Well if you could change the source code on the voting machine it's something as simple as you show the Voter their choice to confirm on the frontend but the backend has been manipulated to always vote for someone else instead. The voter would never know their choice was altered. An example is as simple as the following:
def selectCandidate(voter_id, choice):
if choice == 1:
return {"Voter":voter_id, "Displaychoice": "Candidate 1", "Choice": "Candidate 1"}
if choice == 2:
return return {"Voter":voter_id, "Displaychoice": "Candidate 2", "Choice": "Candidate 1"}
if choice == 3:
return return {"Voter":voter_id, "Displaychoice": "Candidate 3", "Choice": "Candidate 1"}
raise Exception("Should never happen :-p")
