The enhancement of the encryption properties of chaotic signals generated by a semiconductor laser subject to optical feedback is numerically demonstrated by applying subcarrier modulation. The numerical analysis shows that the message can be very efficiently encrypted when the radio frequency carrier is within the frequency range where the chaos power density is maximized. Decoding performance is also numerically assessed considering both open- and closed-loop schemes at the receiver side. The impact of subcarrier modulation on system's performance under the influence of parameter mismatch is highlighted.