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Truly Verifiable
    Voting
          Ben Adida
        Harvard University

  MSR Voting Technology Workshop
          19 March 2010
“If you think
   cryptography
  is the solution
to your problem....

         2
... then you
don’t understand
 cryptography...
... and you don’t
understand your
     problem.”
        3
Yet, cryptography solves
 problems that initially
appear to be impossible.

           4
There is a
potential paradigm shift.

       A means of
  election verification
   far more powerful
 than other methods.
            5
“But with cryptography, you’re just
moving the black box. Few people really
      understand it or trust it.”

            Debra Bowen
  California Sec. of State, 7/30/2008
             (paraphrased)

                   6
DRE
       code




                         Election
                         Results



time

              election

                   7
Three Points
1. Voting is a unique trust problem.
2. Cryptography is not just about secrets,
   it enables collaboration w/o blind trust,
   it democratizes auditing processes.
3. Truly Verifiable Voting
   is closing in on practicality.


                     8
1.
Voting is a unique
 trust problem.


        9
“Swing Vote”

terrible movie.
hilarious ending.

        10
Wooten got the news from his wife, Roxanne,
  who went to City Hall on Wednesday
        to see the election results.

  "She saw my name with zero votes by it.
      She came home and asked me if
       I had voted for myself or not."



                    11
12
13
Bad Analogies

Not just that
ATMs and planes are vulnerable
(they are, but that’s not the point)
It’s that voting is much harder.



                   14
Bad Analogies
Adversaries
➡ pilots vs. passengers (airline is on your side, I think.)
➡ banking privacy is only voluntary:
  you are not the enemy.
Failure Detection & Recover
➡ plane crashes & statements vs. 2% election fraud
➡ Full banking receipts vs. destroying election evidence


Imagine
➡ a bank where you never get a receipt.
➡ an airline where the pilot is working against you.


                            15
Ballot secrecy
conflicts with auditing,
    cryptography
 can reconcile them.


           16
http://www.cs.uiowa.edu/~jones/voting/pictures/   17
/*          1
                                                    * source
                                                    * code
     Polling                  Voting                */
                   3                           2
    Location                 Machine
                                                   if (...

                                                                   Vendor
        4

Alice
                                                                   Results
               5                                         6           .....


                       Ballot Box Collection
                            Black Box

                                 18
Chain of Custody




       19
20
2.
  Cryptography is not
    just about secrets,
it enables collaboration
      w/o blind trust.

           21
Initially,
  cryptographers
     re-created
 physical processes
in the digital arena.

          22
Then, a realization:
cryptography enables a
 new voting paradigm

Secrecy + Auditability.

           23
Public Ballots
            Bulletin Board

         Alice:         Bob:
         Obama         McCain

                  Carol:
                  Obama
                                  Tally

                                Obama....2
                                McCain...1
Alice
                  24
Encrypted Public Ballots
                               Bulletin Board

                        Alice:                 Bob:
                         Rice                 Clinton

                                     Carol:
Ali
      ce                              Rice                         ta  lly
                                                                 e
                                                                hTally
           ver
               ifie                                       ifie st
                   s                                 ever
                       he                         on
                            rv
                              ote         E   very          Obama....2
                                                            McCain...1
Alice
                                     25
End-to-End Verification
                                      /*
                                       * source
                                       * code
                    Voting             */
                   Machine
                                      if (...

                                                      Vendor

                              Ballot Box /                 Results
     Polling                 Bulletin Board
    Location                                                   .....




                  1                               2
Alice
               Receipt




                              26
Democratizing Audits
Each voter is responsible for checking
their receipt (no one else can.)
Anyone, a voter or a public org,
can audit the tally and
verify the list of cast ballots.
Thus, “open-audit” or
truly-verifiable voting

                   27
NO!
Increased transparency
   when some data
  must remain secret.
          28
So, yes, we encrypt,
and then we work with the
 encrypted data in public, so
      everyone can see.

In particular, because the vote
  is encrypted, it can remain
  labeled with voter’s name.
               29
“Randomized” Encryption
Keypair consists of a public key pk and a secret key sk .

          "Obama"         Enc pk       8b5637



          "McCain"        Enc pk       c5de34



          "Obama"        Enc pk        a4b395


                           30
Threshold Decryption
      Secret key is shared amongst multiple parties:
all (or at least a quorum) need to cooperate to decrypt.

