7.1 Limitations
Theattackhasanumberoflimitationswhicharerelated
to:
Adobe Acrobat/Reader Updates: : The e attack takes
advantageofbufferoverflowvulnerabilityinAdobeAc-
robat/Reader. ThisvulnerabilityhasappearedinAdobe
Acrobat/Readerversions7.0.0to8.1.2and9.0.0,andit
wasclaimedtohavebeenfixedinlaterversionsofthe
software.Asaresult,initscurrentform,theattackwill
failwhenthevoterupdatedtheAdobeAcrobat/Reader
tothelatestversion.
OtherPDF ViewerSoftware: : Thereare e otherPDF
viewertoolsthathavesimilarfunctionalitiesas Adobe
Acrobat/Reader. These e tools do not support Acrobat
JavaScriptAPI.Sincetheattackexploits AdobeAcro-
bat/Readervulnerabilities,votersthatuseotherapplica-
tions,thanAdobe,toviewthePDFfile,cannotbetar-
getedusingourapproach.
TheattackonlytargetsFirefoxbrowser: Inourfea-
sibilitystudy,themaliciousextensionwasdesignedand
developedforFirefox.Ontheotherhand,thereareother
popularbrowsers. Accordingto[26], , around70%of
clientsusedInternetExplorer.Asaresult,thecandidate
mayloseaconsiderableproportionofvotesunaffected
bytheproposedattack.
Theattackisplatformspecific: Theexecutableisim-
plementedinawaythatinstallsthemaliciousextension
onlyonmachineswhereWindowsXPistheoperating
system.However,around50%[40]ofclientsusesother
operatingsystems,suchasLinux,Macetc.,whichwere
notconsideredwhendesigningtheattack.
WindowsVistaSecurityMeasures: Duetosecurity
measures consideredinWindows Vista, , themalicious
extensioncannotbeinstalledonthis operatingsystem
usingthebufferoverflowvulnerabilitiesinAdobeAc-
robat/Reader. When n the malicious s PDF F document is
openedonthevictim’smachine,AdobeAcrobat/Reader
crashesbuttheoperatingsystempreventstheexecution
ofthepayload. As s aresulttheexecutablewillnotbe
downloadedandexecutedbythepayload. However,if
theattackeremploysavirusorawormtoinstalltheex-
tension,he/shecanmaketheattackworkonthisplatform
aswell.
7.2 Defenses
Therearepossibleways todefeattheproposedattack,
including:
Disable the e JavaScript in n Adobe Acrobat/Reader.
AdobeAcrobat/Readervulnerabilitiescanbeexploited
only when the e JavaScript t is enabled in the software.
Clients candisablethisoptioninthepreferences’sec-
tionoftheapplication(bydefaultitisenabled).Whena
documentcontainingJavaScriptcodeisopenedwiththe
software,wheretheJavaScriptisdisabled,theprogram
popsupadialogboxsaying“Thisdocumentcontains
JavaScripts.DoyouwanttoenableJavaScriptsfromnow
on? Thedocumentmaynotbehavecorrectlyifthey’re
disabled”.Ifthevotertruststhemaliciouscandidatethen
s/hemayclickon‘OK’buttontoturnontheJavaScript
andasaresulttheattackwillbelaunched.Otherwisethe
attackwillnotworkonhis/hermachine.
ThirdPartyVerifiers. AsmentionedinSection3.1.1,
auditingplaysanimportantroleintheHeliosapplica-
tion.Contributionofthirdpartytrusteestoauditandver-
ifythevoter’sballotsmayhelptodetectthealterationof
itscontentswhichissimilartowhathasbeensuggested
inHelios3.0(seeSection5.1). However,ifthosesys-
temsuseweb-basedtechnologytodisplaytheresultsof
verification,thevotingapproachwillbediscussedin[18]
shouldbeused.
MalwareAnalysisofCandidacyStatements. Thee-
voteadministratorcananalyzethecandidacystatements
to check whether they contain n malware or r malicious
8
JavaScript.However,theattackercanuploadaninnocent
documenttohis/herwebsite. So,theadministratorwill
notbecomesuspicious.Ontheotherhand,themalicious
PDFdocumentmaybeemailedtovotersbyendorsersor
athirdparty.
Usinghack-freededicatedhardware Aspointedout
byareferee,onesolutionistouseacertifiedhack-free
devicethatallowstochecktheresultofthehashfunc-
tion.CurrentversionsofHeliosdonotcomewithsucha
device.
AvoidingHelios Letusconsideranalternativesolution
toInternete-voting. “CodeVoting”hasbeenproposed
in[14]. Webrieflyexplainitsideasanddiscussitsad-
vantagesanddisadvantages.
Avoterneedstoreceivebypostorotherout-of-band
channelalistwithauniquecodeforforeverycandi-
date.Tovotethevoterjustentersthecodereceivedcor-
respondingtohis/hercandidateofchoice.
At first glance our attack can barely work, as s the
client-sideinterfaceofthesystemiscompletelydiffer-
entandmakeslaunchingaclient-sideattackagainstsuch
system more difficult, , ifnot impossible. . It t seems a
hackedmachinecannotchangethevotetoanotherone
(even a random one). . Code e Votingalso protects the
anonymity(privacy)betterthanHelios(seeSection8for
moredetailsonprivacyconcerns). However,thereare
assumptions,whichmustbesatisfiedtoguaranteethese
properties.
