On the Security of Signcryption Scheme with Key Privacy

The concept of a signcryption scheme is proposed by Zheng in 1997 [5]. Since then, there are many signcryption schemes proposed. It is only recently that a formal security proof model [1] is formalized providing security proof for Zheng’s scheme [5] in the random oracle model. In 2003, Boyen [3] proposed a secured identity-based signcryption scheme with ciphertext anonymity and provable secure in the random oracle model. Their security proof model is slightly different from that of [1] which includes the ciphertext anonymity. In 2004, Libert and Quisquater [4] modified Boyen’s security proof model to non-identity based signcryption scheme and proposed a signcryption scheme. They proved that their signcryption scheme is secure in the random oracle model with the following properties: semantically security against adaptive chosen ciphertext attacks, ciphertext anonymity and key invisibility. In this paper, we show that none of the above properties are achieved under their defined attacks games.