Due to, ah, reasons which will soon become apparent, I’ve been interested in opening up the voting data from elections which use more appropriate voting systems than FPTP. Unfortunately it turns out that this is a bad idea.
You should read the paper. It’s short, well written and very interesting. But the summary of it is that when you’ve got something like Majority Judgment or a ranked voting system like AV there are a lot of redundant bits of information in your vote – so much so that this means that pretty much any individual vote will be unique. Malicious parties can then force you to encode messages in your vote – e.g. by saying “Vote for this person first, then vote for the remainder in this distinctive order”. If they then have access to the anonymised set of votes cast they can look for that distinctive pattern in the results and thus confirm that you have voted as you were told to vote. Its absence proves that you didn’t, and its presence proves that with very high probability you did.
This attack is pretty universal – it holds for almost any voting system that allows you to express a complicated enough set of preferences to produce a fair result. I do however feel the need to point out that there is at least one fair voting system that is immune to this…
P.S. This post represents two new things I’m planning to try with this blog in the near future: More promotion of “this is a thing that I found interesting” that is a little bit better curated than you just reading my pinboard feed and shorter blog posts which just point something out rather than constituting a fully formed essay. We’ll see how this plan goes.