**5. Conclusions**

Network transmission is a vital part of our modern life. With the convenience it brings, information security issues follow. Information hiding has made grea<sup>t</sup> progress. Considering the problems of the previous work requires a large number of training databases, has the low hiding rate, and occupies a big space. This work adopts a two-level mechanism, look-up table, and reversible information hiding technology based on high-quality information hiding. Through reversible information hiding, the additional storage and transmission burden is solved. In addition, to further improve the security of the secret message, a new encryption model is designed. Compared with different methods in the experiment, our proposed model achieves high capacity and perfect hiding rate under the premise of ensuring image quality. The robustness is achieved in the attack tests. To verify the security, the PDH analysis is also performed. The test results prove that the method is undetectable by PDH. Therefore, our algorithm can achieve better results in terms of image quality, capacity, and security compared with other methods. The proposed symmetry model is outstanding and efficient.

Furthermore, we can combine different methods with higher capacity and better robustness in future work. In addition, more diverse hash codes or other feature values can be designed to hide the secret message. The method is efficient in the paper, but once an attacker masters our model, hidden information can be extracted or even tampered with. So, to further improve the security of secret messages, asymmetric mechanisms are also the focus of our future research.

**Author Contributions:** Conceptualization, X.-X.S. and J.-S.P.; formal analysis, J.-S.P., X.-X.S., V.S. and S.-C.C.; methodology, J.-S.P., X.-X.S., S.-C.C. and H.Y.; validation, J.-S.P., H.Y. and V.S.; writing— original draft preparation, X.-X.S.; writing—review and editing, J.-S.P., X.-X.S., S.-C.C., V.S. and H.Y. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Data Availability Statement:** Not applicable.

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