By Warren D. Smith 16 May 2011
Ralf Küsters, Tomasz Truderung, and Andreas Vogt (all at University of Trier, Germany) in a preliminary (2011) paper titled Verifiability, Privacy, and Coercion-Resistance: New Insights from a Case Study, found an unexpected bust of the Rivest-Smith low-tech "secure/antiFraud voting" schemes "ThreeBallot" and "VAV."
However, in practice this "bust" looks to be not as severe a problem as it might at first seem, and at present it still looks likely that Rivest-Smith would, if implemented, raise voting in large elections to new levels of security far beyond what humanity ever accomplished previously. Unfortunately the KTV attack seems serious in small secret-ballot elections.
I will now try to explain the bust, and then explain considerations which appear, for large elections, to "bust the bust."
They assume there are a large (but minority) set of dishonest voters and (collaborating with them) a dishonest central election authority. The dishonest voters each cast (in an A versus B election using "ThreeBallot")
keeping the first SingleBallot for receipt purposes. Dishonest Voter phones her dishonest contact at the central authority to inform them of her SingleBallot's serial number. The dishonest central authority uses that information to mispost her xo SingleBallot as ox, but the dishonest voter agrees never to complain about that mispost. The corrupt central authority counts this as three votes for B, not (which it honestly should have been) one vote for B (in the form, more precisely, of 2 for B and 1 for A). That is, the dishonest voter effectively is permitted to submit
as her vote (which if she'd tried that directly, would have been rejected by the checker machine).
The full set of SingleBallots that the corrupt election authority then falsely claims exists remains a valid set provided the minority of colluding cheating pro-B voters is smaller than the set of pro-A honest voters.
So... it looks to me like they are right: this is a valid attack.
KTV then point out that there are different criteria for (what they call) "election verifiability". Rivest & Smith's protocols satisfied the criteria that
Those seemed good to Rivest+Smith, but as KTV point out are not good enough to grant (what they call) "global" verifiability, which means
Question: Is global verifiability achievable at all? (Apparently Kuesters et al do not offer an opinion on this question?)
Frauding elections must be considered from a cost and benefit point of view. E.g. consider the "Tweed Ring" which used election fraud to take over New York City (and to a lesser extent New York State) during about 1865-1872. These fraudsters were easily and reliably able to accomplish numerically-enormous frauds, and the rewards for these fraudsters were tremendous. Tweed diverted a large fraction of New York City's finances into his own and his friends and co-conspirators' pockets, becoming one of the richest men in the country. (He founded his own bank in order to store his loot.) Furthermore, he was able to commit the frauds with the aid of a fairly small number of conspirators. The number of Boss Tweed's co-conspirators is not clear and depends on definitions... arguably there were only 8 or so key conspirators, but also arguably perhaps as many as 1000 participated. Tweed eventually was jailed, escaped, but re-jailed and died in jail.
In the KTV attack, to shift the B-over-A margin by 2M votes, you need at least M colluding conspirators. That is a very small
ratio (2), compared to the Tweed ring (more like 60000). That makes the risks much larger and the benefits much smaller for KTV's Fraudsters. Furthermore, suppose even just one of KTV's colluding cheating voters, happened to be an agent provacateur (or just later had a change of heart). She tells the corrupt election authority "I'll be happy to join in your election rigging scheme." She submits her vote as per Kuester et al's instructions, sends them her serial number so they can modify her vote... and all is fine... except then she reneges on the deal and complains that they altered her SingleBallot!
Even if she sends them her SingleBallot for verified destruction by them, she still could keep a copy of it (thanks to digital signatures)... which alone would suffice to prove the fraud in court!!!
So it seems to me, yes, the Kuesters collusion-attack works, but they all must have total trust in each other. If just one collaborator actually is a double agent... then they're busted. And, they have to worry about that risk even if there is no double agent.
Therefore, this attack in practice may not hurt much. I doubt it is possible for any Tweedlike Fraud Ring to have total trust in more than 1000 colluders, so they could shift at most 2000 votes from A to B. That's a tiny vote-shift in New York City, accomplished at great risk to them. It would be able to alter only a tiny fraction of elections, and a fraction not predictable in advance.
During 1897-2011, no New York City mayor election has been decided by fewer than 50,000 votes, and only two by fewer than 250,000.
From a cost-benefit point of view this whole fraud was not even worth trying. The KTV attack in this setting seems almost no threat. On the other hand, in a vote with only 20 voters of which, say, 5 (plus the tallier) were corrupt colluders, the KTV attack would be a serious problem. Fortunately, most such small high-stakes votes (e.g. corporate boards, votes on bills in Congress, etc) are not secret ballot, hence for them you could get total security by simply publishing the list of all the (named) voters and their votes.
And we already knew Rivest-Smith was vulnerable to very small vote-total-altering frauds (the probability of detection had been thought to rise with the size of the fraud to approach 100% exponentially, but for a tiny fraud could easily escape detection). So the KTV attack makes that known issue in small elections, worse. For large elections, the KTV attack, at least as currently formulated, seems not really a credible threat, or anyhow a small enough threat that even if every large election fraudable in this way (i.e. every large election with honest margin<2000) was, in fact, frauded, it would not hurt humanity by any detectable amount.
In comparison, I point out that Florida in the Bush-Gore presidential contest 2000, altered the vote by ≈30,000 or more by simply falsely declaring/defining (before the election began) a large number of (predominantly Democrat) voters not to be voters since they were "felons."
Elections in small towns or small secret-ballot elections such as the conclaves electing the Catholic Pope, seem the type most hurt by the KTV attack.
Bruno Parga writes:
I was reading [this webpage] and I have a doubt you might be able to answer. The key to the attack, it seems, is this: "Dishonest Voter phones her dishonest contact at the central authority to inform them of her SingleBallot's serial number. The dishonest central authority uses that information to mispost her xo SingleBallot as ox, but the dishonest voter agrees never to complain about that mispost." Wouldn't that be made a lot harder, hard enough for the attack never to be worth it, if every precinct published its own ballots instead of letting a central authority do that?
Bruno Parga's point is, the "dishonest contact at the central authority" would now need to be multiple dishonest contacts, one in each precinct the dishonest voters are in. That makes it harder. In my (W.D.Smith's) opinion (pending comments by others) this would make the KTV attack less effective for large elections (I've argued it already was not a useful attack on large elections, but with Parga's defense that is even more true). Also, individual-precinct publication was a good idea anyhow with Rivest-Smith since it would mean that when a fraud does happen and is detected, we get to know, with somewhat more confidence, where the fraud was happening. (We'd already know that pretty well by knowing which voters were the ones complaining about their receipts disagreeing with the votes posted on the bulletin board, but not if those complainer-voters tried hard to remain anonymous. With precinct-stating bulletin boards, we'd know where the fraud was happening even with entirely-anonymous complainer-voters.) However, unfortunately, KTV remains a good attack on small secret ballot elections.
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