In 1993, Zheng and Seberry proposed three methods for strengthening public key cryptosystems. These methods aimed to obtain schemes that are secure against adaptively chosen ciphertext attacks. One method was improving security by using digital signatures. Zheng and Seberry gave an example scheme that employs this method. However, they were not able to prove 1ND-CCA security of their cryptosystem. In this paper, we modify this cryptosystem by employing a Schnorr signature scheme and prove this new scheme to be IND-CCA secure in the random oracle model. (C) 2013 Elsevier B.V. All rights reserved.