Civitas Toward a Secure Voting System

1. CivitasToward a Secure Voting System Michael ClarksonCornell University AFRL Information Management Workshop October 22, 2010

3. Secret Ballot

10. Florida 2000:Bush v. Gore

11. “Flawless”

13. Security FAIL

14. Analysis of an electronic voting system[Kohno et al. 2003, 2004] • DRE trusts smartcards • Hardcoded keys and initialization vectors • Weak message integrity • Cryptographically insecure random number generator • ...

15. California top-to-bottom reviews[Bishop, Wagner, et al. 2007] • “Virtually every important software security mechanism is vulnerable to circumvention.” • “An attacker could subvert a single polling place device...then reprogram every polling place device in the county.” • “We could not find a single instance of correctly used cryptography that successfully accomplished the security purposes for which it was apparently intended.”

16. Why is this so hard?

17. VERIFIABILITY PRIVACY

18. VERIFIABILITY …not just correctness …even if everyone cheats

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

20. PRIVACY …more than secrecy …even if almost everyone cheats

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

22. ROBUSTNESS Tally availability

23. ROBUSTNESS VERIFIABILITY PRIVACY

24. ROBUSTNESS VERIFIABILITY PRIVACY Remote (including Internet)

25. H.R. 2647 Sec. 589 Military and Overseas Voter Empowerment Act

26. How can we vote securely,electronically,remotely?

27. Cornell Voting Systems • CIVS (ca. 2005) [Myers & Clarkson]http://www.cs.cornell.edu/andru/civs.html • Civitas 0.7 (ca. 2007) [Clarkson, Chong & Myers]http://www.cs.cornell.edu/projects/civitasPublished Oakland 2008 + 2 Masters projects • Civitas 1.0 (started fall 2010) [Clarkson et al.]

28. Cornell Voting Systems • CIVS (ca. 2005) [Myers & Clarkson]http://www.cs.cornell.edu/andru/civs.html • Civitas 0.7 (ca. 2007) [Clarkson, Chong & Myers]http://www.cs.cornell.edu/projects/civitasPublished Oakland 2008 + 2 Masters projects • Civitas 1.0 (started fall 2010) [Clarkson et al.]

29. Security Properties Original Civitas: • Universal verifiability • Eligibility verifiability • Coercion resistance Masters projects: • Voter verifiability • Tally availability …under various assumptions

30. KEY PRINCIPLE: Mutual Distrust

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

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

33. 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

34. Credentials • Verifiable • Unsalable • Unforgeable • Anonymous

35. 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

36. Resisting Coercion:Fake Credentials

37. Resisting Coercion

38. 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

39. 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.

40. 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

41. 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

42. 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

43. Trust Assumptions

44. 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.

45. 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

46. 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

47. 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

48. 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

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

50. 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