International Association for Cryptologic Research (IACR)
Abstract
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 sincryption 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
