An identity based ring signcryption scheme with public verifiability

Signcryption is a cryptographic primitive which offers authentication and confidentiality simultaneously with a cost lower than signing and encrypting the message independently. Ring signcryption enables a user to anonymously signcrypt a message on behalf of a set of users including himself. Thus a ring signcrypted message has anonymity in addition to authentication and confidentiality. Ring signcryption schemes have no centralized coordination: any user can choose a ring of users, that includes himself and signcrypt any message without any assistance from the other group members. Ring Signcryption is useful for leaking trustworthy secrets in an anonymous, authenticated and confidential way. To the best of our knowledge, ten identity based ring signcryption schemes are reported in the literature. Three of them were proved to be insecure in (Li et al., 2008a), (Zhang et al., 2009a) and (Vivek et al., 2009). Four of them were proved to be insecure in (Selvi et al., 2009). In this paper, we show that one among the remaining three schemes, (Zhang et al., 2009b) is not secure against confidentiality, existential unforgeability and anonymity attacks. We propose a new anonymous ring signcryption scheme which is an extension to (Selvi et al., 2009) and give formal security proofs for our system in the random oracle model. Our scheme is publicly verifiable which none of the existing unbroken schemes can achieve.