Security Analysis of Signcryption Scheme from q-Diffie-Hellman Problems

Abstract

In this paper, we analyse the Libert-Quisquater's q-DH signcryption scheme proposed in SCN'2004. Although the paper proved that their scheme is secure against adaptive chosen ciphertext attacks in the random oracle model, we disprove their claim and show that their scheme is not even secure against non-adaptive chosen ciphtertext attacks, which is the weaker security than the adaptive chosen ciphertext attacks. We further show that the semantically secure symmetric encryption scheme defined in their paper is not sufficient to guarantee their signcryption scheme to be secure against adaptive chosen ciphertext attacks.