                 Dec sk1    b739cb

                 Dec sk2    261ad7
     8b5637                                  "Obama"
                 Dec sk3    7231bc

                 Dec sk4    8239ba


                           31
Homomorphic
      Encryption
Enc(m1 ) × Enc(m2 ) = Enc(m1 + m2 )

      g   m1
               ×g   m2
                          =g   m1 +m2


         then we can simply
  add “under cover” of encryption!

                     32
Mixnets



c = Encpk1 (Encpk2 (Encpk3 (m)))
          Each mix server “unwraps”
       a layer of this encryption onion.

                      33
Proving certain details while
   keeping others secret.

   Proving a ciphertext
 encodes a given message
    without revealing
    its random factor.
              34
Zero-Knowledge Proof
                                       President:
                                         President:
                                      Mickey Mouse
                                          President:
                                       Mickey Mouse
                                            President:
                                         Mickey Mouse
                                             President:
                                          Mickey Mouse
                                               President:
                                            Mickey Mouse
                                                Vote For:
                                             Mickey Mouse
                                                Obama
Vote For:
 Obama




                This last envelope
            likely contains “Obama”

                      35
Zero-Knowledge Proof
    President:                 President:
      President:
   Mickey Mouse                  President:
                              Mickey Mouse
       President:
    Mickey Mouse                  President:
                               Mickey Mouse
         President:
      Mickey Mouse                  President:
                                 Mickey Mouse
          President:
       Mickey Mouse                  President:
                                  Mickey Mouse
            President:
         Mickey Mouse                  President:
                                    Mickey Mouse
             Vote For:
          Mickey Mouse                  Vote For:
                                     Mickey Mouse
             Obama                      McCain
                                         Paul




    Open envelopes don’t prove
      anything after the fact.

                         36
A little bit more math
      y=g       x
                         mod p

      S=g   r
                    mod p


                     c

        t = xc + r                t ?
                                 g = Sy c



                    37
does this prove anything?
               y=g       x
                                  mod p

               S=g   r
                             mod p


       c                                       c

  t = xc + r                              t = xc + r




                             38
what’s so special about it?
        y=g   x
                        mod p

         S=g   r
                        mod p

                                t ?
                   c            g = Sy   c


           t = xc + r




                   39
Electronic Experience
                                      Voter interacts with a voting
                                      machine
Voting Machine
                              Alice
                                      Obtains a freshly printed receipt
                                      that displays the encrypted ballot
             Encrypted Vote

                                      Takes the receipt home and uses it
                                      as a tracking number.
                                      Receipts posted for public tally.


                                             40
David
                            Adam
                                                Paper Experience
                                Bob
                           Charlie
    David _______
    Adam _______
     Bob _______                                 paper ballots with indirection
                                                 between candidate and choice
 Charlie _______
                           _______
               8c3sw
                           _______
                           _______
                           _______
                                8c3sw
                                                 break the indirection (tear, detach)
                        Adam - x
                                      8c3sw
                                                 for effective encryption
                         Bob - q
                       Charlie - r
                        David - m                take receipt home and use it
 Adam - x
  Bob - q
              8c3sw
                                                 as tracking number.
Charlie - r

q
q
 David - m
     r
     r   m
         m     x
               x
                                        8c3sw
                                                 receipts posted for public tally.

                       q    r     m      x




                                                         41
3.
Cryptography-based Voting
  (Truly Verifiable Voting)
is closing in on practicality.


              42
Benaloh Casting
                                   "Obama"

                                   Encrypted
                                     Ballot
                    Alice



        "AUDIT"                                         "CAST"
           Decrypted                                      Signed
             Ballot                                      Encrypted
                                                           Ballot
Alice                                           Alice

        Encrypted      Decrypted
          Ballot         Ballot
                                                         Signed
                                                        Encrypted
                                                          Ballot

            VERIFICATION
                                                Alice




                                           43
Many more great ideas
Neff ’s MarkPledge
➡   high-assurance, human-verifiable, proofs of correct encryption
Prêt-à-Voter by Ryan et al.
➡   elegant, simple, paper-based
STV: Ramchen, Teague, Benaloh & Moran.
➡   handling complex election styles
Scantegrity I & II
➡   closely mirrors opscan voting


                               44
Deployments!

Scantegrity II @ Takoma Park
real municipal elections
Université catholique de Louvain
25,000 voters
Scratch, Click & Vote


                  45
Three Points
1. Voting is a unique trust problem.
2. Cryptography is not just about secrets,
   it enables collaboration w/o blind trust,
   it democratizes the auditing process.
3. Truly Verifiable Voting
   is closing in on practicality.


