Cryptanalysis and Improvement of Zheng's Signcryption Technique

The signcryption technique was first proposed by Y. Zheng, where two cryptographic operations digital signature and message encryption are made combinedly. We cryptanalyze the technique and observe that the signature and encryption become vulnerable if the forged public keys are used. This paper proposes an improvement using modified DSS (Digital Signature Standard) version of ElGamal signature and DHP (Diffie-Hellman key exchange protocol), and shows that the vulnerabilities in both the signature and encryption methods used in Zheng's signcryption are circumvented. DHP is used for session symmetric key establishment and it is combined with the signature in such a way that the vulnerabilities of DHP can be avoided. The security and performance analysis of our signcryption technique are provided and found that our scheme is secure and designed using minimum possible operations with comparable computation cost of Zheng's scheme.

I. Introduction

There is a widespread need to safeguard sensitive information communicated. Receiver should be able to authenticate the source. Sender should be able to furnish reliable security measures before communicating and the respective recipient should be able to properly decrypt it. The signcryption technique proposed by Zheng is a new cryptographic primitive that combines digital signature and encryption/decryption for message integrity and confidentiality, respectively [1]. It is done in one compact step with the processing cost much lesser than the usual signature-then-encryption method used. The steps involved are much less [2]. In signcryption, Zheng uses a variation of ElGamal's signature and proposes/uses two signatures in his signcryption scheme. It computes a session secret key, and splits into two keys for calculation of keyed hash digest and ciphertext of a message. In unsigncryption, the key recovery and splitting are done, and the ciphertext is decrypted and accepted if the signature is verified. A short review of other signcryption techniques are given now.