Civitas Security and Transparency for Remote Voting

1. CivitasSecurity and Transparencyfor Remote Voting Michael ClarksonCornell University with Stephen Chong (Harvard) and Andrew Myers (Cornell) Swiss E-Voting Workshop September 6, 2010

2. TRANSPARENCY SECURITY

3. VERIFIABILITY PRIVACY

4. VERIFIABILITY PRIVACY Remote (including Internet)

5. KEY PRINCIPLE: Mutual Distrust

6. VERIFIABILITY Universal verifiability Voter verifiability Eligibility verifiability UV: [Sako and Killian 1994, 1995] EV & VV: [Kremer, Ryan & Smyth 2010]

7. PRIVACY Coercion resistance better than receipt freenessor simple anonymity RF: [Benaloh 1994] CR: [Juels, Catalano & Jakobsson 2005]

8. ROBUSTNESS Tally availability

9. Security Properties Original system: • Universal verifiability • Eligibility verifiability • Coercion resistance Ongoing projects: • Voter verifiability • Tally availability …under various assumptions

10. JCJ Voting Scheme [Juels, Catalano & Jakobsson 2005] Proved universal verifiability and coercion resistance Civitas extends JCJ

11. Civitas Architecture registration teller registration teller registration teller tabulation teller ballot box bulletinboard ballot box tabulation teller ballot box voterclient tabulation teller

12. tabulation teller ballot box bulletinboard ballot box tabulation teller ballot box tabulation teller Registration registration teller registration teller registration teller voterclient Voter retrieves credential share from each registration teller;combines to form credential

13. Credentials • Verifiable • Unsalable • Unforgeable • Anonymous

14. registration teller registration teller registration teller tabulation teller bulletinboard tabulation teller tabulation teller Voting ballot box ballot box ballot box voterclient Voter submits copy of encrypted choice and credential to each ballot box

15. Resisting Coercion:Fake Credentials

16. Resisting Coercion

17. registration teller registration teller registration teller voterclient Tabulation tabulation teller ballot box bulletinboard ballot box tabulation teller ballot box tabulation teller Tellers retrieve votes from ballot boxes

18. registration teller registration teller registration teller ballot box ballot box ballot box voterclient Tabulation tabulation teller bulletinboard tabulation teller tabulation teller Tabulation tellers anonymizevotes;eliminate unauthorized (and fake) credentials; decrypt remaining choices.

19. registration teller registration teller registration teller voterclient Auditing tabulation teller ballot box bulletinboard ballot box tabulation teller ballot box tabulation teller Anyone can verify proofs that tabulation is correct

20. Universal verifiability:Tellers post proofs during tabulation Coercion resistance:Voters can undetectably fake credentials Civitas Architecture registration teller registration teller registration teller tabulation teller ballot box bulletinboard ballot box tabulation teller ballot box voterclient tabulation teller Security Proofs

21. Protocols • El Gamal; distributed [Brandt]; non-malleable [Schnorr and Jakobsson] • Proof of knowledge of discrete log [Schnorr] • Proof of equality of discrete logarithms [Chaum & Pederson] • Authentication and key establishment [Needham-Schroeder-Lowe] • Designated-verifier reencryption proof [Hirt & Sako] • 1-out-of-L reencryption proof [Hirt & Sako] • Signature of knowledge of discrete logarithms [Camenisch & Stadler] • Reencryption mix network with randomized partial checking [Jakobsson, Juels & Rivest] • Plaintext equivalence test [Jakobsson & Juels] Implementation: 21k LoC

22. Trust Assumptions

23. Trust Assumptions • “Cryptography works.” • The adversary cannot masquerade as a voter during registration. • Voters trust their voting client. • At least one of each type of authority is honest. • The channels from the voter to the ballot boxes are anonymous. • Each voter has an untappable channel to a trusted registration teller.

