A Dynamic Approach to Image Steganography Using the Three Least Significant Bits and Extended Hill Cipher

In this paper we propose a technique for secure communication between sender and receiver. We use both cryptography and steganography. We take image as the carrier to use steganography. We have extended the existing hill cipher to increase its robustness and used it as our cryptography algorithm. By using this extended hill cipher (a new block cipher) which uses a 128 bit key, we encrypt the secret message. Then the cipher text of the secret message is embedded into the carrier image in 6th, 7th and 8th bit locations of some of the selected pixels (bytes). The 8th bit in a pixel (byte) is called as the least significant bit (LSB). The pixel selection is done depending on the bit pattern of the cipher text. So for different messages the embedding pixels will be different. That means to know the pixels of the image where the cipher text is embedded we should know the cipher text bits. Thus it becomes a stronger steganography. As the pixels where we embed are chosen during the run time of the algorithm, so we say that it is dynamic steganography. After embedding the resultant image will be sent to the receiver, the receiver will apply the reverse process what the sender has done and get the secret message.