Paper on variable strength anonymous identity

I read a quite interesting paper a while back, which I've lost the link to, of course, that described methods for creating "strong" cryptographic identities by using the same proof-of-work principles as Bitcoin. Essentially, it eliminates the "sock-puppet" problem with online identity by making a new cryptographically provable identity very expensive to create. That is, having invested time and money in creating such an identity, one would be highly dis-incentivized to abuse it by using it to defraud, or other abuse others.

But since the strength of the identity can be chosen arbitrarily, relatively "weak" identities can be created to do things that don't require long-term identity.

Services wishing to create high value can then require that users wishing to create an account present an identity with a given minimum strength so as to weed out the trolls, hackers and abusers beforehand.

If anyone recalls this paper or any similar, or can suggest some appropriate google search terms, I'd very much appreciate your sending them along. I've tried a bunch of google searches to no avail so far.

Thanks!