Complete Double Layered Embedding Scheme for Information Hiding

Chin-Feng Lee1, Jau-Ji Shen2 and Hung-Lin Chao1

1Department of Information Management Chaoyang University of Technology

168 Jifong E. Rd., Wufong Township, Taichung County 41349, Taiwan, R.O.C. E-mail: {lcf, s9614620}@cyut.edu.tw

2Department of Management Information Systems National Chung Hsing University

250, Kuo Kuang Rd., Taichung 402, Taiwan, R.O.C. E-mail: jjshen@nchu.edu.tw

Abstract

Double-Layered Embedding was proposed by exploiting the least significant bit replacement on gray pixels which are not specified by wet paper codes. DLE adds one to or subtracts one from pixel value when the secret bit to be embedded is not matched the least significant bit of that pixel. However, DLE requires extra space like location map to indicate if the pixel has been changed by adding and subtracting 1. Though the location map gives a hand to correctly recovery the secret message into its original appearance, it is an overhead. Without the requirements of location map for secret message extracting, we propose an improved scheme called Complete Double-Layered Embedding to provide higher capacity compared to DLE scheme does and maintain good image quality.

Keywords: Information hiding, wet paper code, double-layered embedding, LSB

1. Introduction

As the rapidly development and improvement in digital information technology, any information can be presented and exchanged in the form of digital base through the Internet. Therefore, it is a critical concern currently to deliver message safely during the process of transmittance. Illegal information grabber might seize, alter and forge the message through the transmitting process. Therefore, the information hiding technique is widely used to solve such problem [1-7]. The digital media like images or videos act as the embedding carriers for transmitting the secret message successfully and safely. In such case, how to embed

the secret message into the digital media more efficiently with no aware of detection by the grabber is the critical issue for current researches.

As the original LSB Replacement can be easily analyzed by the analysis of statistical techniques [2][3],Zhang et al. therefore proposed Double-Layered Embedding (also called DLE for short) scheme based on Wet Paper Code techniques [1] to embed the secret message into the 1st LSB or the 2nd LSB of pixel values through adding and subtracting 1 in order to carry bit. DLE requires a location map to record all the locations of pixels that are added and subtracted by 1 in order to recovery the correct message later. This will not only enhance the safety of secret message but also increase the capacity of hiding information. On the basis of DLE scheme, here CDLE which is short for Complete Double Layered Embedding is proposed to improve the embedding capacity compared to that of DLE scheme. The experimental result shows that the proposed scheme indeed outperforms the DLE scheme in the capacity. Moreover, the average PSNR value of the stego images can maintain up to 46.37dB.

The organization of this paper is as follows. Some notations are listed in Section 2 for simplifying the introduction of following schemes. Double-Layered Embedding scheme is reviewed in Section 3. The introduction to the proposed scheme is presented in Section 4. The experimental analysis and discussion is shown in Section 5 and the conclusions are stated in Section 6.

2. Notations

Prior to the introduction of DLE and CDLE schemes, we define the relevant notations used in this paper below.

Eighth International Conference on Intelligent Systems Design and Applications

978-0-7695-3382-7/08 $25.00 © 2008 IEEE

DOI 10.1109/ISDA.2008.105

525

1 2( , , ..., )nI a a a : A grayscale image I with cover pixels and each pixel value can the denoted as

, .

n

ia a0 2i≤ ≤

1 2 1

2 1 2 1 2

2 1 2 1 2

2 1

( ) ( )

1 and ( ) 0;

