Improvements of Image Sharing with Steganography and Authentication Ching-Nung Yang, Tse-Shih Chen, Kun Hsuan Yu, Chung-Chun Wang Journal of Systems and software, Vol. 80, Issue 7, pp. 1070-1076, July, 2007 Reporter: 周美君

Outline Introduction Lin–Tsai scheme Proposed scheme Experimental results Conclusions

Introduction An example of Shamir's secret sharing Ex: (2,3) threshold system (The secret key s =4) F(x) = 4 + 2x 3 shares: (1,6), (5,14), (8,20) x y F(x) (1,6) (5,14) (8,20) F(0)=4

Lin–Tsai scheme Using (2, 3)-threshold Watermark signal p=1 Parity Check Secret image Camouflage images 27 31 28 61 60 59 58 200 198 197 60 F(xi) = 60 + m1 × xi

27 25 30 26 27 31 28 61 62 61 60 59 58 200 198 197 200 203 194 193 (∵ p=1, ∴parity check b=0) 27 0001 1111 27 25 30 26 01 0001 1011 0001 1100 110 010

27 25 30 26 61 62 27 0001 1001 61 0011 1110 0001 1110 0001 1010 0011 1110 0001 1110 Check the Parity Check (p=1) is correct or not. F(27) = s + m × 27 = 011100102 = 114 F(61) = s + m × 27 = 101101102 = 182 } F(0) = s = 60 60

Improving Lin-Tsai scheme Authentication hash function with the secret key K Quality of stego image rearranging 17 bits ( 8 bit input xi + 8 bit output qi + 1 check bit Pi ) Non-lossless secret image mod Galois Field GF(28) g(x) = ( x8 + x4 + x3 + x + 1 )

Proposed scheme Camouflage images Secret image 23 20 15 11 27 31 28 61 60 59 58 60 F(xi) = 60 + m1 × xi F(23) = 60 + 2 × 23 mod (238+234+233+23+1) = 106 F(27) = 60 + 2 × 27 mod (278+274+273+27+1) = 114 F(61) = 60 + 2 × 61 mod (618+614+613+61+1) = 182

21 18 14 10 23 20 15 11 25 27 28 30 27 31 28 62 59 53 58 61 60 59 58 F(23) = 60 + 2 × 23 = 106 = 01101010 01 ( illustration at next slide ) 0001 0111 0001 0100 21 18 14 10 11 10 0000 1111 0000 1011 10 10

0001 0111 0001 0100 0001 0Pi10 0000 1010 0000 1110 0001 0101 106 = 01101010 0000 1111 0000 1011 Hk( 00010101 || 00010 10 || 00001110 || 00001010 || Block ID || Image ID )= h Pi = XOR h

Experimental Results Lin–Tsai scheme 38.54 38.49 38.31 Proposed scheme Secret image Experimental Results Lin–Tsai scheme 38.54 38.49 38.31 Proposed scheme 41.02 41.08 40.59

Conclusions This paper improves Lin–Tsai scheme: to strengthen authentication to have batter quality of stego image to have lossless recovery image.