Cryptanalysis of two identity-based signcryption schemes

Encryption and signature schemes are fundamental cryptographic tools for providing privacy and authenticity. In the last few years, a new separate primitive - called signcryption - has emerged to model a process simultaneously achieving privacy and authenticity. In this paper, we analyze two signcryption schemes: Yu-Yang-Sun-Zhu's scheme (2007) and Li-Xin-Hu's scheme (2008). To Yu-Yang-Sun-Zhu's scheme (2007), we give two attack methods that correspond to the semantic security and existential unforgeability. To Li-Xin-Hu's scheme (2008), we show that the scheme can be broken by the attacker. Besides the receivers, the attacker can also decrypt any ciphertexts. The receiver can find the targeted master secret key.