In this paper, the concept of secure synchronization of chaotic systems using adaptive and robust techniques , has been discussed and then a new secure communication scheme, based on secure synchronization of a general class of chaotic systems called Generalized Lorenz System, are presented. This communication scheme is combination of conventional cryptographic methods and chaotic modulation method. Analytical and simulation results using sine and voice signal and with unknown constant propagation delay between transmitter and receiver , have been presented. Also, robustness of this scheme against Gaussian channel noise with variance of , and security analysis of this communication system from brute-force viewpoint has been evaluated. By using this communication scheme, the very great key length has been obtained.