applied cryptography - protocols, algorithms, and source code in c

authors bruce schneier isbn 0471128457 publication

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Unformatted text preview: t of votes signed by the CTF. (These votes are signed but unencrypted, so the voter can easily see which vote is “yes” and which is “no.”) (5) The voter chooses one of the votes (ah, democracy) and encrypts it with the CTF’s public key. (6) The voter sends his vote in. (7) The CTF decrypts the votes, checks the signatures, checks its database for a duplicate identification number, saves the serial number, and tabulates the votes. It publishes the results of the election, along with every serial number and its associated vote. A malicious voter, call him Mallory, cannot cheat this system. The blind signature protocol ensures that his votes are unique. If he tries to send in the same vote twice, the CTF will notice the duplicate serial number in step (7) and throw out the second vote. If he tries to get multiple votes signed in step (2), the CTF will discover this in step (3). Mallory cannot generate his own votes because he doesn’t know the facility’s private key. He can’t intercept and change other people’s votes for the same reason. The cut-and-choose protocol in step (3) is to ensure that the votes are unique. Without that step, Mallory could create a set of votes that are the same except for the identification number, and have them all validated. A malicious CTF cannot figure out how individuals voted. Because the blind signature protocol prevents the facility from seeing the serial numbers on the votes before they are cast, the CTF cannot link the blinded vote it signed with the vote eventually cast. Publishing a list of serial numbers and their associated votes allows voters to confirm that their vote was tabulated correctly. There are still problems. If step (6) is not anonymous and the CTF can record who sent in which vote, then it can figure out who voted for whom. However, if it receives votes in a locked ballot box and then tabulates them later, it cannot. Also, while the CTF may not be able to link votes to individuals, it can generate a large numbe...
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This note was uploaded on 10/18/2010 for the course MATH CS 301 taught by Professor Aliulger during the Fall '10 term at Koç University.

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