An Efficient and Provably Secure ID-Based Threshold Signcryption Scheme

Signcryption is a cryptographic primitive that performs digital signature and public key encryption simultaneously, at a lower computational costs and communication overheads than the signature-then-encryption approach. Recently, two identity-based threshold signcryption schemes[12],[26] have been proposed by combining the concepts of identity-based threshold signature and signcryption together. However, the formal models and security proofs for both schemes are not considered. In this paper, we formalize the concept of identity-based threshold signcryption and give a new scheme based on the bilinear pairings. We prove its confidentiality under the Decisional Bilinear Diffie-Hellman assumption and its unforgeability under the Computational Diffie-Hellman assumption in the random oracle model. Our scheme turns out to be more efficient than the two previously proposed schemes

Analysis and Improvement of Authenticatable Ring Signcryption Scheme

Ring signcryption is an anonymous signcryption which allows a user to anonymously signcrypt a message on behalf of a set of users including himself. In an ordinary ring signcryption scheme, even if a user of the ring generates a signcryption, he also cannot prove that the signcryption was produced by himself. In 2008, Zhang, Yang, Zhu, and Zhang solve the problem by introducing an identity-based authenticatable ring signcryption scheme (denoted as the ZYZZ scheme). In the ZYZZ scheme, the actual signcrypter can prove that the ciphertext is generated by himself, and the others cannot authenticate it. However, in this paper, we show that the ZYZZ scheme is not secure against chosen plaintext attacks. Furthermore, we propose an improved scheme that remedies the weakness of the ZYZZ scheme. The improved scheme has shorter ciphertext size than the ZYZZ scheme. We then prove that the improved scheme satisfies confidentiality, unforgeability, anonymity and authenticatability