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Civitas Verifiability and Coercion Resistance for Remote Voting

Civitas Verifiability and Coercion Resistance for Remote Voting. Michael Clarkson The George Washington University. with Stephen Chong (Harvard) and Andrew Myers (Cornell). University of South Alabama August 15, 2012. CONFIDENTIALITY. INTEGRITY. CONFIDENTIALITY. INTEGRITY. Remote.

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Civitas Verifiability and Coercion Resistance for Remote Voting

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  1. CivitasVerifiability and Coercion Resistancefor Remote Voting Michael ClarksonThe George Washington University with Stephen Chong (Harvard) and Andrew Myers (Cornell) University of South Alabama August 15, 2012

  2. CONFIDENTIALITY INTEGRITY

  3. CONFIDENTIALITY INTEGRITY Remote (including Internet)

  4. KEY PRINCIPLE: Mutual Distrust

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

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

  7. AVAILABILITY Tally availability

  8. Security Properties Original system: • Universal verifiability • Eligibility verifiability • Coercion resistance Follow-up projects: • Voter verifiability • Tally availability …under various assumptions

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

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

  11. 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

  12. Credentials • Verifiable • Unsalable • Unforgeable • Anonymous

  13. 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

  14. Resisting Coercion:Fake Credentials

  15. Resisting Coercion

  16. 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

  17. 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.

  18. 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

  19. 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

  20. 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

  21. Trust Assumptions

  22. 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.

  23. Trust Assumptions Universal verifiability Coercion resistance • “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. Coercion resistance

  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.

  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.

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

  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.

  28. Eliminating Trust in Voter Client VV: Use challenges (like Helios, VoteBox) CR: Open problem

  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.

  30. 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.

  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.

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

  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.

  34. Trusted procedures?

  35. Time to Tally

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

  37. 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)

  38. Assurance Security proofs (JCJ, us) Secure implementation (Jif)

  39. Ranked Voting

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

  41. Technical Issues • Web interfaces • BFT bulletin board • Threshold cryptography • Anonymous channel integration

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

  43. CivitasVerifiability and Coercion Resistancefor Remote Voting Michael ClarksonThe George Washington University with Stephen Chong (Harvard) and Andrew Myers (Cornell) University of South Alabama August 15, 2012

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