Lossless Image Cryptography Algorithm Based on Discrete Cosine Transform

The science of cryptography has recently attracted significant attention, as progressively more information is stored and transmitted in electronic form. Cryptography is the discipline of using codes to encrypt data into an unreadable format that only the targeted recipients can decrypt and read. Encryption methods can be divided into two categories: lossy and lossless. In lossy encryption methods, the decrypted image details are vulnerable to distortion. Lossless encryption methods are more relevant when marginal distortion is not tolerable. In this research, the authors propose a novel lossless encryption/decryption technique. In the proposed algorithm, the image is transformed into the frequency domain, where low and high frequencies are processed in a way that guarantees a secure, reliable, and an unbreakable form. The encryption algorithm uses the discrete cosine transform to convert the target image into the frequency domain, after which the encryption involves scattering the distinguishable DC value using a reversible weighting factor amongst the rest of the frequencies. The algorithm is designed to shuffle and reverse the sign of each frequency in the transformed block before the image blocks are transformed back to the pixel domain. The results show a total change in the encrypted image pixel values, concealing the image details. The decryption algorithm reverses the encryption steps and returns the image to its original form without any loss in the pixel values. Based on the application's requirements, the decryption algorithm can perform with or without a decryption key, The encryption algorithm is suitable for applications that require secure transport of high quality data.