Biblio

A key exchange protocol is an important primitive in the field of information and network security and is used to exchange a common secret key among various parties. A number of key exchange protocols exist in the literature and most of them are based on the Diffie-Hellman (DH) problem. But, these DH type protocols cannot resist to the modern computing technologies like quantum computing, grid computing etc. Therefore, a more powerful non-DH type key exchange protocol is required which could resist the quantum and exponential attacks. In the year 2013, Lei and Liao, thus proposed a lattice-based key exchange protocol. Their protocol was related to the NTRU-ENCRYPT and NTRU-SIGN and so, was referred as NTRU-KE. In this paper, we identify that NTRU-KE lacks the authentication mechanism and suffers from the man-in-the-middle (MITM) attack. This attack may lead to the forging the authenticated users and exchanging the wrong key.

Signcryption is a cryptographic primitive that simultaneously realizes both the functions of public key encryption and digital signature in a logically single step, and with a cost significantly lower than that required by the traditional “signature and encryption” approach. Recently, an efficient certificateless signcryption scheme without using bilinear pairings was proposed by Zhu et al., which is claimed secure based on the assumptions that the compute Diffie-Hellman problem and the discrete logarithm problem are difficult. Although some security arguments were provided to show the scheme is secure, in this paper, we find that the signcryption construction due to Zhu et al. is not as secure as claimed. Specifically, we describe an adversary that can break the IND-CCA2 security of the scheme without any Unsigncryption query. Moreover, we demonstrate that the scheme is insecure against key replacement attack by describing a concrete attack approach.