The chaos-based cryptography techniques are used widely to protect digital information from intruders. The chaotic systems have some of special features that make them suitable for the purpose of encryption. These systems are highly unpredictable and are highly sensitive or responsive to the initial conditions, also known as butterfly effect. This sensitive dependence on initial conditions make these systems to exhibit an intricate dynamical behaviour. However, this dynamical behaviour is not much complex in simple one-dimensional chaotic maps. Hence, it becomes easy for an intruder to predict the contents of the message being sent. The proposed work in this paper introduces an improved method for encrypting images, which uses cosine transformation of 3-D Intertwining Logistic Map (ILM). The proposed approach has been split into three major parts. In the first part, Secure Hash Function-256 (SHA-256) is used with cosine transformed ILM (CT-ILM) to generate the chaotic sequence. This chaotic sequence is used by high-efficiency scrambling to reduce the correlations between the adjacent pixels of the image. In the second part, the image is rotated to move all the pixels away from their original position. In the third part, random order substitution is applied to change the value of image pixels. The effectiveness of the proposed method has been tested on a number of standard parameters such as correlation coefficient, Entropy and Unified average change in intensity. The proposed approach has also been tested for decryption parameters like mean square error and peak signal to noise ratio. It can easily be observed from the obtained results that the proposed method of image encryption is more secure and time efficient than some earlier proposed techniques. The approach works for both color and grey scale images.

基于混沌的加密技术被广泛用于保护数字信息免受入侵者的侵害。混沌系统具有一些特殊功能，使其适合于加密目的。这些系统是高度不可预测的，并且对初始条件高度敏感或敏感，也称为蝴蝶效应。对初始条件的敏感依赖使这些系统表现出复杂的动态行为。但是，这种动态行为在简单的一维混沌图中并不复杂。因此，入侵者很容易预测正在发送的消息的内容。本文提出的工作介绍了一种改进的图像加密方法，该方法使用3-D交织逻辑映射（ILM）的余弦变换。提议的方法已分为三个主要部分。在第一部分中，将安全哈希函数256（SHA-256）与余弦变换的ILM（CT-ILM）结合使用以生成混沌序列。高效加扰使用此混沌序列来减少图像相邻像素之间的相关性。在第二部分中，旋转图像以将所有像素移离其原始位置。在第三部分中，应用随机顺序替换来更改图像像素的值。该方法的有效性已经在许多标准参数上进行了测试，例如相关系数，熵和强度的统一平均变化。所提出的方法也已针对解密参数（如均方误差和峰值信噪比）进行了测试。从获得的结果可以容易地观察到，与一些较早提出的技术相比，所提出的图像加密方法更安全，更省时。该方法适用于彩色和灰度图像。