Secret image communication scheme based on vector quantisation
Step 3: Embed the parameters and the compressed message of secret images into the selected codewords by directly modifying the least significant bits (LSBs) of codeword elements.
S.D. Lin and S.-C. Shie
A modified codebook C 0 is obtained after all the compressed message of secret images and the information of related parameters are embedded into the original codebook C. For security, the modified codebook C 0 is then encrypted by the DES cryptosystem [4] to create an encrypted message. Finally, the encrypted data stream covering a set of secret images is generated. Fig. 1 illustrates the embedding procedure of the proposed scheme.
A novel secret image communication scheme capable of delivering several images by a small cover medium is proposed. To reduce the volume of secret images to be delivered, a codebook is generated and the secret images are compressed based on the VQ encoding system. The compressed message is then embedded into the VQ codebook by an adaptive least-significant-bits modification technique. Finally, the slightly modified codebook is encrypted into a meaningless data stream by the DES cryptosystem for security.
Introduction: Secret image communication or transmission has attracted much attention in past years, and many researches about image hiding have been proposed in the literature [1, 2]. Image hiding involves embedding a large amount of image data into another cover medium with minimal perceptible degradation. However, the embedding capacity and the distortion of the cover medium are a trade-off since more embedded images always result in greater degradation of the cover medium. Recently, Hu proposed a new image hiding scheme in which several secret images can be embedded into a cover image with the same image size [2]. To date, Hu’s scheme is the best when considering the proportion of hiding capacity (the total file size of secret images) to the file size of the cover medium. Nevertheless, the quality of extracted secret images at the receiver is not good. Further, the distortion of secret image becomes more serious when more secret images have to be embedded into the cover medium. This is because for Hu’s scheme it is necessary to reduce the codebook size for decreasing the volume of each compressed secret image. In this Letter, we propose a novel secret image communication scheme that focuses both on the hiding capacity of the cover medium and on the quality of extracted secret images. To design a high-capacity and high-quality secret image communication scheme, we incorporate the vector quantisation (VQ) technique [3] into our scheme to compact the volume of secret images. Moreover, to guarantee the visual quality of extracted secret images at the receiver, the VQ codebook utilised in the encoding procedure is adopted as the cover medium.
Proposed scheme: The goal of the proposed scheme is to deliver a set of secret images with an encrypted data stream. The size of this encrypted data stream is relatively small compared with the file size of all secret images. Assume that there are t secret images to be delivered and these images are grey-level images of w h pixels. To compress the secret images with VQ, a codebook should be generated before the encoding procedure. Let the size of the VQ codebook be Nc and the codeword be composed of m n elements. These Nc codewords of the codebook are generated based on the iterative LBG algorithm [3] using the secret images as a training set. After the codebook C is generated, each of these secret images is partitioned into blocks of m n pixels. Each block is then encoded into a binary index of codewords. The compressed message of all secret images is obtained by merging these binary indexes. Therefore the overall size l of the compressed message for secret images can be defined by: l ¼ t dw=me dh=ne log2 Nc
Fig. 1 Flow chart of proposed secret image communication scheme
The procedure of secret image extraction is quite simple at the receiver. To reconstruct the secret images, the received data stream is first decrypted based on the DES decryption procedure. After the decryption process, the modified codebook C 0 is directly obtained. Therefore, all the parameters, including the number of secret images t, the image size w and h, the codeword size m and n and the codebook size Nc, can be easily extracted from the first p codewords of C 0. According to the extracted parameters, the compressed message of secret images can be directly obtained from the next k codewords of C 0. The compressed message is then decoded by performing the table lookup operation on the modified codebook C 0. Finally, all the secret images are reconstructed. Simulation results: In the computer experiment, our proposed scheme was performed on five secret images (Lena, Pepper, Boats, Goldhill and Toys) with size 512 512 pixels and 256 levels per pixel. Here, the codeword size is 8 8 pixels and the codebook size is 4096. Consequently, there are 262 144 (8 8 4096) elements in this codebook and the length of each codeword index is 12 bits. Therefore, the compressed message for each secret image is 49 152 (12 (512 512)=(8 8)) bits. To embed the five secret images, we select 3840 codewords from the codebook and modify the least significant bit of each codeword element to embed the compressed message. Table 1 lists the quality of extracted secret images at the receiver, together with the original quality of VQ-compressed images at the transmitter. The peak signal-to-noise ratio (PSNR) criterion is applied to evaluate the quality of the secret image. This experimental result shows that the degradation of image quality before and after secret message embedding is very small. It also reveals that the visual quality of the secret image at the receiver is quite good.
