Provably Secure Convertible Nominative Signature Scheme

Nominative signature, introduced by Kim, Park and Won, is a useful cryptographic primitive to limit the publicly verifiable property of ordinary digital signature. In a nominative signature scheme, a nominator and a nominee jointly generate a signature in such a way that only the nominee can check the validity of the signature and further convince a third party of the fact. An extended concept, convertible nominative signature, was introduced by Huang and Wang. In the new concept, the nominee can convert a nominative signature into a publicly verifiable one. In this paper, we first propose selectively and universally convertible nominative signatures so that the nominee can publish a selective proof to convert a nominative signature into a publicly verifiable one, or issue a universal proof to make all nominative signatures with respect to the nominator and the nominee publicly verifiable. Then, we present a security model for convertible nominative signatures. Furthermore, we propose a concrete scheme based on bilinear pairings and give the security analysis in the random oracle model.