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Hugh Thompson, an adjunct computer science professor at the Florida Institute of Technology … and Harri Hursti, a Finnish computer scientist, were able to change votes on the Diebold machine without leaving a trace. … The hack Thompson and Hursti performed involves a memory card that's inserted in the Diebold machines to record votes as officials scan ballots. According to Thompson, data on the cards isn't encrypted or secured with passwords. Anyone with programming skills and access to the cards -- such as a county elections technical administrator, a savvy poll worker or a voting company employee -- can alter the data using a laptop and card reader.
The vote is rigged by installing a card that starts with a certain number of votes for one candidate and the same number of votes subtracted from the count for a second candidate. The total number of votes will be correct. After the candidate with the initially negative total receives that number of votes, the evidence of tampering is essentially gone. The card can also be rigged to report that it is initially blank even though it isn't.