Universal Designated Verifier Ring Signature (Proof) Without Random Oracles

This paper first introduces the concept of universal designated verifier ring signature (UDVRS), which not only allows members of a group to sign messages on behalf of the group without revealing their identities, but also allows any holder of the signature (not necessary the signer) to designate the signature to any designated verifier. According to whether the designator has a registered public key, two kinds of UDVRS are proposed. In order to distinguish the two types of UDVRS, we call it UDVRS Proof (UDVRSP) if the designator has not a registered public key, and this protocol is interactive. We give the formal security definitions and notions of UDVRS and UDVRSP. Then, we propose a UDVRS and a UDVRSP scheme, with rigorous security proofs without random oracles.