In the medical sector, the digital image is multimedia data that contain secret information. However, designing an efficient secure cryptosystem to protect the confidential images in sharing is a challenge. In this work, we propose an improved chaos-based cryptosystem to encrypt and decrypt rapidly secret medical images. A complex chaos-based PRNG is suggested to generate a high-quality key that presents high randomness behaviour, high entropy, and high complexity. An improved architecture is proposed to encrypt the secret image that is based on permutation, substitution, and diffusion properties. In the first step, the image’s pixels are randomly permuted through a matrix generated using the PRNG. Next, pixel’s bits are permuted using an internal condition. After that, the pixels are substituted using two different S-boxes with an internal condition. In the final step, the image is diffused by XORing pixels with the key stream generated by the PRNG in order to acquire an encrypted image. R rounds of encryption can be performed in a loop to increase the complexity. The cryptosystem is evaluated in depth by his application on several medical images with different types, contents, and sizes. The obtained simulation results demonstrate that the system enables high-level security and performance. The information entropy of the encrypted image has achieved an average of 7.9998 which is the most important feature of randomness. The algorithm can take full advantage of parallelism and pipeline execution in hardware implementation to meet real-time requirements. The PRNG was tested by NIST 800-22 test suit, which indicates that it is suitable for secure image encryption. It provides a large key space of 2¹⁹² which resists the brute-force attack. However, the cryptosystem is appropriate for medical image securing.

中文翻译：

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用于医学图像加密和解密的改进的基于混沌的密码系统

在医疗领域，数字图像是包含秘密信息的多媒体数据。然而，设计一种有效的安全密码系统以保护共享中的机密图像是一个挑战。在这项工作中，我们提出了一种改进的基于混沌的密码系统来对快速秘密的医学图像进行加密和解密。建议使用基于混沌的复杂PRNG来生成高质量的密钥，该密钥具有高随机性，高熵和高复杂性。提出了一种改进的体系结构，用于基于置换，替换和扩散属性对机密图像进行加密。第一步，通过使用PRNG生成的矩阵对图像的像素进行随机排列。接下来，使用内部条件对像素的位进行置换。之后，使用两个不同的S替换像素-具有内部条件的盒子。在最后一步中，通过对像素与PRNG生成的密钥流进行XOR运算来扩散图像，以获取加密图像。[R可以在一个循环中执行几轮加密，以增加复杂性。他通过对几种具有不同类型，内容和大小的医学图像进行应用，对加密系统进行了深入评估。所获得的仿真结果表明，该系统具有较高的安全性和性能。加密图像的信息熵平均达到7.9998，这是随机性的最重要特征。该算法可以在硬件实现中充分利用并行性和流水线执行，以满足实时需求。PRNG已通过NIST 800-22测试服测试，表明它适用于安全的图像加密。它提供2 ¹⁹²的较大密钥空间抵抗强力攻击。但是，密码系统适用于医学图像保护。