A well-known problem with anonymity is that it allows trolls to ply their unwelcome trade. So, pretty much every decent cryptographic anonymity scheme proposed has some mechanism for blacklisting. Basically these work by some kind of zero-knowledge proof that you aren’t on the blacklist – and once you’ve done that you can proceed.
However, this scheme suffers from the usual problem with trolls: as soon as they’re blacklisted, they create a new account and carry on. Solving this problem ultimately leads to a need for strong identification for everyone so you can block the underlying identity. Obviously this isn’t going to happen any time soon, and ideally never, so blacklists appear to be fundamentally and fatally flawed, except perhaps in closed user groups (where you can, presumably, find a way to do strong-enough identification, at least sometimes) – for example, members of a club, or employees of a company.
So lately I’ve been thinking about using “blacklists” for reputation. That is, rather than complain about someone’s behaviour and get them blacklisted, instead when you see someone do something you like, add them to a “good behaviour blacklist”. Membership of the “blacklist” then proves the (anonymous) user has a good reputation, which could then be used, for example, to allow them to moderate posts, or could be shown to other users of the system (e.g. “the poster has a +1 reputation”), or all sorts of other things, depending on what the system in question does.
The advantage of doing it this way is that misbehaviour can then be used to remove reputation, and the traditional fallback of trolls no longer works: a new account is just as useless as the one they already have.
There is one snag that I can see, though, which is at least some anonymity systems with blacklisting (e.g. Nymble, which I’ve somehow only recently become aware of) have the side-effect of making every login by a blacklisted person linkable. This is not good, of course. I wonder if there are systems immune to this problem?
Given that Jan Camenisch et al have a presentation on upside-down blacklisting (predating my thinking by quite a long way – one day I’ll get there first!), I assume there are – however, according to Henry, Henry and Goldberg, Camenisch’s scheme is not very efficient compared to Nymble or Nymbler.