On the security of a group signcryption scheme from distributed signcryption scheme

Signcryption denotes a cryptographic method, which can process encryption and digital signature simultaneously. So, adopting such schemes, computational cost of encryption and signature compared to traditional signature-then- encryption can be reduced to a great extent. Based on the existing distributed signcryption schemes, Kwak and Moon proposed a new distributed signcryption scheme with sender ID confidentiality and extended it to a group signcryption. Their scheme is more efficient in both communication and computation aspects. Unfortunately we will demonstrate that their scheme is insecure by identifying some security flaws. Exploring these flaws, an attacker without any secret can mount universal forging attacks. That is, anyone (not necessary the group member) can forge valid group signatures on arbitrary messages of his/her choice.