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Both Color and Gray Scale Secret Images Hiding in a Color Image

This article discusses a scheme for hiding secret images, both in color and grayscale, within a host image while maintaining image quality and accommodating a large amount of secret information.

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Both Color and Gray Scale Secret Images Hiding in a Color Image

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  1. Both Color and Gray Scale Secret Images Hiding in a Color Image Min-Hui Lin, Yu-Chen Hu and Chin-Chen Chang, International Journal of Pattern Recognition and Artificial Intelligence, Vol. 16, No. 6, 2002, pp. 697-713. Advisor：ProfessorChangChin Chen Student：Chen Yan Ren Date：2003/02/18

  2. Outlines • Introduction • PSNR Calculation and Palette Generation • The Proposed Scheme • Experimental Results • Conclusions

  3. Introduction • Data hiding technology: to protect data safety and to keep from exposing the value of data. • Hiding one secret image that needs to be protected into another meaningful image, called a host image. • Most of the methods use a gray scale image as the host image. • The authors propose an color image hiding scheme that maintains host image quality and accommodates a large quality of secret information. • This scheme can apply to both color and gray scale secret images.

  4. PSNR (Peak-Signal-to-Noise Ratio) Calculation • The authors assume that there are two color images, S (secret image) and H (host image). • The image size of S and H is m×m. (1) (2) • MSE：Mean-Square Error. • n：the number of bits per pixel. • m：the width and height of the host image • zij：the original pixel value • hij： the pixel value after substitution of the image at the location of (i,j).

  5. Palette Generation • Produce a palette for a color image, the easiest way is uniform sampling. • Choose R=6,G=7,B=6 samples and four random colors to make a 256-color palette. (6×7×6+4=256 colors) Fig.1. The relationship between an image IM and its palette

  6. The Proposed Scheme • Basic Concept • Color Image Hiding Procedure • Substitution • Palette Design • Index Mapping • Encryption • Replacement • An Example

  7. Basic Concept • Achieve three goals • Cause minimum distortion to the host and the retrieved secret images. • Minimize extra data volume during image transmission. • Maximize the difficulty level which the interceptors extract the secret image. • Five Steps • Set the rightmost bits of host image to be zero. • Create a palette on the transformed host image. • Covert the color secret image into index values. • Use DES to encrypt the transformed secret image. • Embed the encrypted secret image into transformed host image.

  8. Color Image Hiding Procedure • We assume that there are two color images with a size of 512×512. • Both images use RGB channels, and every channel uses 8 bits. Fig. 2. The flowchart of embedding a secret image into a host image

  9. H H Host Image Host Image 8 8 8 8 8 ( 8 ( bits) bits) ( ( 24 24 - - bits/pixel) bits/pixel) Original Pixel Original Pixel R R G G B B Rules: Rules: R: the rightmost three R: the rightmost three bits bits ’ ’ value =0 value =0 Substitution Substitution G: the rightmost two bits G: the rightmost two bits ’ ’ value =0 value =0 B:the rightmost three bits B:the rightmost three bits ’ ’ value=0 value=0 H H ' ' Substituted Image Substituted Image 5 3 6 2 5 3 ( 5 3 5 2 5 3 ( bits) bits) Substituted Pixel Substituted Pixel ( ( 24 24 - - bits/pixel) bits/pixel) 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 Substitution • We decide to use the rightmost 3 bits of R, the rightmost 2 bits of G, and rightmost 3 bits of B in host images. • We set the values of these bits as zero, then get a transformed host image H’. Fig. 3. The flowchart of transforming H image into H’

  10. Palette Design • Identify maximum and minimum values of all pixels in every one of the R, G, and B channels from H’ image. • Calculate the channel range. • RangeR=maxR-minR; RangeG=maxG –minG; RangeB=maxB-minB. • Locate the representation values from R, G, and B channels, six values from R and B, and seven values from G. • ValueR[j]=(RangeR∕6) × (j+0.5) + minR, 0≤j<6; • ValueG[j]=(RangeG∕7) × (j+0.5) + minG, 0≤j<7; • ValueB[j]=(RangeB∕6) × (j+0.5) + minB, 0≤j<6. • Use values of R, G, B to create 252 combinations as the 252 color indexes on the palette, i.e. CP[0], CP[1],…, CP[251]. • Determine the final four colors.

