Signcryption is a cryptographic primitive that fulfill the functionalities of digital signature and public key encryption simultaneously, at a cost significantly lower than that required by the traditional sign-then-encrypt or encrypt-then-sign approach. In this paper, we address the question whether it is feasible to construct a PKI based signcryption scheme with tight security reduction in the insider security model of signcryption without pairing. This question seems to have never been

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... sed in the literature before. We answer the question positively in this paper. We give a novel PKI based signcryption scheme and the security is based on CDH-assumption. Ours is the first scheme of its kind which is secure in insider security model proved with tight security reduction. All other PKI based systems without pairing neither have insider security nor have tight reduction. In-spite of a slightly higher count of exponentiation, our scheme is the most efficient one currently, thanks to the tight reduction we have established to our scheme.