May 30, 2024

Archives for October 2006

ThreeBallot and Tampering

Let’s continue our discussion (1; 2) of Rivest’s ThreeBallot voting system. I’ve criticized ThreeBallot’s apparent inability to handle write-in votes. More detailed critiques have come from Charlie Strauss (1; 2) and Andrew Appel. Their analysis (especially Charlie’s) is too extensive to repeat here, so I’ll focus on just one of Charlie’s ideas.

Recall that ThreeBallot requires each voter to mark three ballots. Each candidate must be marked on at least one ballot (call this the mandatory mark); and the voter can vote for a candidate by marking that candidate on a second ballot. I call these rules (each candidate gets either one or two marks, and at most one candidate per race gets two marks) the Constraints. Because of the Constraints, we can recover the number of votes cast for a candidate by taking the total number of marks made for that candidate, and subtracting off the number of mandatory marks (which equals the number of voters).

ThreeBallot uses optical scan technology: the voter marks paper ballots that can be read by a machine. A voter’s three ballots are initially attached together. After filling out the ballots, the voter runs them through a checker machine that verifies the Constraints. If the ballots meet the Constraints, the checker puts a red stripe onto the ballots to denote that they have been checked, separates the three ballots, and gives the voter a copy of one ballot (the voter chooses which) to take home. The voter then deposits the three red-striped ballots into a ballot box, where they will eventually be counted by a tabulator machine.

Charlie Strauss points out there is a window of vulnerability from the time the ballots are checked until they’re put in the ballot box. During this time, the voter might add marks for his desired candidate, or erase marks for undesired candidates, or just put one of the ballots in his pocket and leave. These tactics are tantamount to stuffing the ballot box.

This kind of problem, where a system checks whether some condition is true, and then later relies on that assumption still being true even though things may have changed in the meantime, is a common cause of security problems. Security folks, with our usual tin ear for terminology, call these “time of check to time of use” or TOCTTOU bugs. (Some people even try to pronounce the acronym.) Here, the problem is the (unwarranted) assumption that if the Constraints hold when the ballots are put into the checker, they will still hold when the ballots are later tabulated.

One way to address this problem is to arrange for the same machine that checks the ballots to also tabulate them, so there is no window of vulnerability. But ThreeBallot’s security depends on the checker being dumb and stateless, so that it can’t remember the ballot-sets of individual voters. The tabulator is more complicated and remembers things, but it only sees the ballots after they are separated and mixed together with other voters’ ballots. Rivest makes clear that the checker and the tabulator must be separate mechanisms.

We might try to fix the problem by having the checker spit out the checked ballots into a sealed, glass-sided chute with a ballot box at the bottom, so the voter never gets to touch the ballots after they are checked. This might seem to eliminate the window of vulnerability.

But there’s still a problem, because the ballots are scanned by two separate scanner devices, one in the checker and one in the tabulator. Inevitably the two scanners will be calibrated differently, so there are some borderline cases that look like a mark to one scanner but not to the other. This means that ballot-triples that meet the Constraints according to one scanner won’t necessarily meet them according to the other scanner. A clever voter can exploit these differences, regardless of which scanner is more permissive, to inflate the influence of his ballots. He can make his mandatory marks for disfavored candidates faint, so that the checker just barely detects them, in the hope that the tabulator will miss them. And he can add a third mark for his favored candidate, just barely too faint for the checker to see, in the hope that the tabulator will count it. If either of these hopes is fulfilled, the final vote count will be wrong.

This is just a small sample of Charlie Strauss’s critique of ThreeBallot. If you want to read more, check out Charlie’s reports (1; 2), or Andrew Appel’s.

Spamhaus Tests U.S. Control Over Internet

In a move sure to rekindle debate over national control of the Internet, a US court may soon issue an order stripping London-based spamhaus.org of its Internet name.

Here’s the backstory. Spamhaus, an anti-spam organization headquartered in London, publishes ROKSO, the “Register of Known Spam Operations”. Many sites block email from ROKSO-listed sites, as an anti-spam tactic. A US company called e360 sued Spamhaus, claiming that Spamhaus had repeatedly and wrongly put e360 on the ROKSO, and asking the court to award monetary damages and issue an injunction ordering e360’s removal from ROKSO.

Spamhaus lost the case, apparently due to bad legal maneuvering. Faced with a U.S. lawsuit, Spamhaus had two choices: it could challenge the court’s jurisdiction over it, or it could accept jurisdiction and defend the case on the merits. It started to defend on the merits, but then switched strategies, declaring the court had no jurisdiction and refusing to participate in the proceedings. The court said that Spamhaus had accepted its jurisdiction, and it proceeded to issue a default judgment against Spamhaus, ordering it to pay $11.7M in damages (which it apparently can’t pay), and issuing an injunction ordering Spamhaus to (a) take e360 off ROKSO and keep it off, and (b) post a notice saying that previous listings of e360 had been erroneous.

