Unforgeability of an improved certificateless signature scheme in the standard model

Certificateless signature is an interesting cryptographic primitive which does not suffer from the inherent key escrow problem of identity-based cryptography and the costly certificate management problem of traditional public key cryptography. Since security proofs in the random oracle model can only be viewed as heuristic arguments and cannot ensure the security in the real implementation, certificateless signature schemes with security proofs in the standard model (i.e. without random oracles) is more desirable. Some attempts have been devoted to propose certificateless signature schemes in the standard model, whereas all of these schemes are later shown to be either insecure or flawed in the security proofs. Recently, a new certificateless signature scheme in the standard model has been proposed. However, in this study the authors show that this scheme cannot resist the key replacement attack, and hence it is not existentially unforgeable.