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Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).
Histograms of the plain images and their corresponding cipher images. The first and the second columns are the plain images and their corresponding histograms, respectively. The third and the fourth columns are the cipher images and their corresponding histograms, respectively.