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To improve the embedding capacity and provide an imperceptible visual quality, a novel steganographic method based on four-pixel differencing and modified least significant bit (LSB) substitution is presented. The average difference value of a four-pixel block is exploited to classify the block as a smooth area or an edge area. Secret data are hidden into each pixel by the k-bit modified LSB substitution method, where k is decided by the level which the average difference value falls into. Readjustment will be executed to guarantee the same level that the average difference value belongs to before and after embedding, and to minimize the perceptual distortion. By proving that the readjusting procedure works, a theoretical proof is given to justify our method succeeded in embedding and extracting. Our experimental results have shown that the proposed method not only has an acceptable image quality but also …