Modified Huang-Wang´s Convertible Nominative Signature Scheme

At ACISP 2004, Huang and Wang first introduced the concept of convertible nominative signatures and also proposed a concrete scheme. However, it was pointed out by many works that Huang-Wang´s scheme is in fact not a nominative signature. In this paper, we first present a security model for convertible nominative signatures. The properties of unforgeability, invisibility, non-impersonation and non-repudiation in the setting of convertible nominative signatures are defined formally. Then we modify Huang-Wang´s scheme into a secure one. Formal proofs are provided to show that the modified Huang-Wang´s scheme satisfies all the security properties under some conventional assumptions in the random oracle model.