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Sorry for the long delay,
well, first of all we have to say that this is a Proof Of Concept. In a real environment things would be different.
In order to prevent bad actors from voting multiple times, I image this scenario.
When someone wants to vote, the ByteStamp server assigns a unique vote code. This code vode is assigned by ByteStamp, but in a real environment it would be assigned by an institution.
Let's say you go to an office where there is an officer who attests that you can vote.
Let's say there is a box full of electoral codes in that office. Whoever has the right to vote randomly picks a single electoral code from the box. No one knows which electoral code is taken, as well as no one knows who is the owner of a Bitcoin address. But everyone can see that the electoral code is a valid code.
Then the voter votes using that code and then destroy the code itself.
Now, the unique electoral code is recorded in the blockchain along with the vote. In this way no one can vote multiple times because only the first vote will be valid. In order to vote multiple times you would need multple electoral codes (that are issued by an Authority).
To prevent to buy/sell votes, let me say it's arguable.
One can say "I have voted with the code 1234 so give me your money".
But everyone can see on the blockchain how voted the code 1234 and everyone could say that 1234 is his/her own vote code.
Lat's say that the vode codes are simple paper pieces that anyone could print with a PC at home.
No one can prove that 1234 is its own vote code.
In my opinion, the risk of buy/sell votes is higher now that peoples take photos with smartphone on how they voted.
Thank You for your interest in ByteStamp
By ByteStamp - MIDA SRLS -
http://www.bytestamp.net - 29/04/2017
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The file is timestamped at UTC Date/Time:
2017-04-29 14:46:17 UTC in block number 1784978 - ID transaction: 9caf6293f66b27d06c4f93986579533bbd7349fdd9e67cc5d2b934926e83bcf8 #ByteStampProof
http://www.bytestamp.net/docs/qdoc/en/76849f912edd14aa53cc7f04092aab60