Cryptanalysis of Li et al.'s Identity-Based Threshold Signcryption Scheme

Signcryption is a cryptographic primitive that aims at providing confidentiality and authentication simultaneously. Recently in May 2008, a scheme for identity based threshold signcryption was proposed by Fagen Li and Yong Yu. They have proved the confidentiality of their scheme and have also claimed the unforgeability without providing satisfactory proof. In this paper, we show that in their signcryption scheme the secret key of the sender is exposed (total break) to the clerk during signcryption and hence insecure in the presence of malicious clerks. Further, we propose a corrected version of the scheme and formally prove its security under the existing security model for signcryption.