24. Trust Assumptions • “Cryptography works.” • The adversary cannot masquerade as a voter during registration. • Voters trust their voting client. • At least one of each type of authority is honest. • The channels from the voter to the ballot boxes are anonymous. • Each voter has an untappable channel to a trusted registration teller. Universal verifiability Coercion resistance Coercion resistance

25. Trust Assumptions • “Cryptography works.” • The adversary cannot masquerade as a voter during registration. • Voters trust their voting client. • At least one of each type of authority is honest. • The channels from the voter to the ballot boxes are anonymous. • Each voter has an untappable channel to a trusted registration teller. UV + CR CR

26. Trust Assumptions • “Cryptography works.” • The adversary cannot masquerade as a voter during registration. • Voters trust their voting client. • At least one of each type of authority is honest. • The channels from the voter to the ballot boxes are anonymous. • Each voter has an untappable channel to a trusted registration teller. UV + CR CR

27. Trust Assumptions • “Cryptography works.” • The adversary cannot masquerade as a voter during registration. • Voters trust their voting client. • At least one of each type of authority is honest. • The channels from the voter to the ballot boxes are anonymous. • Each voter has an untappable channel to a trusted registration teller. UV + CR CR

28. Registration In person. In advance. Con: System not fully remote Pro:Credential can be used in many elections

29. Trust Assumptions • “Cryptography works.” • The adversary cannot masquerade as a voter during registration. • Voters trust their voting client. • At least one of each type of authority is honest. • The channels from the voter to the ballot boxes are anonymous. • Each voter has an untappable channel to a trusted registration teller. UV + CR CR

30. Eliminating Trust in Voter Client UV: Use challenges (like Helios) CR: Open problem

31. Trust Assumptions • “Cryptography works.” • The adversary cannot masquerade as a voter during registration. • Voters trust their voting client. • At least one of each type of authority is honest. • The channels from the voter to the ballot boxes are anonymous. • Each voter has an untappable channel to a trusted registration teller. UV + CR CR

32. Trust Assumptions` • “Cryptography works.” • The adversary cannot masquerade as a voter during registration. • Voters trust their voting client. • At least one of each type of authority is honest. • The channels from the voter to the ballot boxes are anonymous. • Each voter has an untappable channel to a trusted registration teller. UV + CR CR

33. Trust Assumptions • “Cryptography works.” • The adversary cannot masquerade as a voter during registration. • Voters trust their voting client. • At least one of each type of authority is honest. • The channels from the voter to the ballot boxes are anonymous. • Each voter has an untappable channel to a trusted registration teller. UV + CR CR

34. Untappable Channel Minimal known assumption for receipt freeness and coercion resistance Eliminate? Open problem. (Eliminate trusted registration teller? Also open.)

35. Trust Assumptions • “Cryptography works.” • The adversary cannot masquerade as a voter during registration. • Voters trust their voting client. • At least one of each type of authority is honest. • The channels from the voter to the ballot boxes are anonymous. • Each voter has an untappable channel to a trusted registration teller. UV + CR CR

36. Trusted procedures?

37. Time to Tally

38. Tabulation Time # voters in precinct = K, # tab. tellers = 4, security strength ≥ 112 bits [NIST 2011–2030]

39. Summary Can achieve strong security and transparency: • Remote voting • Universal (voter, eligibility) verifiability • Coercion resistance Security is not free: • Stronger registration (untappable channel) • Cryptography (computationally expensive)

40. Assurance Security proofs (JCJ) Secure implementation (Jif)

41. Ranked Voting

42. Open Problems • Coercion-resistant voter client? • Eliminateuntappable channel in registration? • Credential management? • Application-level denial of service?

43. http://www.cs.cornell.edu/projects/civitas (google “civitas voting”)

44. CivitasSecurity and Transparencyfor Remote Voting Michael ClarksonCornell University with Stephen Chong (Harvard) and Andrew Myers (Cornell) Swiss E-Voting Workshop September 6, 2010