In this paper, the author propose a method, SD-EI, for image encryption, which basically has two stages: 1) In first stage, each pixel of image is converted to its equivalent eight bit binary number and in that eight bit number, the number of bits, which are equal to the length of password are rotated and then reversed; 2) In second stage, extended hill cipher technique is applied by using involutory matrix, which is generated by same password used in second stage of encryption to make it more secure. The technique presented in this paper is very good for encrypting any type of images, and from the encrypted image it is very difficult to extract the real image without proper authorization and algorithm implementation. The proposed approach is implemented for different images and the results were very satisfactory. The proposed technique, SD-EI, can be implemented by any person or organization to encrypt images, which are to be kept secret, or can encrypt the images, where data (messages) are already hidden using steganography.
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