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Secret Ballot Receipts: True Voter Verifiable Elections

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Secret Ballot Receipts: True Voter Verifiable Elections Author: David Chaum Published: IEEE Security & Privacy Presenter: Adam Anthony – PowerPoint PPT presentation

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Title: Secret Ballot Receipts: True Voter Verifiable Elections


1
Secret Ballot ReceiptsTrue Voter Verifiable
Elections
  • Author David Chaum
  • Published IEEE Security Privacy
  • Presenter Adam Anthony

2
Outline
  • Paper Selection Criteria
  • Secret Ballot Discussion
  • Electronic vs. Handwritten ballots
  • Summary of Results
  • Physical Receipt Characteristics
  • Verifying Votes
  • Properties of the system
  • Encoding, decoding, tallying votes
  • Conclusion

3
Paper Selection
  • Google Scholar 25 Citations
  • Published in IEEE Security and Privacy 2004
  • David Chaum founded the International
    Association for Cryptographic Research, has filed
    25 separate cryptography related patents
  • Referenced directly in Wednesdays paper
  • Scored 1,545,673 out of a possible 1,545,674
    points on the Adam Anthony thinks its a really
    neat paper scale

4
Secret Ballots
  • Required by free democracies
  • Basic premise The voter brings nothing out of
    the polling place that he didnt bring in that
    would provide information as to who he voted for.
  • Buttons, T-Shirts, etc. allowed
  • Copy of ballot, plaintext ballot materials, not
    allowed

5
Trust Issues
  • Handwritten Ballots are the Gold Standard of
    voting
  • Electronic voting machines are considered insecure

6
Summary of Results
  • Use visual encryption to produce a
    zero-information ballot receipt
  • Eliminates the need for proprietary black box
    systems
  • Setup
  • A normal computer running openly published,
    verifiable software
  • A special receipt printer
  • User may take part of the encrypted receipt with
    him which can be used (personally, or by his
    party affiliation officials) to verify the
    correctness of his ballot
  • Additionally, correctness can be verified without
    revealing who he voted for
  • Tallying of votes is also quickly verifiable

7
Printer Requirements
  • Printer fundamentally appears to be a simple cash
    register receipt printer
  • Printer heads are positioned to print on both the
    front and back of a clear polymer tape
  • The tape is actually 2 laminated pieces of tape
  • The bottom inch contains instructions for
    separating the tape

8
Receipts, continued
9
Encoding a Receipt
  • Generate one pad of random pixel symbols (white
    sheet)
  • The second pad is created by choosing the correct
    symbol to either allow transparency or opacity
    (red sheet)
  • Transparent portions produce the type-set report
  • Swap every other pixel symbol between the two
    sheets so that either layer can be chosen as the
    receipt

10
Verifying Receipts
  • Handheld scanners can be used to verify ballot
    consistency outside the polling place
  • Digital copies of the receipts are sent to the
    main server
  • Online Enter the serial number at the bottom of
    the receipt and verify the image on record is
    identical to your own
  • Eventually, all ballots are decrypted and posted
    online as well, to verify the count

11
Properties
  1. If your receipt is correctly posted, you can be
    sure (with acceptable probability) that your vote
    will be included correctly in the tally
  2. No one can decode your receipt or otherwise link
    it to your vote except by breaking the code or
    decrypting it using all the secret keys, each of
    which is assigned to a different trustee
  3. There are only 3 ways a system could change a
    voters ballot without direct detection
  4. Print an incorrect layer, gambling theyll choose
    the other layer
  5. Use the same serial number for 2 different
    receipts, hoping the 2 voters choose the same
    layer
  6. Perform a tally process step incorrectly, taking
    the chance that the step will escape selection
    during the audit
  7. There is a 50/50 chance that any of the above
    fraud attempts will succeed, per ballot

12
Meat, Potatoes, Hold the Vegetables
  • Where weve been
  • System Hardware Specification
  • Encoding Receipts
  • Verifying Receipts
  • Properties of the system
  • Where were going
  • Mathematical model of the voting process
  • Mathematical model of the tallying process
  • Proof of system properties

13
About Dolls
  • Author uses the Russian Doll analogy to explain
    the decryption process.
  • A Doll consists of a set of random pads, added
    together (mod 2)
  • The largest doll is used to create the
    background sheet
  • There is a set of private keys that opens one
    of each of the dolls.
  • The output of the decryption yields a partially
    decrypted message, as well as the value of the
    next doll
  • Several trustees oversee each phase of
    decryption, basic key management schemes protect
    against missing/corrupt trustees

14
Voting Phase
  1. The voter supplies a ballot image B
  2. The system responds by providing two 4-tuples
    ltLz,q,Dt,Dbgt - this is the data printed on each
    separate layer
  3. The voter visually verifies that Lt ? Lb B and
    that q, Dt, Db are identical on both layers
  4. Voter aborts if there is a problem, or selects
    x t or b for his choice of the top or bottom
    layer

15
Voting Phase, cont.
  1. The system makes two digital signatures, and
    provides them as a 2-tuple ltsx(q), ox(Lz,q,Dt,Db,
    sx(q)) gt
  2. The voter (or a designate) performs a consistency
    check to ensure that the digital signatures of
    the 2-tuple check, using agreed public inverses
    of the systems private signature functions sx
    and ox,with the unsigned version of the
    corresponding values of the selected 4-tuple (as
    printed) on the selected layer, and that sx(q)
    correctly determines Dx and the half of the
    elements of Lx that it should determine

16
Yet more on the voting phase
  • Remember that each layer contains an equal amount
    of red bits (the message) and white bits (the
    sum of dolls)
  • Let Rz and Wz be matrices representing the set of
    red and white bits for layer image Lz
  • Let h and h be pseudo-random functions of q
  • ei is a public key corresponding to a trustees
    private key di
  • Lti,2j - (i mod 2) Rt
  • Lti,2j - (i 1 mod 2) Wt
  • Lbi,2j - (i 1 mod 2) Rb
  • Lbi,2j - (i mod 2) Wb
  • Rx ? Wy Bx
  • Wzi,j (dzk ? dzk-1 ? ? dz1)
  • dzl h(sz(q),l)
  • dzl h(dzl)
  • Dzl el(dzl e2(dz2(e1(dz1))
  • The final Doll, Dz Dzk

17
Decryption to Plaintext
  • Input Lx and Dy , refer to them as Bk, Dk
  • Compute dl from Dk using the proper private key
  • Dk-1 Dk/ dl
  • Find dl using h
  • Compute Bk-1 Bk ? dl
  • B0 Bz, the plaintext ballot

18
More important than decryption
19
Conclusion
  • Reduces the cost of integrity while raising its
    level dramatically
  • Voters are able to assure their own vote
  • Voting can be more accessible due to the better
    handling of provisional ballots
  • Hardware system costs are lower than current
    black-box systems, cost of printers should be
    less than the money saved
  • Simpler maintenance, easier upgrade, multiple
    uses
  • Open code means opposing parties will work hard
    to assure its integrity, and the government can
    fund the operation as well
  • The auditing of trustees and system integrity is
    easily automated, and mathematically sound
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