Analysis and Improvements of Two Identity Based Anonymous Signcryption Schemes for Multiple Receivers

Anonymous signcryption provides anonymity of the sender with the advantages of signcryption. When a sender wants to send a message to multiple receivers in the confidential and authenticated way, multi receiver signcryption is needed. In 2010, Zhang et al. proposed an identity based anonymous signcryption scheme for multiple receivers which is proved secure in the standard model. At the same time, Lal et al. designed another identity based anonymous signcryption scheme for multiple receivers in the random oracle model. Unfortunately, we show that the two schemes do not satisfy the semantic security. Then we improve their corresponding anonymous signcryption schemes that remedy the weaknesses of the above two schemes. Our proposed schemes satisfy the semantic security, unforgeability, signcrypter identity´s ambiguity, and public authenticity.