I have several problems approaching the following problem: Let's assume that we have Alice, Bob and Cathy. Alice wants to present a message to Cathy, showing her that someone else has signed it (Bob), but without disclosing Bob's identity. She needs to show that the signature does not belong to her.
If we use blind signatures. then Alice could be the one who signed it, so we cannot be sure that it was signed from someone else.
By using group signatures, more or less we have the same problem, sure there's the group manager who could certify that, but we break the anonymity...
With ring signatures, we have the needed anonymity, Bob presents a message having himself along with others, however, if you want to make it efficient, the ring has to be quite small.
Any other ideas?