Brainstorm tradeoffs from moving to 2 (or even 1) guards
There are now many conflicting issues to consider when changing the default number of guards. I'd like to write a proposal suggesting we move to 2 (or even 1), but I don't think I'm ready to write the analysis section yet.
Here's a start:
Pro 1: Reduces chance of using an adversary's guard. This argues for 1, but 2 would still be a lot better. See Tariq's WPES 2012 paper for details.
Pro 2: Reduces impact from guard fingerprinting: if the adversary learns that you have the following n guards, and later sees an anonymous user with the same guards, how likely is it to be you? Said another way, a trio of guards produces a cubic, whereas a duo of guards produces a quadratic. Somebody should do the math to sort out the chance of having all possible trios of guards, followed by the expected uniqueness of a trio. I expect moving to 2 gives the majority of the benefit here.
Con 1: Increases the variance of performance. The more guards you have, the closer to average performance you'll be. Whereas if you have just one guard, your performance will be impacted a lot by that choice. It would seem that we need to raise the bar on getting the Guard flag if we move people to having just one guard.
Con 2: Moving to 1 guard will rule out a Conflux-style design. But 2 guards would still work fine.
What did I miss?