Case 1: ( ) 0, ( ) 0 and ( ) 0, ( ) 1,

1;Case 2: ( ) 0, ( ) 1 and ( ) 1, ( ) 0,

1;Case 3: ( ) 1,

i j

i i j j

i

i i j j

i

i

if LSB a S b

j j L ielse

LSB a LSB a S b S b

do aLSB a LSB a S b S b

do aLSB a LSB

−

−

−

=

= + ←

= = =

+= = =

+= 2 1 2

2 1 2 1 2

2 1 2 1 2

2 1

( ) 0 and ( ) 1, ( ) 1,

1;Case 4: ( ) 1, ( ) 1 and ( ) 0, ( ) 0,

1;Case 5: ( ) 0, ( ) 0 and ( ) 1, ( ) 1,

1;Case 6: ( ) 0, ( )

i j j

i

i i j j

i

i i j j

i

i i

a S b S b

do aLSB a LSB a S b S b

do aLSB a LSB a S b S b

do aLSB a LSB a

−

−

−

= =

+= = =

+= = =

−= = 2 1 2

2 1 2 1 2

2 1 2 1 2

1 and ( ) 0, ( ) 0,

1;Case 7: ( ) 1, ( ) 0 and ( ) 0, ( ) 1,

1;Case 8: ( ) 1, ( ) 1 and ( ) 1, ( ) 0,

1. 2 and ( ) 1,

j j

i

i i j j

i

i i j j

i

S b S b

do aLSB a LSB a S b S b

do aLSB a LSB a S b S b

do aj j L i

end else

−

−

−

= =

−= = =

−= = = =

−= + ←

;iend while

+ +

55=

=

=

=

=

=

1 2( , , ..., )nI g g g : A grayscale image I with stego pixels and each pixel value can the denoted as

n

ig , .0 2i≤ ≤ 55g

1 2( , , ..., )mS b b b : A secret message S with bits, and each secret bit can be denoted as

m

jb ,

where .{0, 1}∈jb

1( )iLSB a : The 1st least significant bit value of the cover pixel .ia

2 ( )iLSB a : The 2nd least significant bit value of the cover pixel .ia

1( )iLSB g : The 1st least significant bit value of stego pixel ig .

2 ( )iLSB g : The 2nd least significant bit value of stego pixel ig .

1( , )i jLSB a b : A procedure to produce a stego pixel ig

by using the secret bit to replace the 1st

least significant bit of a .jb

i

( )L i : A binary value in the ith position of location map. ( ) 0L i = , if the stago pixel ig is equal to the cover

pixel ; , otherwise. ia ( ) 1L i = As the receiver obtains the stego image, the secret message can be extracted according to the indication of location map. If is the secret bit to be extracted; If concatenating with

are two secret bit to be extracted.

1( ) 0, ( )iL i LSB g=

1( ) 1, ( )iL i LSB g=

2 ( )iLSB g

3. Related Work

Double-Layered Embedding (DLE) scheme was proposed by Zhang et al [4][7]. DLE will firstly check if the secret bits to be embedded are equal to the 1st

LSB of those cover pixels. Wet paper codes are used to specify the pixels whose 1st LSBs are not matched to the secret bits. For all the pixel values with wet paper codes, DLE will make the pixel values increased or decreased by one by modifying the 1st or 2nd LSBs. The quality of the stego image is fairly high after secret message is embedded such that DLE has the ability against statistic analysis. Accordingly, the DLE algorithm is presented as follows:

4. Proposed Scheme

Complete Double Layered Embedding (CDLE) scheme based on the Double-Layered Embedding scheme is proposed in order to raise the embedding capacity. Instead of reserving a location map which is usually occupied huge overhead in space, DLE scheme performs the embedding procedure like DLE scheme does to alter the bit values of 1st LSB and 2nd LSB of cover pixel such that the cover pixel values are added or subtracted one. CDLE scheme is promising because it enhances the embedding capacity substantially without the requirement of location map. Moreover, the stego image quality of CDLE is as high as that of DLE scheme such that CDLE can withstand the statistic analysis. The description to CDLE scheme is as the following two phases.

1 2

1 2

1 2

: the cover image ( , , ..., ) and a secret bitstream ( , , ..., ).

: the stego image ( , , ..., ).int 1, 1;

( )

n

m

n

Input I a a aS b b b

Output I g g gi j

while j m= =

≤

526

4.1. Embedding phase

The CDLE scheme algorithm can be presented as follows:

: A cover imageInput 1 2( , , ..., )nI a a a

1 2 , ..., )mb b and a secret

bitstream .( ,S b

1 2: The corresponding stego image ( , , ..., ).nOutput I g g gStep1: Initial 1i = and 1j = .Step2: Perform the complement for these secret bits

2( )jS b ; change 1's to 0's and 0's to 1's,

1, 2, ..., for i = k'2( )

such that they are changed

into jS b .

