**6. Conclusions**

In this paper, a detailed security analysis of a color image encryption algorithm named CIEA-FOHS using a fractional-order chaos was performed. From the perspective of cryptanalysis, this paper found that CIEA-FOHS can be broken by a chosen-plaintext attack method, owing to its some inherent security defects. Theoretical analysis and experimental simulations show that the attack method is both effective and efficient for attacking CIEA-FOHS. Although the fractional-order chaotic system has complex dynamics, the algorithm defects may cause insecurity. The reported results would help the designers of chaotic cryptography pay more attention to the gap between complex chaotic system and secure cryptosystem.

**Author Contributions:** Methodology, H.W.; Software, H.W. and J.K.; Validation, H.W., L.H. and C.Z.; Supervision, C.Z.; Project Administration, C.Z. and D.X.; Funding Acquisition, C.Z., H.W. and D.X. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was supported partly by the National Science Foundation of China (62071088, 61571092), Project for National Key RD Program of China (2018YFB1801302), Project for Innovation Team of Guangdong University (2018KCXTD033), Project for Zhongshan Social Public Welfare Science and Technology (2019B2007), Zhongshan Innovative Research Team Program (180809162197886), Research Project for Talent of UESTC Zhongshan Institute (418YKQN07, 419YKQN23), Natural Science Project for Young Innovative Talents by the Department of Education of Guangdong Province (2019KQNCX191), Characteristic Innovation Project of Department of Guangdong Province (2017GWTSCX010).

**Data Availability Statement:** No applicable.

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