**7. Conclusions**

In this paper, we presented the novel dirty modulation method based on the phase drift, which is intended to create covert channels in radio transmissions using the *N*-ary PSK or QAM modulations. The method is based on the DCM approach that was previously used to watermark audio signals. In the proposed solution, a random change of the dirty constellation parameters is possible. It ensures its greater resistance to detection. On the other hand, it is possible to adapt these parameters to the modulation type and propagation conditions in the current radio channel.

In the paper, we described the idea of dirty modulation and developed a solution. Next, we introduced the criteria of the covert channel evaluation. Based on BER and EVM, we analyzed the proposed method using simulation studies and practical implementation, including comparison with other solutions. The obtained results showed that our dirty modulation method could be a valuable supplement to the existing steganographic methods.

In the near future, we want to focus on developing an adaptive method of selecting dirty modulation parameters, including the centroid distance and phase drift step, as well as multiple repetitions, i.e., averaging of secret transmission subsymbols. This will increase the undetectability of the proposed method. We also consider the effectiveness of this method in the case of additional use of the OFDM signal. Additionally, practical implementation of the method in the wireless part of the SDN within the aforementioned SOFTANET project is planned. In the near future, we want to implement ML algorithms (e.g., [39,40,47]) to increase the detection correctness of secret transmissions. These algorithms can be also used for time-variant selection of the developed dirty constellation parameters to improve their undetectability via various steganalysis techniques.

**Author Contributions:** Conceptualization, Z.P. and J.M.K.; methodology, K.G., Z.P. and J.M.K.; software, K.G.; validation, K.G.; formal analysis, K.G., Z.P. and J.M.K.; investigation, K.G.; resources, Z.P.; data curation, K.G. and Z.P.; writing—original draft preparation, K.G., Z.P. and J.M.K.; writing—review and editing, K.G. and J.M.K.; visualization, K.G.; supervision, Z.P. and J.M.K.; project administration, Z.P.; funding acquisition, Z.P. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the National Centre for Research and Development, gran<sup>t</sup> number CYBERSECIDENT/381319/II/NCBR/2018 on "The federal cyberspace threat detection and response system" (acronym DET-RES) as part of the second competition of the CyberSecIdent Research and Development Program—Cybersecurity and e-Identity.

**Data Availability Statement:** The data presented in this study are available on request from the corresponding author. The data are not publicly available due to project restrictions.

**Acknowledgments:** The authors would like to express their grea<sup>t</sup> appreciation to the *Electronics* journal editors and anonymous reviewers for their valuable suggestions, which have improved the manuscript quality.

**Conflicts of Interest:** The authors declare no conflict of interest.