Step3: Perform the procedure 2

' '1( ) ( , )

ji iI a LSB a b=

Then the modified pixel such that

2

'1( ) ( , )

ji iI g LSB a b=

2

'

by using the secret bit

jb

ia for

i

to replace the 1st least significant bit of

cover secret , and the stego pixel

1, 2, ..., i k=g conceals the complement of

secret bit .2ibStep4: While j is less than k , where 2k m≤ .

Perform one of the following cases depending on the values of , ,

and .1( )iLSB a 2 ( )iLSB a

2 1( jS b − )

=

=

=

=

=

=

=

2( )jS b

2 1 2 1 2

2 1 2 1 2

2 1 2 1 2

Case 1: ( ) 0, ( ) 0 and ( ) 0, ( ) 1,

1;Case 2: ( ) 0, ( ) 1 and ( ) 1, ( ) 0,

1;Case 3: ( ) 1, ( ) 0 and ( ) 1, ( ) 1,

1;Case 4:

i i j j

i

i i j j

i

i i j j

i

LSB a LSB a S b S b

do aLSB a LSB a S b S b

do aLSB a LSB a S b S b

do a

−

−

−

= = =

+= = =

+= = =

+

2 1 2 1 2

2 1 2 1 2

2 1 2 1 2

2

( ) 1, ( ) 1 and ( ) 0, ( ) 0,

1;Case 5: ( ) 0, ( ) 0 and ( ) 1, ( ) 1,

1;Case 6: ( ) 0, ( ) 1 and ( ) 0, ( ) 0,

1;Case 7: ( )

i i j j

i

i i j j

i

i i j j

i

i

LSB a LSB a S b S b

do aLSB a LSB a S b S b

do aLSB a LSB a S b S b

do aLSB a

−

−

−

= = =

+= = =

−= = =

−

1 2 1 2

2 1 2 1 2

1, ( ) 0 and ( ) 0, ( ) 1,

1;Case 8: ( ) 1, ( ) 1 and ( ) 1, ( ) 0,

1.

i j

i

i i j j

i

LSB a S b S b

do aLSB a LSB a S b S b

do a

−

−

= = =

−= = =

−

j =

Step5: Reset 1i i= + , 1j j= + and repeat Step2 until all secret bits are embedded.

Example 4.1

Assume four grayscale cover pixel values to image I are presented by be and the secret message to be embedded is .Firstly, the proposed CDLE algorithm transforms

10(10, 11, 22, 13)I(10110111S 2)

Iinto its binary representation which is

Theembedding procedure is then presented by the following 3 steps:

20110, 00001101) .( , , 00 010 00001011 0001I 001

Step1: Convert secret bits in even position to theircomplement values but remain the secret bit in odd position unchanged. That is, = S(0

1 1 1) are converted into =S(1 0 0 0). 2( )jS b

)'2( jS b

Step2: Embed all bits in '2( )jS b into the 1st LSB of

cover pixels in I such that 'I is a provisional stego image with four stego pixel values

whose binary representation are

.

1022 )

,0010

'( , , 11 10 , 2 1I

'( ,00001011 00I 210 000101 000011 )10, 00Step3: Separate S into four secret pairs such that

1 2 3 4 3 4 7 810, 11, 11, and 11.b b b b b b b b= = = =

These four secret pairs will be embedded into four cover pixels of image 'I in turn. When embedding

into the first cover pixel of 1 2b b 1a 'I , we perform the eighth case (case 8) to subtracting by one, i.e.,

When embedding into the second cover pixel of

1ab b1 1 1 10.= − =g a 3 4

2a 'I , we perform the third case (case 3) to add by one, i.e., When embedding into the third cover pixel of

2a 2 2g a 1 11.=

3a= +

5 6b b 'I ,we perform the seventh case (case 7) to subtracting by one, i.e., g a When embedding into the fourth cover pixel of

3a

7 8b b3 3= −1 21.

