Cryptanalysis of Two Identity Based Signcryption Schemes

Recently, Yu et al. proposed two identity based signcryption schemes, one is an identity based signcryption scheme for multiple receivers and the other is an identity based signcryption scheme without random oracles. They showed their former scheme is secure against adaptive chosen ciphertext attacks and unforgeable in the random oracle model, and the latter scheme is secure against adaptive chosen ciphertext attacks and unforgeable in the standard model. In this paper, however, we show that their first scheme is vulnerable to universal forgery attack and their second scheme is not secure against adaptive chosen ciphertext attack.