                     46
My Fear:
  computerization of
   voting is inevitable.
without true verifiability,
 the situation is grim.

            47
My Hope:
   public auditing proofs
will soon be as common as
 public-key crypto is now.

            48
Challenges




Ed Felten: “you have no voter privacy, deal with it.”


                          49
Questions?

             50

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Beginners Guide to TikTok for Search - Rachel Pearson - We are Tilt __ Bright...
 

Truly Verifiable Elections

  • 1. Truly Verifiable Voting Ben Adida Harvard University MSR Voting Technology Workshop 19 March 2010
  • 2. “If you think cryptography is the solution to your problem.... 2
  • 3. ... then you don’t understand cryptography... ... and you don’t understand your problem.” 3
  • 4. Yet, cryptography solves problems that initially appear to be impossible. 4
  • 5. There is a potential paradigm shift. A means of election verification far more powerful than other methods. 5
  • 6. “But with cryptography, you’re just moving the black box. Few people really understand it or trust it.” Debra Bowen California Sec. of State, 7/30/2008 (paraphrased) 6
  • 7. DRE code Election Results time election 7
  • 8. Three Points 1. Voting is a unique trust problem. 2. Cryptography is not just about secrets, it enables collaboration w/o blind trust, it democratizes auditing processes. 3. Truly Verifiable Voting is closing in on practicality. 8
  • 9. 1. Voting is a unique trust problem. 9
  • 11. Wooten got the news from his wife, Roxanne, who went to City Hall on Wednesday to see the election results. "She saw my name with zero votes by it. She came home and asked me if I had voted for myself or not." 11
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  • 13. 13
  • 14. Bad Analogies Not just that ATMs and planes are vulnerable (they are, but that’s not the point) It’s that voting is much harder. 14
  • 15. Bad Analogies Adversaries ➡ pilots vs. passengers (airline is on your side, I think.) ➡ banking privacy is only voluntary: you are not the enemy. Failure Detection & Recover ➡ plane crashes & statements vs. 2% election fraud ➡ Full banking receipts vs. destroying election evidence Imagine ➡ a bank where you never get a receipt. ➡ an airline where the pilot is working against you. 15
  • 16. Ballot secrecy conflicts with auditing, cryptography can reconcile them. 16
  • 18. /* 1 * source * code Polling Voting */ 3 2 Location Machine if (... Vendor 4 Alice Results 5 6 ..... Ballot Box Collection Black Box 18
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  • 21. 2. Cryptography is not just about secrets, it enables collaboration w/o blind trust. 21
  • 22. Initially, cryptographers re-created physical processes in the digital arena. 22
  • 23. Then, a realization: cryptography enables a new voting paradigm Secrecy + Auditability. 23
  • 24. Public Ballots Bulletin Board Alice: Bob: Obama McCain Carol: Obama Tally Obama....2 McCain...1 Alice 24
  • 25. Encrypted Public Ballots Bulletin Board Alice: Bob: Rice Clinton Carol: Ali ce Rice ta lly e hTally ver ifie ifie st s ever he on rv ote E very Obama....2 McCain...1 Alice 25
  • 26. End-to-End Verification /* * source * code Voting */ Machine if (... Vendor Ballot Box / Results Polling Bulletin Board Location ..... 1 2 Alice Receipt 26
  • 27. Democratizing Audits Each voter is responsible for checking their receipt (no one else can.) Anyone, a voter or a public org, can audit the tally and verify the list of cast ballots. Thus, “open-audit” or truly-verifiable voting 27
  • 28. NO! Increased transparency when some data must remain secret. 28
  • 29. So, yes, we encrypt, and then we work with the encrypted data in public, so everyone can see. In particular, because the vote is encrypted, it can remain labeled with voter’s name. 29
