Biblio

Forward secure proxy signature (FoSPS) solves the security drawback of private key exposure problem of generating the private key of each time interval. Directed signature scheme solves the public signature verification problem in traditional digital signature by designating the constant one as the signature verifier. Due to excellent properties, the two signature schemes have attracted the research of many experts. Recently, based on the Elliptic curve cryptography (ECC), a new FoSPS scheme and directed signature scheme were proposed. In this paper, we analyze the two schemes and present which the either of both schemes is insecure and do not satisfy the unforgeability. In other words, anyone is able to forge a valid signature but the one does not know the signer's secret key. In the same time, we give the main reasons why the enemy is able to forge the signature by analyzing the two schemes respectively. And we also present a simple improvement idea to overcome existing problems without adding extra computational cost which can make them applied in some environments such as e-medical information system.