An algorithm optimized towards encrypting a specific region in an image is proposed. The encryption of specific region(s) in an image is often of more practical relevance than encrypting the entire image, thus avoiding wastage of time and processing power. The algorithm proposed is ideal for encrypting images with relatively small Region of Interest (ROI). It makes novel use of the XOR operation and the relative visual redundancy of the blue-plane components in a RGB color image. A pseudorandom number generator is used as a basic security feature. Furthermore, Cipher Block Chaining (CBC) is also suggested as an improvement to further enhance the security of the algorithm. The algorithm may be classified as lossy since some visual data is sacrificed in the decrypting process. However, in case of encryption algorithms, the high security of the encrypted image is often given priority over faithful reproduction of images.

A non-format compliant JPEG encryption algorithm is proposed which is based on a modification of the RSA encryption system. Firstly, an alternate form of entropy coding is described, which is more suited to the proposed algorithm, instead of the zigzag coding scheme used in JPEG. The algorithm for the encryption and decryption process is then elaborated. A variant of the algorithm, also based on the RSA algorithm is also described, which is faster than the original algorithm, but expands the bit stream slightly. Both the algorithms are shown to be scalable and resistant to 'sketch' attacks. Finally, the encrypted file sizes for both the algorithms are compared with the unencrypted JPEG compressed image file size. The encrypted image is found to be moderately expanded, but which is justified by the high security and most importantly, the scalability of the algorithm.