Comment on an ID-Based Broadcast Signcryption Scheme

Broadcast encryption is an important cryptographical technique. It allows a center to deliver the encrypted data to a large set of users so that only a particular subset of privileged users can decrypt it. Most of previous broadcast encryption schemes only provide confidentiality, while they cannot provide authentication and non-repudiation. Recently,Li et.al proposed a broadcast signcryption scheme to achieve authentication and non-repudiation. And they claimed that their scheme was secure against outside attack and inside attack. Unfortunately, we show that their scheme is insecure by analyzing its security and cannot be against outside attack and inside attack. Thus, their scheme cannot achieve authentication and non-repudiation.