1 視覺密碼學 chair professor chin-chen chang ( 張真誠 ) national tsing hua university (...
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視覺密碼學視覺密碼學
Chair Professor Chin-Chen Chang ( 張真誠 )
National Tsing Hua University ( 清華大學 ) National Chung Cheng University ( 中正大學 ) Feng Chia University ( 逢甲大學 ) http://msn.iecs.fcu.edu.tw/~ccc
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Outlines Concept of Secret Sharing Concept of visual cryptography Visual cryptography for binary images Visual cryptography for grayscale images
Meaningful visual cryptography for grayscale images
Meaningful and progressive visual cryptography Visual cryptography for color images Authenticatable VSS Scheme
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shadows
secret message
Traditional (t, n)-secret sharing (Shamir’s Method 1979)
secret retrieval
(2, 3)-secret sharing
Alice KenBob
ShamirShamir, A. (1979): How to Share a Secret, CACM, Vol. 22, Nov. 1979, pp. 612-613., A. (1979): How to Share a Secret, CACM, Vol. 22, Nov. 1979, pp. 612-613.
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Ex: (2, 3)-threshold Scheme (The secret k=7)
7mod112xf(x)
3 shadows: (1, 9), (2, 0), (3, 2)
x
y
7)0( f
(1, 9)
(2, 0)
(3, 2)
How to Share a Secret (t, n)-threshold Scheme
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Ex: (3, 5)-threshold Scheme (t=3, n=5, k=13, p=17)
8)1( f
13mod1710x2xf(x) 2
Alice KenBob Jesse Eva
ID1=1 ID2=2 ID3=3 ID4=4 ID5=5
7)2( f 10)3( f 0)4( f 11)5( f
(1, 8)
(3, 10)
(5, 11)
13mod1710x2xf(x) 2
Lagrange Interpolation
)35)(15(
)3)(1(11
)53)(13(
)5)(1(10
)51)(31(
)5)(3(8)(
xxxxxx
xf
k
How to Share a Secret (t, n)-threshold Scheme
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Concept of Visual Cryptography
Cryptography Traditional Cryptography Symmetric: DES, AES Asymmetric: RSA
The encryption and decryption processes need high computation costs
Need secret key for decoding Visual Cryptography
low computation Fast decoding Without the secret key
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Hidden in Hidden in
The Basis Image: Mickey The Counterfeit Image: Minnie
The Secret Image: Airplane
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The Basis Image: Mickey The Counterfeit Image: Minnie
The Expanded Basis Image: Mickey-3X3
The Expanded Counterfeit Image: Minnie-3X3
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Airplane
Mickey-3X3 Minnie-3X3
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Naor and Shamir (1994) proposed a (k, k)–VSS scheme Extend a secret pixel into a block of 22 s
ub-pixels Contain two white pixels and two black p
ixels for each block White pixel: transparent Black pixel: black
Visual Cryptography for Binary Image
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Secret pixel
stacked result
Secret image
Share 1
Share 2
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Transfer the grayscale image to the halftone image
Utilize the density of black dots to simulate the content of grayscale image White area: low density of black dots Black area: high density of black dots
Visual Cryptography for Grayscale Image
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Share 1 Share 2
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shadow 1 shadow 2
camouflage
cover image
stego image1 stego image2
secret image
Meaningful Visual Cryptography for Grayscale Image
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0 1
1 0
152
167
140
137
cover image
shadow1
140
transparent
stego image1
0 1
1 0
170
97
150
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0 1
1 0
1 0
0 1
140
167 170
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shadow2
stego image2
167 97
150
shadow1
shadow2
17
140
167
83
170
140 150
167 97
stego image1
stego image2
stacked result
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Experimental Resultsshadow1 cover image1 stego image1
shadow2 cover image2 stego image2
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stego image1
stego image2
stacked result
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shadow1 cover image1 stego image1
shadow2 cover image2 stego image2
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stego image1
stego image2
stacked result
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Meaningful and Progressive Visual Cryptography
Meaningful shares: friendly content Progressive visual cryptography
Improve the stacked result progressively
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Progressive Visual Cryptography (t, 6)
(2, 6) (3, 6) (4, 6) (5, 6) (6, 6)
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Meaningful shares: friendly content
Progressive & Meaningful Visual Cryptography
shadow 1 shadow n
camouflage
cover image
stego image1 stego image n
…
…
secret image
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secret image
cover image
secret image cover image shadow imageextended secret image
shadow image 1 shadow image 2 shadow image 3 shadow image 4
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secret image
cover image
t =2 t = 3
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(2, 6) (3, 6) (4, 6) (5, 6) (6, 6)
Experimental Results
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RGB: TV’s and Monitors CMY: Color Printing Press
Visual Cryptography for Color Image
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shadow image 1 shadow image 2 stacked image 2
Experimental Results
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Participant 1
Participant 2
Share 1
Share 2
Stack
Recovered secret image
Hacker
Authentication
Original secret image
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Share 1
Share 2
Correct stacking result
Share 1 Share 2
Correct stacking result
Legal extra confidential image
Legal revealed secret image
Authentication
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Tampered share 1
Share 2
Tampered share 1
Share 2
Tampered secret imageTampered secret image
NothingTampered revealed secret image
Authentication
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Grayscale image Lena Grayscale image Baboon Binary main secret
Halftone image of Lena Halftone image of Baboon Binary extra confidential image
Authenticatable VSS Scheme for Binary Secret
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Share image A Share image B
Stacking result Revealed extra confidential image
Authenticatable VSS Scheme for Binary Secret
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Color image Lena
Color image Baboon
Color main secret
Color halftone image of Lena
Color halftone image of Baboon Color halftone extra confidential image
Authenticatable VSS Scheme for Color Secret
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Share image A Share image B
Stacking result Revealed extra confidential image
Authenticatable VSS Scheme for Color Secret
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Visual cryptography (Cont.)
Share 1 Share 2
Secret 1 “VISUAL” Secret 2 “SECRET”
Stacking Stacking
Rotating 40o
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Steganography (Cont.) Text 壞的毛病 壞毛病 山東的蘋果 山東蘋果 我的媽媽 我媽媽 主要的問題 主要問題 另外的一件事 另外一件事 其他的問題 其他問題 北京的大學 北京大學 Video Sound
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Conclusions An alternative for hiding visual secret Less secret Low computation cost
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