In this paper, a secure communication system based on cryptograph and chaotic synchronization is developed. In the proposed scheme, four chaotic systems: the main transmitter, the main receiver and two slave systems are specially designed, with only a scalar signal transmitted on the public channel. Given certain structural conditions of a class of the chaotic systems, the main transmitter and an observer-based main receiver with an appropriate observer gain are constructed to synchronize with each other. In addition, these two slave systems, driven simultaneously by the transmitted signal, are designed to synchronize and generate secret keys. Two well-known chaotic systems: Rössler - like and Chua's circuit, respectively, are associated with two especially designed slave systems and used as illustrative examples to demonstrate the effectiveness of the proposed scheme. Moreover, the synchronization and stability of the overall system are guaranteed by the Lyapunov approach.