Table 1: Quality (in dB) of secret images before and after secret message embedding Secret images
Boats
Goldhill
Toys
32.574 33.071 31.554
30.806
34.226
Extracted image (at receiver) 32.513 33.001 31.506
30.767
34.136
ð1Þ
VQed image (at transmitter)
To deliver a set of secret images with a small and encrypted data stream, we propose a novel idea to embed the compressed message of secret images into the codebook associated with these images. In addition, the parameters used in this scheme, t, w, h, m, n and Nc, have to be preserved well for future use at the receiver. These parameters are also embedded into the codebook. The embedding procedure at the transmitter is accomplished based on the following steps. Step 1: Sort the codewords of the codebook into ascending order based on the referred frequency in the procedure of secret image encoding. Step 2: Select the first p þ k codewords of the codebook based on l (the size of compressed message for secret images). The first p codewords are taken for embedding the information of related parameters, while the rest of the k codewords are used for embedding the compressed message of secret images.
Lena
Pepper
To verify the performance of our proposed secret image communication scheme, the embedding capacity (the maximum number of secret images) with respect to the size of data stream (the file size of the cover medium) is listed in Table 2. Here, each secret image file is of size 256 kbytes. This Table also illustrates that more embedding capacity can be provided by enlarging the codebook size or modifying more LSBs within a single codeword element in the proposed scheme. However, to provide better visual quality for secret images at the receiver, the number of modified LSBs within each codeword element should not exceed three. This is because, in terms of mean-squared-error (MSE), the degradation of image quality between the VQ-compressed secret image at the transmitter and the extracted secret image at the receiver is bounded
ELECTRONICS LETTERS 8th July 2004 Vol. 40 No. 14
by (2r 1)2. Here, r is the number of modified LSBs within each codeword element.
Table 2: Embedding capacity (maximum number of secret images) of proposed scheme Codebook size Size of data stream Modified LSBs 1
2
3
2048
128 kbyte
2
5
8
4096
256 kbyte
5
10
16
8192
512 kbyte
9
19
29
To compare the proposed scheme with Hu’s scheme, we first discuss the quality of extracted secret images at the receiver for each scheme. In the experiment of [2], three secret images (Lena, Pepper and Toys) are extracted after embedding, and the quality of these images is 31.072, 30.948 and 30.567 dB, respectively. However, in our scheme five images (including Lena, Pepper and Toys) are extracted after embedding, and the quality of the three secret images (Lena, Pepper and Toys) is 32.513, 33.001 and 34.136 dB, respectively. This result demonstrates that our scheme provides an impressive improvement in the quality of extracted secret images, even when more secret images are embedded in the cover medium. As for the hiding capacity (the maximum number of secret images) of Hu’s scheme, seven secret images at most can be embedded into the cover medium in the experiment of [2]. Nevertheless, with the same number of modified LSBs (3 bits per codeword element) on the cover medium, 16 secret images can be embedded into the cover medium by our proposed scheme. Note that both of the cover media are of the same file size (256 kbytes). By contrast, our scheme gives a good improvement in the hiding capacity of the cover medium as well.
of the following reasons: (i) The secret images are included into the training set in the codebook generation procedure. (ii) The volume of secret images to be delivered is greatly reduced by applying the VQ technique on these images. (iii) The cover medium is composed of all the VQ codewords associated with these secret images. (iv) The compressed message of secret images is embedded into cover medium based on an adaptive LSBs substitution technique. In addition, the modified cover medium is further encrypted into a meaningless data stream by the DES cryptosystem for more security. Simulation results demonstrate that the proposed scheme provides significant improvement both in the quality and quantity of covered secret images. Therefore, we conclude that the proposed scheme is feasible for secret image communication.
Acknowledgments: The authors are supported by NDHU-ROC and NSC-ROC with project no. NSC 92-2213-E-259-018. # IEE 2004 Electronics Letters online no: 20040568 doi: 10.1049/el:20040568
S.D. Lin and S.-C. Shie (Department of Computer Science and Information Engineering, National Dong Hwa University, Hualien, Taiwan, Republic of China) E-mail:
[email protected] References 1 2
Conclusions: A novel secret image communication scheme capable of delivering a set of secret images has been introduced in this Letter. The proposed scheme provides impressive improvement both in the visual quality of extracted secret images and in the hiding capacity of the cover medium. This scheme outperforms Hu’s scheme because
17 March 2004
3 4
Du, W.-C., and Hsu, W.-J.: ‘Adaptive data hiding based on VQ compressed images’, IEE Proc., Vis. Image Signal Process., 2003, 150, (4), pp. 233–238 Hu, Y.C.: ‘Grey-level image hiding scheme based on vector quantisation’, Electron. Lett., 2003, 39, (2), pp. 202–203 Linde, Y., Buzo, A., and Gray, R.M.: ‘An algorithm for vector quantizer design’, IEEE Trans. Commun., 1980, 28, pp. 84–95 ‘DES Encryption Standard (DES)’, National Bureau of Standards (US), Federal Information Processing Standards Publication 46, National Technical Information Service, Springfield, VA, April 1997
ELECTRONICS LETTERS 8th July 2004 Vol. 40 No. 14
