Reversible hiding in DCT-based compressed images Authors:Chin-Chen Chang, Chia-Chen Lin, Chun-Sen Tseng and Wei-Liang Tai Adviser: Jui-Che Teng Speaker: Gung-Shian Lin Date:2009/12/17

2 Outline 1. Introduction 2. Related works 3. Proposed scheme 4. Experimental results 5. Conclusions

3 Introduction Lossless and reversible steganography scheme for hiding secret data in each block of quantized DCT coefficients in JPEG images.

4 Introduction In 2001, Fridrich et al. proposed Invertible authentication watermark for JPEG images. In 2004, Iwata et al. proposed Digital steganography utilizing features of JPEG images.

5 Related works RGB transformation for JPEG RGB Image Transformation RGB→YCbCr Composition MCU 2-D DCT Quantization Table Runlength coding Huffman coding Huffman Table JPEG Image

6 Proposed scheme Embedding procedure R5R5 R9R9 R8R8 R7R7 R6R6 R1R1 R2R2 R4R4 R3R3 b i be the length of ceaseless zeros z i,1 represents the zero value of the lowest frequency

7 Proposed scheme R5R5 R4R4 R3R3 R2R2 R1R1 s i be the secret bit we want to embed into set R i

8 Proposed scheme The embedding strategies and elimination measures for ambiguous conditions are as follows: Case 1:If b i ≧ 2, we use the value of z i,2 to indicate the hidden secret bit in set R i (1 ≦ i ≦ 9). We modify the value of z i,2 to hide secret bit by using the Eq. where 1 or -1 is randomly selected.

9 Proposed scheme Ambiguous condition A and its remedial measure 。 1 0 x R1R1

10 Proposed scheme where 3 ≦ (j － 1) ≦ k i R2R2

11 Proposed scheme Case 2: If b i < 2 and both z i,1 and z i,2 do not exist, none secret bits can be hidden in a set R i. Two ambiguous conditions may exist, and therefore two remedial measures for eliminating them are described below.  Ambiguous condition B and its remedial measure  Ambiguous condition C and its remedial measure

12 Proposed scheme Example of embedding:assume four secret bits, 0, 0, 1 and 1

13 Proposed scheme Extracting procedure Step 1. Obtain non-overlapping 8 * 8 blocks of quantized DCT coefficients of the Y components from a JPEG stego-image after Huffman decoding and runlength decoding. Step 2. Scan each block according to a predetermined order.Step 3. For each set R i in a block, let r i,j be the highest frequency non-zero component, where 1 ≦ i ≦ 9 and 1 ≦ j ≦ k i. Step 4. Extract si from set Ri by using the following rules:Step 5. Repeat Steps 3 and 4 until all blocks are processed. RiRi r i,j ＝ 1 or -1 r i,j does not existr i,j ≠1 or -1 r i,j+1 ＝ 0j≦2j≦2 r i,j-1 ＝ 0 and r i,j-2 ＝ 0 r i,j+1 ≠0 r i,j-1 ＝ 0 and r i,j-2 ＝ 0 j≦2j≦2 s i =0 mark r i,j-2 as z i,2 s i does not exist s i =0 mark r i,1 as z i,2 s i does not exist s i =0 mark r i,j-2 as z i,2 s i =1 mark r i,j as z i,2

14 Proposed scheme Restoring procedure  Rule 1: If s i exists and r’ i,j+3 ＝ 0, where 4 ≦ (j ＋ 3) ≦ k i, then the original value of r’ i,j+2 is restored by using Eq.  Rule 2:If s i does not exist and the two highest coefficients (r’ i,1 ， r’ i,2 ) of set R i equals (x, 0), where x≠0, then the original value of r’ i,1 is restored by using Eq. where 3 ≦ (j ＋ 2) ＜ k i.

15 Proposed scheme  Rule 3: If s i does not exist and the pair having the three highest coefficients (r’ i,1, r’ i,2, r’ i,3 ) of set R i equals (0,x,0), where x≠0, then the original value of r’ i,2 is restored by using Eq.

16 Proposed scheme

17 Proposed scheme Modifying quantization table for better image quality and hiding capacity

18 Experimental results

19 Experimental results

20 Experimental results

21 Conclusions The scheme provides stego-images with acceptable image quality and similar hiding capacity can be achieved with the Iwata et al. scheme 。 The scheme can withstand visual and statistical attacks 。