4a=

'I , we perform the fifth case (case 5) to subtracting by one, i.e.,

Eventually, the stego image is 4a

4 4 1 11.g a= − = 11, 21, 11)10(10,I .

4.2. Extracting phase

The secret message can be extracted by performing such that 2 1 2 2 1( ) ( ), ( ) ( )j i jS b LSB g S b LSB g− = =

2 1 1 1 2 2 1{ ( ), ( ), ( ), (S LSB g LSB g LSB g LSB g=i

2 2), ..., ( ),kLSB g

527

1( )} , 1, 2, ..., .kLSB g for i j k=

10(10, 11, 21, 11)I( , , 00001010 00001011 000I

2 1 1 1 2 2( ( ), ( ), ( )S LSB g LSB g LSB g

1 3 2 4 1 4( ), ( ), ( )) (LSB g LSB g LSB g =

Example 4.2

According Example 4.1, the extracting procedure of CDLE scheme is firstly to transform the stego image

into its binary representation which is 210101, 00001011) .

Then the secret message can be extracted from 8 least significant bits. Therefore, the secret message is

1 2 2 3, ( ), ( ),LSB g LSB g

210110111) .

5. Experimental Results

In order to evaluate the performance of the proposed scheme, we implemented 2LSB replacement, DLE as well as CDLE schemes by using MATLAB 7.1 software that works on the Intel Pentium IV 3.0GHz CPU with 1.5GB RAM hardware platform.

In order to evaluate the performance of different schemes, embedding capacity to calculate the number of bits which can be embedded and visual quality estimated by Peak Signal to Noise Ratio (PSNR) are utilized as two evaluation factors. Lena, Babala, Baboon, Jet(F16), Pepper, Tiffany, Sailboat, GoldHill are eight test images sized 512×512 pixels used in our experiments.

The experimental comparison results are as shown in Table 1 as follows.

Table 1. Performance comparison in image quality and embedding capacity among 2LSB Replacement,

DLE and the propose scheme

2LSBReplacement DLE Propose CDLE Schemes

Image PSNR Capacity PSNR Capacity PSNR Capacity

Lena 44.16 524288 46.37 393291 46.37 524288Babala 44.15 524288 46.37 392751 46.37 524288Baboon 44.15 524288 46.37 393179 46.37 524288Jet(F16) 44.13 524288 46.37 393085 46.36 524288Pepper 44.14 524288 46.36 393032 46.36 524288Tiffany 44.15 524288 46.36 393116 46.37 524288Sailboat 44.14 524288 46.35 392625 46.38 524288GoldHill 44.15 524288 46.36 393245 46.36 524288Location

Map No Yes No

Average 44.14 524288 46.36 393041 46.37 524288

In Table 1, the average embedding capacity of DLE is 524288 bits with the average PSNR value 44.16dB; the average embedding capacity of DLE is 393245 bits with the average PSNR value 46.36dB; the average embedding capacity and PSNR value of CDLE scheme

are 524288 bits and 46.37 dB, respectively. The results show that CDLE scheme has the equal embedding capability as 2LSB replacement does but has higher visual quality compared to 2LSB replacement. Besides, CDLE scheme reaches the equal embedding capability as 2LSB replacement but has higher visual quality compared to 2LSB replacement. Since DLE scheme possesses fairly high quality of the stego image, it has the ability against statistic analysis. Like DLE, CDLE scheme has good visual quality, so it is more robust as statistic analysis than LSB replacement.

6. Conclusions

It has always been one critical issue between the image quality and the information hiding capacity. We improve the Double-Layered Embedding technique in the embedding capacity with no location map requirement. Compared to DLE scheme, the experimental result shows that CDLE increases almost 25% in the capacity with rather good image quality.

References

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[4] W. Zhang, X. Zhang, and S. Wang, “A double layered “plus-minus one” data embedding scheme”, IEEE Signal Processing Letters, Vol. 14, No. 11, pp. 848-851, 2007.

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