On the security of two ring signcryption schemes

Ring signcryption is a cryptographic primitive, that allows an user to send a message in confidential, authentic and anonymous way, i.e. the recipient of the message is convinced that the message is valid and it comes from one of the ring member, but does not know the actual sender. In this paper, we show attacks on ring signcryption schemes by Li et al. (Li et al., 2008b) and Chung et al. (Chung et al., 2006). We demonstrate anonymity and confidentiality attack on the scheme by Li et al. (Li et al., 2008b) and confidentiality attack on the scheme by Chung et al. (Chung et al., 2006).