1 A Combined Approach to Integrity Protection and Verification of Palette Images Using Fragile Watermarks and Digital Signatures Source: IEICE TRANS. On FUNDAMENTALS, vol. E87-A, no. 6, pp , Jun Author: Chih-Hsuan Tzeng and Wen-Hsiang Tsai Speaker: Na-I Wu ( 吳男益 ) Data: 2004/09/16
2 Outline Introduction The proposed method (1) Fragile watermarking embedding (2) Generate “digital signatures” Experimental results
3 Introduction An indexed image consists of data matrix, X, and a colormap matrix, map. Size: 129 x 126 Ex: 256 色 Indices: form 1 to 256 File Size: Color map: 129 x 126 x 8 Indices: 256 x 3 x k bytes
4 Introduction
5 The proposed method Fragile watermarking embedding To classify pixels into embeddable and non-embeddable ones. Embed authentication signals Generate Digital signature
6 Fragile watermarking embedding Raster Scanning Manner : to find “embeddable”, and “non-embeddable” x8 Raster Scanning Manner : to find “embeddable”, and “non-embeddable”
7 Raster Scanning Manner : to find “embeddable”, and “non-embeddable” P1(i-1, j-1)P2(i, j-1)P3(I+1, j-1) P4(i-1, j)X(i,j) Raster Scanning Fragile watermarking embedding α: the number of distinct colors of pixels in Prec(X) β: The maximum color distance d max (X, Prec(X))
8 How to compute α, β values 12 60α = α = 3 Fragile watermarking embedding 12 (50, 60, 70) 轉成 L, a, b 顏色模式 (69, 10, 5) 10 (90, 10, 60) 轉成 L, a, b 顏色模式 (10, 7, 6) 兩者的距離 =[(69-10) 2 +(10-7) 2 +(5-6) 2 ] 1/2 L: luminance a: green to red B: blue to yellow
9 Fragile watermarking embedding The values of T c and T d be set by user EX1: T c =2, T d =10 12 α = 0, β = α = 1, β = 48 “non-embeddable”
10 The values of T c and T d be selected by user EX2: T c =2, T d =10 “embeddable” α = 3, β = α = 4, β = 8 Fragile watermarking embedding
11 Palette Mapping Function M Fragile watermarking embedding 1(120,50,30) 2(111,51,21) 3(112,52,32) 4(123,13,33) 5(134,4,34) 6(25,55,35) Color palette 重排列 f(u)=2 16 xr +2 8 xg +b 換算權重值
12 Palette Mapping Function M Fragile watermarking embedding K: secret key Generate a random bit stream 若為 n 色,則產生 n bits
13 Fragile watermarking embedding Palette Mapping Function M 1(120,50,30) 1 2(111,51,21) 1 3(112,52,32) 0 4(123,13,33) 0 5(134,4,34) 1 6(25,55,35) 1 attack 1(120,50,30) 1 3(112,52,32) 1 2(111,51,21) 0 4(123,13,33) 0 5(134,4,34) 1 6(25,55,35) 1 將 pixel 值 2 變 3 , 3 變 2
14 EX2: T c =2, T d =10 , and want to hide “0” α = 3, β = 6 Fragile watermarking embedding K: secret key No modify Authentication Data: K: secret key
15 EX2: T c =2, T d =10 , and want to hide “1” α = 3, β = 6 Fragile watermarking embedding K: secret key (1) 去找週遭 4 個的 Lab 顏色 模式,其差最小的那一個 and (2) M(i)==secret data 修改完之必需符合 “Embeddable pixels” 的規定 α > T c and T d < β 16
The proposed method Generate “digital signatures” EX1: T c =2, T d =10 12 α = 0, β = 0 “non-embeddable” 0 Authentication Data: K: secret key XOR operator 0 Generate “digital signatures”
17 The proposed method 每個 8x8 區塊的 “ digital signatures ” 的長度不一樣, 故每個 簽章的前面先放 ceil(nxn) bits ,用來 記錄每個 小區塊的 簽章的長度。 將每個小區塊的簽章的長度,合併起來來, 最後放在檔頭。
18 The Authentication Process EX2: T c =2, T d = α = 3, β = K: secret key K: Authentication Data secret data “1” temper α = 3, β = 4 12
19 The Authentication Process EX1: T c =2, T d =10 12 α = 0, β = 0 “non-embeddable” K: secret key K: Authentication Data “digital signatures” XOR operator temper α = 1, β = 1 1
20 Experimental Results Block size = 8 × 8 T c = 2, T d = 15 Size: 560 × palette colors Digital signatures: 7938bytes The degrees of average distortion in the embeddable pixels of a block: ADEPB(max): 8.25
21 Experimental Results Size: 488 × palette colores Digital signatures: 11907bytes ADEPB(max): 9.76