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The Recent Developments in Visual Cryptography
Chair Professor Chin-Chen Chang (張真誠) National Tsing Hua University National Chung Cheng University Feng Chia University
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Outlines 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
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Concept of Visual 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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The Secret Image: Airplane
Hidden in Hidden in The Basis Image: Mickey The Counterfeit Image: Minnie
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The Basis Image: Mickey
The Counterfeit Image: Minnie The Expanded Counterfeit Image: Minnie-3X3 The Expanded Basis Image: Mickey-3X3
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Mickey-3X3 Minnie-3X3 Airplane
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Visual Cryptography for Binary Image
Naor and Shamir (1994) proposed a (k, k)–VSS scheme Extend a secret pixel into a block of 22 sub-pixels Contain two white pixels and two black pixels for each block White pixel: transparent Black pixel: black
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Secret pixel Share 1 Secret image stacked result Share 2
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Visual Cryptography for Grayscale Image
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
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Share 1 Share 2
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Meaningful Visual Cryptography for Grayscale Image
secret image cover image shadow 1 shadow 2 camouflage stego image1 stego image2
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shadow1 shadow1 transparent shadow2 shadow2 stego image1 stego image2
1 1 167 97 140 150 shadow2 shadow2 stego image1 1 1 167 170 83 140 stego image2 152 167 140 137 170 97 150 83 cover image
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140 150 167 97 stego image1 stacked result 140 167 83 170 stego image2
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Experimental Results shadow1 cover image1 stego image1 shadow2
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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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Progressive & Meaningful Visual Cryptography
Meaningful shares: friendly content … shadow 1 shadow n secret image camouflage cover image … stego image1 stego image n
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extended secret image secret image cover image shadow image secret image cover 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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Experimental Results (2, 6) (3, 6) (4, 6) (5, 6) (6, 6)
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Visual Cryptography for Color Image
RGB: TV’s and Monitors CMY: Color Printing Press
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Experimental Results shadow image 1 shadow image 2 stacked image 2
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Conclusions An alternative for hiding visual secret Less secret
Low computation cost
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