Spamhaus has ignored the injunction. As I understand it, courts have broad authority to enforce their injunctions against noncompliant parties. In this case, the court is considering (but hasn’t yet issued) an order that would revoke Spamhaus’s use of the spamhaus.org name; the order would require ICANN and the Tucows domain name registry to shut off service for the spamhaus.org name, so that anybody trying to go to spamhaus.org would get a domain-not-found error. (ICANN says it’s up to Tucows to comply with any such order.)

There are several interesting questions here. (1) Is it appropriate under U.S. law for the judge to do this? (2) If the spamhaus.org is revoked, how will spamhaus and its users respond? (3) If U.S. judges can revoke domain name registrations, what are the international implications?

I’ll leave Question 1 for the lawyers to argue.

The other two questions are actually interrelated. Question 3 is about how much extra power (if any) the US has by virtue of history and of having ICANN, the central naming authority, within its borders. The relevance of any US power depends on whether affected parties could work around any assertion of US power, which gets us back to Question 2.

Suppose that spamhaus.org gets shut down. Spamhaus could respond by registering spamhaus.uk. Would the .uk registry, which is run or chartered by the UK government, comply with a US court order to remove Spamhaus’s registration? My guess would be no. But even if the .uk registry complied and removed spamhaus.uk, that decision would not depend on any special US relationship to ICANN.

The really sticky case would be a dispute over a valuable name in .com. Suppose a US court ordered ICANN to yank a prominent .com name belonging to a non-US company. ICANN could fight but being based in the US it would probably have to comply in the end. Such a decision, if seen as unfair outside the US, could trigger a sort of constitutional crisis for the Net. The result wouldn’t be pretty. As I’ve written before, ICANN is far from perfect but the alternatives could be a lot worse.

(via Slashdot)

ThreeBallot and Write-Ins

Yesterday I wrote about Ron Rivest’s ThreeBallot voting system. Today I want to start a discussion of problems with the system. (To reiterate: the purpose of this kind of criticism is not to dump on the designer but to advance our collective understanding of voting system design.) Charlie Strauss and Andrew Appel have more thorough criticisms, which I’ll get to in future posts. Today I want to explain what I think is the simplest problem with ThreeBallot: it has no natural way to handle write-in votes.

(For background on how ThreeBallot works, see the previous post.)

The basic principle of ThreeBallot voting is that each voter fills out three ballots. Every candidate’s name must be marked on either one or two of the three ballots – to vote for a candidate you mark that candidate on exactly two of the three ballots; all other candidates get marked on exactly one of the three ballots. The correctness of ThreeBallot depends on what I’ll call the Constraint: each voter creates at least one mark, and no more than two marks, for each candidate.

But how can we maintain the Constraint for write-in candidates? The no-more-than-two part is easy, but the at-least-one part seems impossible. If some joker writes in Homer Simpson on two of his ballots, does that force me and every other voter to write in Homer on one of my three ballots? And how could I, voting in the morning, know whether somebody will write in Homer later in the day?

We could give up on the Constraint for write-in candidates. But the desirable features of ThreeBallot – the combination of auditability and secrecy – depend on the Constraint.

In particular, it’s the at-least-one part of the Constraint that allows you to take home a copy of one of your ballots as a receipt. Because you have to mark at least one ballot for every candidate, a receipt showing that you marked one ballot for a particular candidate (a) doesn’t force you to help that candidate, and (b) doesn’t prove anything about how you really voted – and that’s why it’s safe to let you take a receipt. If we throw out the at-least-one rule for write-ins, then a receipt showing a write-in is proof that you really voted for that write-in candidate. And that kind of proof opens the door to coercion and vote-buying.

Alternatively, we can declare that people who cast write-in votes don’t get to take receipts. But then the mere existence of your receipt is proof that you didn’t vote for any write-in candidates. I don’t see any way out of this problem. Do you?

There’s an interesting lesson here about election security, and security in general. Systems that work well in the normal case often get in trouble when they try to handle exceptional or unusual cases. The more complicated the system is, the more likely such problems seem to be.

In the next post I’ll talk about some other instructive problems with ThreeBallot.

ThreeBallot

ThreeBallot is a new voting method from Ron Rivest that is supposed to make elections more secure without compromising voter privacy. It got favorable reviews at first – Michael Shamos even endorsed it at a congressional hearing – but further analysis shows that it has some serious problems. The story of ThreeBallot and its difficulties is a good illustration of why voting security is hard, and also (I hope) an interesting story in its own right.

