An Improved Identity-Based KCDSA Signcryption Scheme

KCDSA (Korean Certificate-based Digital Signature Al- gorithm) is the Korean digital signature standard. In 2006, Ryu, Jeong, and Seo proposed three identity-based sign- cryption schemes based on KCDSA. However, their schemes provide either public verifiability or forward security. That is, the properties of public verifiability and forward secu- rity are mutually exclusive in their schemes. In this paper, we propose an improved scheme that provides both public verifiability and forward security. We give a formal proof of semantic security under the Decisional Bilinear Diffie- Hellman assumption for this improved scheme in the ran- dom oracle model.