Unformatted text preview: o Alice, Carol, and Dave. Each vote now looks like this: SB(EC(ED(V,R1))) (10) Carol verifies and deletes Bob’s signatures. She decrypts all the votes with her private key, checks to see that her vote is among the set of votes, signs all the votes, and then sends the result to Alice, Bob, and Dave. Each vote now looks like this: SC(ED(V,R1)) (11) Dave verifies and deletes Carol’s signatures. He decrypts all the votes with his private key, checks to see that his vote is among the set of votes, signs all the votes, and then sends the result to Alice, Bob, and Carol. Each vote now looks like this: SD(V,R1) (12) All verify and delete Dave’s signature. They check to make sure that their vote is among the set of votes (by looking for their random string among the votes). (13) Everyone removes the random strings from each vote and tallies the votes. Not only does this protocol work, it is also self-adjudicating. Alice, Bob, Carol, and Dave will immediately know if someone tries to cheat. No CTF or CLA is required. To see how this works, let’s try to cheat. If someone tries to stuff the ballot, Alice will detect the attempt in step (3) when she receives more votes than people. If Alice tries to stuff the ballot, Bob will notice in step (4). More devious is to substitute one vote for another. Since the votes are encrypted with various public keys, anyone can create as many valid votes as needed. The decryption protocol has two rounds: round one consists of steps (3) through (7), and round two consists of steps (8) through (11). Vote substitution is detected differently in the different rounds. If someone substitutes one vote for another in round two, his actions are discovered immediately. At every step the votes are signed and sent to all the voters. If one (or more) of the voters noticed that his vote is no longer in the set of votes, he immediately stops the protocol. Because the votes are signed at every step, and because everyone can backtrack through the second round of t...
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- Fall '10
- Cryptography, Bruce Schneier, Applied Cryptography, EarthWeb, Search Search Tips