To guaranteeprivacy, , one needs to o guaranteethere
isnocollaborationbetweenthepostalservicewiththe
returningofficer. Moreover, , ifthepartythatgenerates
these codes iscorrupted, , integritycanbe e undermined
(e.g.,usingballotstuffing).Onecouldobservethatthere
isnoinherentverifiabilitytothisscheme.(Onecancom-
parethiswiththeverifiabilityHelios2.0provides/does
notprovideinpractice,seefromSection4.3on.)
8 FutureWorkandGeneralization
Wefirstdiscusspotentialextensionsofourattack. The
attackcanbeadaptedtotargetmorecomputersofvoters,
e.g.,toincludemachinesrunningMacandUNIXoper-
atingsystems.AlthoughFirefoxisplatform-specific,the
extensionsareplatform-independent. AdobeReaderis
availableforseveraloperatingsystemsandsomeofthese
versionsarealsovulnerabletobufferoverflowattacks.
This impliestheattackercaninstallthesameBrowser
Rootkitonaboveplatforms.Ontheseplatformsadiffer-
entpayloadshouldbeexecutedafterthebufferoverflow.
Thesecondgeneralizationwouldtargetvotersusing
InternetExplorer(IE).BrowserHelperObject(BHO)is
aDLLmoduleusedtoextendandcustomizeIE. Like
Firefox extensions, BHO can n fully access s an n HTML
DocumentObjectModel(DOM)treeandmonitoruser’s
navigations.
Besides attackingtheintegrity, , theBrowserRootkit
canbefurtherextendedtomountanattackagainstthe
anonymity ofthe votersinHelios. The extensioncan
bedesignedtocapturethenameoftheselectedcandi-
datebeforetheencryptionofthe ballotandtheemail
ofthevoter, atthe authenticationstage. Bydevelop-
inga clientemail, usingtheSMTPservice, themali-
ciousextensioncanemailthe recordedinformationto
theattackerfrom thevoter’sbrowser. . Anotheroption,
aspointedoutby[38],isthatthemaliciousextension
sendstheinformationtotheattackerusingHTTPScon-
nection,sonothingcanbedetectedfromthecommuni-
cation. NotethattheHelios3.0remainsvulnerableto
thistypeofattackonprivacy.
Inourtextweassumedthatthecandidateperforms
theattackandourattackfocusesonexploitingAdobe
Acrobat/Readervulnerabilitiestolaunchanattack. We
cametotheconclusionthatthisisthemostrealisticway
toperformtheattack,sinceHeliosallowsURLspoint-
ingtothecandidacystatements,whichcanbeassumed
tobeinaPDFformat. However,theattackcouldeas-
ily be launched by anonymoussupportersofa candi-
date. TheycoulduseSocialEngineeringtechniquesto
getthevoter’smachineinfected,e.g.,usingmailorlur-
ingavoterintoopeningaPDFfilewrittenbyasupporter.
Theattackercanalsodevelopmalware,suchasavirus,
aworm,etc.,toinstallthemaliciousextensionanddis-
tributeitusingaUSBdevice,oraDOCfile.
Anothergeneralizationisthatinourattack,weassume
thatthevoter’svoteismodifiedintooneforaparticular
candidate. Otherattacksincludetomodifyavoteintoa
randomone[38]. Thismaypreventthefavoritecandi-
datetowinandmightbehardertodetect. Theattacker
mayusethetechniquesmentionedearliertoinstallthe
maliciousextensiononthevoter’smachines. Foradis-
cussiononthesecurityofHelios3.0,see[18].
9 Conclusion
Thispaperdemonstratesthefeasibilityofhackingapre-
liminaryversionofHelios. Onlytwoweekswerespent
ondevelopingthis attack. . Due e tothis limited effort,
ourattackisquiteplatformdependent. However,with
therightincentives realworldhackersmayworkona
broaderattackaffectingmoreplatforms.Indeed,botnets
haveshowntheprogresshackershavemade.
AlthoughHelios2.0wasmodifiedintoHelios3.0due
toourattack,severalissuesremain. Aspointedoutby
ananonymousreferee,itisnotclearhowmanyvoters
willexploitthenewsecuritymechanisms(seeSection5).
9
We therefore leaveitto the reader to decidewhether
thefundamentaldifferencebetweennon-verifiableand
verifiableinternete-votingsystems,pointedoutinSec-
tion2.1,hasanypracticalrelevance. Moreover,Helios
privacy(anonymity)protectionremainsitsAchilles’heel
(seeSection8).
Althoughmanyengineershavearguedagainstusing
internetvotingforlargepoliticalelections,severalcoun-
tries(e.g., Estonia [34], Finland[1]etc.) ) arealready
movingtowards this. . In n the light ofthis, webelieve
onlyhistorywillbeabletojudgewhetherourfeasibil-
itystudywasanoverlycautiouswarning,orwhether,to
quoteBollyn[13],internetvotingwillbethe“Deathof
Democracy.”
10 Acknowledgment
We thankStelios ErotokritouandRebeccaWright for
permissiontociteunpublishedwork. Wethankanony-
mousrefereesforsuggestionsonhowtoimproveonthe
attackandJoshBenalohforusefuldiscussions.Wethank
Mohamed-AliKaafaratINRIAGrenoble-Rhˆone-Alpes
forhisfeedbackandsupport. Apartofthisworkwas
donewhilethesecondauthorwasinpartfundedbyEP-
SRCEP/C538285/1andbyBT,(asBTChairofInfor-
mationSecurity).
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/WorkshoponTrustworthyElections,August2009.
10
Documents you may be interested
Documents you may be interested