Electronic voting -- context

Transcription

1 Eletroni voting: theory and pratie Eletroni voting -- ontext Mark D. Ryan Governments worldwide are investing in e-voting Eletroni voting promises onvenient way of reording and tallying votes seurity against fraud and manipulation transpareny for voters and andidates It ould be used in a variety of kinds of eletions small ommittees or on-line ommunities student eletions, trade unions, loal government full national eletions Eletroni systems potentially allow large sale undetetable fraud In ontrast, fraud in manual systems limited by requirement to generate or dispose of paper, whih is quite hard to do undetetably in presene of TV ameras. There are protools whih are apable of guaranteeing strong properties But few ompanies are marketing them, and few ountries are interested in them for their government eletions Some desired properties of e-voting systems More desired properties 1. Auray: the delared outome orresponds properly to the votes ast. 2. Eligibility: only eligible voters an vote, and only one. 3. Fairness: no early results; i.e. no voter an be influened by votes already made. Question: whih of these properties does the urrent UK system provide? Transpareny 4. Individual verifiability: a voter an verify that her vote was ounted. 5. Universal verifiability: a voter an verify that the published result is the tally of the votes ast. Remark: differene between assurane and ryptographi guarantee

2 Yet more desired properties 6. Privay no-one an find out how Alie voted. Coerion resistane and the voting booth 7. Reeipt-freeness Alie doesn t get a reeipt (or any other by-produt of the voting proess); thus: Voting booth Voting system Alie annot prove afterwards to a oerer how she voted Thus, reeipt-freeness is like privay, but even with Alie s ooperation Published data 8. Coerion-resistane Coerer Alie annot prove how she voted, even if interation with the oerer is allowed during the voting proess Even stronger than reeipt-freeness. Coerion-resistane Alie interats with the oerer (e.g. by mobile phone) during the eletion. The oerer an partiipate in Alie s vote: She an tell him messages she reeives during the proess (although he might not believe her) He an instrut her on what messages to send bak (although she might not obey). He might have independent means of verifying her reports and her ations Some non-funtional requirements 9. Robustness: Voters annot disrupt the eletion. Faulty behaviour tolerated. 10. Vote-and-go: Voters partiipate just one in an eletion.

3 The urrent situation in the USA Situation in the USA US Presidential eletion 2000 and 2004 employed a variety of diret reording eletroni (DRE) systems, the dominant supplier of whih is Diebold. Proprietary system, not based on disiplined protool. Numerous allegations of involvement of equipment supplier with a politial party. I voted party p1 and the system said `Thank you, we have reorded your vote for party p2. (Radio phone-ins, websites) None of our list of desired properties an be guaranteed in any meaningful way. Blak Box Voting signifies voting whih reports a result without allowing proper verifiation or auditability of the votes and totals. Ativist Bev Harris popularised the term in her book with that title and runs the BlakBoxVoting.org website. Suh eletroni voting mahines an be rigged. If a programmer for a private ompany whih makes suh mahines writes ode whih steals votes, it will be diffiult to detet. Afterwards, if eletion results are suspiious, there is no way to do a meaningful reount. Situation in the USA Diebold s ode was leaked on the internet. Aademi omputer seurity researhers Tadayoshi Kohno, Adam Stubblefield, Aviel D. Rubin, and Dan S. Wallah wrote a damning ritique of it, showing lots of naive assumptions and seurity risks. 15 year old in garage ould manufature ards and sell them on the internet that would allow multiple votes [Avi Rubin] The urrent situation in Estonia Tiny former Soviet republi (pop. 1.4M) niknamed "e-stonia" beause of its teh-savvy population Ot 2005 eletion allowed voters nationwide to ast ballots over the Internet. Fewer than 10,000 people (1% of registered voters) partiipated online. Offiials hailed the experiment as a suess. Said they had reeived no reports of flaws or haking attempts. The system is based on Linux. Voters need a speial ID ard, a $24 devie that reads the ard, and a omputer with Internet aess. About 80 perent of Estonian voters have the ID ards, also used sine 2002 for online aess to bank aounts and tax reords.

4 Estonia and oerion-resistane Estonian eletion system allows multiple online votes to be ast by the same person during the days of advane voting, with eah subsequent vote anelling out the previous one. The Estonian system still gives priority to paper ballots, so anyone who voted online an also go to a polling station on eletion day and vote in the traditional way, anelling out the vote they ast online. But ritis laim no system better than manual voting "The benefits don't ome anywhere near the risks," said Jason Kitat, an online onsultant and researher at the University of Sussex in England. "It's a waste of money and a waste of government energy." He aknowledged that Estonia's system was the most seure to date but said no system was "good enough for a politially binding eletion." "Chek out the world s most high-teh abinet room. This e-abinet doesn t just look ool. It is ool and it promotes effiieny and saves money, too." Newsweek, Marh 11, 2002 Eletroni voting protools The systems used in the USA and other ountries are based on proprietary ode Therefore, it is impossible to investigate the properties of those systems properly [FujiokaOkamotoOhta1992] Alie Administrator etor blindommit v,, b 1 A blindommit v,,b 1 D unblind...ommit v, 1 D I Some systemati protools exist ommitv, D 1 II The protools an be systematially evaluated in terms of whether they satisfy properties. Systems an be implemented that omply with the protools. l, publ. l,ommit v, open...v publ. v III

5 FujiokaOkamotoOhta1992 [FujiokaOkamotoOhta1992] Blind signatures To guarantee privay Commitment To guarantee fairness Three phases To avoid traffi analysis attaks Cryptographi guarantees of privay (link between voter and vote kept seret) fairness (no early results) even if the eletion offiials are orrupt. does not satisfy vote-and-go oerion resistane Simplified [LBDKYY 03] Simplified [LBDKYY 03] Uses re-enryption and designated verifier proofs. Re-enryption Randomised enryption: {m}k ontains random oins Alie Signv 1, Alie Trusted smartard etor Re-enryption: hange the random oin E.g., in El Gamal, the iphertext (x,y) is hanged to (xg r,yh r ). Designated verifier proofs S an prove to A that, say, is the enryption of m, but A annot use this proof to onvine someone else. Tehnially this is ahieved by giving A the ability to simulate transripts of the proof Signv 2, Admin DVPv 1 2,v reenrypt Signv 2, Admin

6 Simplified [LBDKYY 03] [Chaum P.Ryan Shneider 2005] Cryptographi guarantees of Administrator Alie T2k-2 T2k-4 T2 T0 privay (link between voter and vote kept seret) fairness (no early results) reeipt-freeness (like oerion resistane, but assuming passive attaker) even if the eletion offiials are orrupt. also satisfies vote-and-go onion offset onion offset v der subtr mix oni off der subtr mix does not satisfy der subtr mix oerion resistane onion g 2k1, g 2k2,..., g 1, g 0,D T 0 T 1... T 2k3T 2k2T 2k1 offseth g 2k1...hg 0 modv d s m v Where will it go from here? Stakes very high. Powerful players. Amerian experiene gives plenty of ause for onern. Estonian experiene signifiantly better. Apparently no real effort at using strong protools. Ron Rivest is leading a standards initiative in the USA. Might not result in anything, but... David Chaum a passionate believer in itizen empowerment, and sells (or used to sell) voting systems through his ompany Votegrity.