One reason secure voting is hard is that it must meet seemingly contradictory goals. On the one hand, votes must be counted as cast, meaning the vote totals reported at the end are the correct summation of the ballots actually cast by the voters. The obvious way to guarantee this is to build a careful audit trail connecting each voter to a ballot and each ballot to the final tally. But this is at odds with the secret ballot requirement, which says that there can be no way to connect a particular voter to a particular ballot. Importantly, this last requirement must hold even if the voter wants to reveal his ballot, because letting a voter prove how he voted opens the door to coercion and vote-buying.

If we were willing to abandon the secret ballot, we could help secure elections by giving each voter a receipt to take home, with a unique serial number and the list of votes cast, and publishing all of the ballots (with serial numbers) on the web after the election. A voter could then check his receipt against the online ballot-list, and complain if they didn’t match. But of course the receipt violates the secret-ballot requirement.

Rivest tries to work around this by having the voter fill out three ballots. To vote for a candidate, the voter marks that candidate on exactly two of the three ballots. To vote against a candidate, the voter marks that candidate on exactly one ballot. All three ballots are put in the ballot box, but the voter gets to take home a copy of one of them (he chooses which one). At the end of election day, all ballots are published, and the voter can compare the ballot-copy he kept against the published list. If anybody modifies a ballot after it is cast, there is a one-third chance that the voter will have a copy of that ballot and will therefore be able to detect the modification. (Or so the theory goes.)

Consider a two-candidate race between George Washington and Benedict Arnold. Alice wants to cast her vote for Washington, so she marks Washington on two ballots and Arnold on one. The key property is that Alice can choose to take home a copy of a Washington ballot or a copy of an Arnold ballot – so the mark on the ballot she takes doesn’t reveal her vote. Arnold’s crooked minions can offer to pay her, or threaten to harm her, if she doesn’t produce an Arnold ballot afterward, and she can satisfy them while still casting her vote for Washington.

ThreeBallot is a clever idea and a genuine contribution to the debate about voting methods. But we shouldn’t rush out and adopt it. Other researchers, including Charlie Strauss and Andrew Appel, have some pretty devastating criticisms of it. They’ll be the topic of my next post.

Dutch E-Voting System Has Problems Similar to Diebold's

A team of Dutch researchers, led by Rop Gonggrijp and Willem-Jan Hengeveld, managed to acquire and analyze a Nedap/Groenendaal e-voting machine used widely in the Netherlands and Germany. They report problems strikingly similar to the ones Ari Feldman, Alex Halderman and I found in the Diebold AccuVote-TS.

The N/G machines all seem to be opened by the same key, which is easily bought on the Internet – just like the Diebold machines.

The N/G machines can be put in maintenance mode by entering the secret password “GEHEIM” (which means “SECRET” in Dutch) – just as the Diebold machines used the secret PIN “1111” to enter supervisor mode.

The N/G machines are subject to software tampering, allowing a criminal to inject code that transfers votes undetectably from one candidate to another – just like the Diebold machines.

There are some differences. The N/G machines appear to be better designed (from a security standpoint), requiring an attacker to work harder to tamper with vote records. As an example, the electrical connection between the N/G machine and its external memory card is cleverly constructed to prevent the voting machine from undetectably modifying data on the card. This means that the strategy used by our Diebold virus, which allows votes to be recorded correctly but modifies them a few seconds later, would not work. Instead, a vote-stealing program has to block votes from being recorded, and then fill in the missing votes later, with “improvements”. This doesn’t raise the barrier to vote-stealing much, but at least the machine’s designers considered the problem and did something constructive.

The Dutch paper has an interesting section on electromagnetic emanations from voting machines. Like other computers, voting machines emit radio waves that can be picked up at a distance by somebody with the right equipment. It is well known that eavesdroppers can often exploit such emanations to “see” the display screen of a computer at a distance. Applied to voting machines, such an attack might allow a criminal to learn what the voter is seeing on the screen – and hence how he is voting – from across the room, or possibly from outside the polling place. The Dutch researchers’ work on this topic is preliminary, suggesting a likely security problem but not yet establishing it for certain. Other e-voting machines are likely to have similar problems.

What is most striking here is that different e-voting machines from different companies seem to have such similar problems. Some of the technical challenges in designing an e-voting machine are very difficult or even infeasible to address; and it’s not surprising to see those problems unsolved in every machine. But to see the same simple, easily avoided weaknesses, such as the use of identical widely-available keys and weak passwords/PINs, popping up again, has to teach a deeper lesson.