  • 30. “Randomized” Encryption Keypair consists of a public key pk and a secret key sk . "Obama" Enc pk 8b5637 "McCain" Enc pk c5de34 "Obama" Enc pk a4b395 30
  • 31. Threshold Decryption Secret key is shared amongst multiple parties: all (or at least a quorum) need to cooperate to decrypt. Dec sk1 b739cb Dec sk2 261ad7 8b5637 "Obama" Dec sk3 7231bc Dec sk4 8239ba 31
  • 32. Homomorphic Encryption Enc(m1 ) × Enc(m2 ) = Enc(m1 + m2 ) g m1 ×g m2 =g m1 +m2 then we can simply add “under cover” of encryption! 32
  • 33. Mixnets c = Encpk1 (Encpk2 (Encpk3 (m))) Each mix server “unwraps” a layer of this encryption onion. 33
  • 34. Proving certain details while keeping others secret. Proving a ciphertext encodes a given message without revealing its random factor. 34
  • 35. Zero-Knowledge Proof President: President: Mickey Mouse President: Mickey Mouse President: Mickey Mouse President: Mickey Mouse President: Mickey Mouse Vote For: Mickey Mouse Obama Vote For: Obama This last envelope likely contains “Obama” 35
  • 36. Zero-Knowledge Proof President: President: President: Mickey Mouse President: Mickey Mouse President: Mickey Mouse President: Mickey Mouse President: Mickey Mouse President: Mickey Mouse President: Mickey Mouse President: Mickey Mouse President: Mickey Mouse President: Mickey Mouse Vote For: Mickey Mouse Vote For: Mickey Mouse Obama McCain Paul Open envelopes don’t prove anything after the fact. 36
  • 37. A little bit more math y=g x mod p S=g r mod p c t = xc + r t ? g = Sy c 37
  • 38. does this prove anything? y=g x mod p S=g r mod p c c t = xc + r t = xc + r 38
  • 39. what’s so special about it? y=g x mod p S=g r mod p t ? c g = Sy c t = xc + r 39
  • 40. Electronic Experience Voter interacts with a voting machine Voting Machine Alice Obtains a freshly printed receipt that displays the encrypted ballot Encrypted Vote Takes the receipt home and uses it as a tracking number. Receipts posted for public tally. 40
  • 41. David Adam Paper Experience Bob Charlie David _______ Adam _______ Bob _______ paper ballots with indirection between candidate and choice Charlie _______ _______ 8c3sw _______ _______ _______ 8c3sw break the indirection (tear, detach) Adam - x 8c3sw for effective encryption Bob - q Charlie - r David - m take receipt home and use it Adam - x Bob - q 8c3sw as tracking number. Charlie - r q q David - m r r m m x x 8c3sw receipts posted for public tally. q r m x 41
  • 42. 3. Cryptography-based Voting (Truly Verifiable Voting) is closing in on practicality. 42
  • 43. Benaloh Casting "Obama" Encrypted Ballot Alice "AUDIT" "CAST" Decrypted Signed Ballot Encrypted Ballot Alice Alice Encrypted Decrypted Ballot Ballot Signed Encrypted Ballot VERIFICATION Alice 43
  • 44. Many more great ideas Neff ’s MarkPledge ➡ high-assurance, human-verifiable, proofs of correct encryption Prêt-à-Voter by Ryan et al. ➡ elegant, simple, paper-based STV: Ramchen, Teague, Benaloh & Moran. ➡ handling complex election styles Scantegrity I & II ➡ closely mirrors opscan voting 44
  • 45. Deployments! Scantegrity II @ Takoma Park real municipal elections Université catholique de Louvain 25,000 voters Scratch, Click & Vote 45
  • 46. Three Points 1. Voting is a unique trust problem. 2. Cryptography is not just about secrets, it enables collaboration w/o blind trust, it democratizes the auditing process. 3. Truly Verifiable Voting is closing in on practicality. 46
  • 47. My Fear: computerization of voting is inevitable. without true verifiability, the situation is grim. 47
  • 48. My Hope: public auditing proofs will soon be as common as public-key crypto is now. 48
  • 49. Challenges Ed Felten: “you have no voter privacy, deal with it.” 49