Convertible designated confirmer partially blind signatures

The concept of partially blind signature is extended to the convertible designated confirmer partially blind signature, which has an additional property that only the designated confirmer and the user can verify and prove the validity of given signatures and convert given signatures into universally verifiable partially blind signatures. A formal definition is given and a practical scheme is proposed. The proposed scheme is efficient and secure, in which its unforgeability is the same as that of the Schnorrs signature scheme and its untransferability relies on the hardness of the Decision-Diffie-Hellman Problem. Furthermore, the proposed scheme can be used as a normal partially blind signature scheme by letting the designated confirmers secret key be a common constant.