This paper proposes a robust method for constructing chaos-based secure communication systems that rely on parameter modulation. Both the transmitter and the receiver have similar structure that is characterized by the ability of performing complete synchronization using the knowledge of a single time series from the transmitter. In addition a single parameter from the transmitter is changed in a piecewise manner that can satisfy the requirements of maintaining the chaotic behavior while having a frequency that is quite close to that of the transmitted signal. This robustifies the design via accomplishing two goals; preventing the breaking of the system security using filtration techniques, and the possibility to use a nonlinear encryption function that uses a state observer at the receiver for which the design is decoupled from the synchronization process. Simulations show that, at the receiver, the signal could be efficiently retrieved only if the secret key is known, even when both the receiver and the transmitter are in perfect synchronization. The proposed technique is demonstrated to have improved security and privacy against intruders, when compared to other techniques reported in the literature, while being simple to implement using both analog and digital hardware.