We introduce and motivate the concept of unclonable group identification, that provides maximal protection against sharing of identities while still protecting the anonymity of users. We prove that the notion can be realized from any one-way function and suggest a more efficient implementation based on specific assumptions. 1 Some earlier works suggest to discourage this by forcing users to either give away all their information, or nothing, but here we are interested in cases where dishonest users in fact have an interest in giving everything away.

Abstract. We propose techniques that allow construction of robust threshold RSA signature schemes that can work without a trusted dealer using known key generation protocols and is as efficient as the best previous schemes. We do not need special conditions on the RSA modulus, extra complexity or set-up assumptions or random oracles. An "optimistic" variant of the scheme is even more efficient in case no faults occur. Some potential more general applications of our basic idea are also pointed out.