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A steganographic scheme for JPEG compressed image with high capacity and with good quality of the stego-image was presented. A quantization table of size 16*16 was used instead of the commonly used size 8*8 in most JPEG compression to obtain higher embedding capacity. In addition, to improve the quality of the stego-image, particle swarm optimization (PSO) was applied to find an optimal substitution matrix to transform the secret data into the best fit for the cover image before embedding. The experimental results show that, for the proposed scheme, the improvement of the quality of the stego-image and a higher capacity of the secret data was achieved.

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Abstract. An improved method for embedding a secret message into a cover image with least significant bit (LSB) substitution in discrete cosine transformation (DCT) domain was proposed. The secret message was first split into partitions, while the cover image was divided into blocks of size 2x2, and DCT was used to convert the blocks from spatial domain to frequency domain. Then, Particle Swarm Optimization (PSO) algorithm was applied to search for an optimal substitution matrix T to transform the split partitions for an optimal embedding. Next, the transformed part of secret message was embedded into the AC coefficients of the transformed image blocks by LSB substitution. Experimental results show the proposed method can keep the quality of the stego-image better, while the security of the hidden secret message is increased by use of the substitution matrix T.

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