  11. x x x y y y z z z i i i i i i i i i Index Index Index ( ( ( R,G,B) Table R,G,B) Table R,G,B) Table ( ( ( R R R ,G ,G ,G ,B ,B ,B ) ) ) I I I 0 0 0 0 0 0 0 0 0 0 0 0 I I I ( ( ( R R R ,G ,G ,G ,B ,B ,B ) ) ) 1 1 1 1 1 1 1 1 1 1 1 1 S S S ( ( ( 24 24 24 - - - bits/pixel) bits/pixel) bits/pixel) I I I (R (R (R ,G ,G ,G ,B ,B ,B ) ) ) 255 255 255 255 255 255 255 255 255 255 255 255 Rule: Rule: Index Mapping Index Mapping Index Mapping If If (x (x - - R R ) ) + + (y (y - - G G ) ) + + (z (z - - B B ) ) is minimum, is minimum, 2 2 2 2 2 2 i i j j i i j j i i j j then then (x (x ,y ,y ,z ,z ) ) =I =I , where , where i i i i i i j j I I 0 0 I<image size, 0 i<image size, 0 j ≦255. j<255. ≦ ≦ ≦ ≦ j j S S S ' ' ' ( ( ( 8 8 8 - - - bits/pixel) bits/pixel) bits/pixel) Index Mapping • Select in due sequence the values of every pixel of image S, (xi, yi, zi), 0≤i<image size, and the 256 sets of RGB values (Rj, Gj, Bj), 0≤j≤255. • Formula  Min (xi-Rj)2+(yi-Gj)2+(zi-Bj)2, 0≤i<image size, and 0≤j≤255. Fig. 4. The flowchart of mapping S image into S’

  12. Encryption • Use 64-bits DES encryption key. • The transformed S’ image is cut from left to right and from top to bottom into 64-bit blocks. • The encrypted blocks are grouped into an encrypted image DS’, which is as large as S’ image.

  13. Replacement • Every 8 bits of the encrypted image DS’ are put into one group from left to right. • Every group is further divided into three subgroups, 3 bits, 2 bits, and 3 bits. • These bits are used to replace those pre-selected positions in the transformed host image H’ to generate an embedded image E.

  14. An Example (b) Lena (a) Sailboat Fig. 5. (a) Sailboat is the secret image and (b) Lena is the host image

  15. Example of Substitution and Palette Design • Substitution • Palette Design • The representation values can be used to generate the corresponding RGB values, CP[0],CP[1],…,CP[251]. • The last four RGB values：CP[252]=(0,0,0), CP[253]=(80,80,80), CP[254]=(160,160,160), and CP[255]=(255,255,255) Host Image H Host Image H’ Table 1. Pixel information from Lena (Host Image H’) RangeR=248-48=200 ValueR[0]= 200/6×(0+0.5)+48=64

  16. Example of Index Mapping, Encryption and Replacement • Index Mapping • Encryption • Replacement (R,G,B) Table Index mapping formula Min (xi-Rj)2+(yi-Gj)2+(zi-Bj)2 Index Secret Image S (xi, yi, zi) Secret Image S’(Ij) Encrypted image DS’ Transformed S’ into 64-bits blocks and encrypts by DES. Secret Image S’ 8 bits of Encrypted Image Encrypted image DS’ Embedded Image E

  17. Six Color Images for First Experiment (a) Sailboat (c) Airplane (e) House (b) Lena (d) Baboon (f) Peppers Fig. 6. Sample images: six original color images of 512512 pixels

  18. Experimental Results Table 2. Quality of secret images after performing color quantization using palettes of different host images Table 3. Quality of host images hiding secret images via our proposed scheme

  19. Eight Gray Scale Images for Second Experiment (a) Airplane (c) Barbara (g) Sailboat (e) Lena (b) Baboon (d) Boat (f) Peppers (h) Toys Fig. 7. The eight gray level 512512 images that are used as the secret images

  20. Experimental Results Table 4. Quality (PSNR) of embedded gray scale images (hidden by color host images)

  21. Conclusions • Propose a new image hiding technique. • A color image hiding a color image. • A color image hiding a gray scale image. • Propose conducting color quantization on the color secret image via palettes to reduce the number of bits. • Use DES to encrypt the transformed secret image. • “A color image hiding a color image”, the average PSNR value of the host image is 39.184. • The lowest PSNR value of the secret image after color quantization is 25.913. • The scheme ensures the quality of the host image and the retrieved secret image for color host images.

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