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Visual cryptography is a method of encrypting an image into several encrypted images. Conventional visual cryptography can display only monochrome images. We previously proposed a color visual cryptography method that uses the interference color of high-order retarder films and encrypts one secret image into two encrypted images. In other words, this method can only encrypt one image at a time. In this paper, we propose a new method that encrypts two color images using interference color.

In recent years, various optical cryptography techniques have been proposed for information security [

Many types of visual cryptography have been proposed [

This section describes the principles of visual cryptography using interference color. In the conventional method, we share a secret image through two encrypted images.

Retarder film type | Rotate angle | Symbolic | Retardation (nm) |
---|---|---|---|

0˚ | 560 | ||

90˚ | −560 | ||

0˚ | 140 | ||

90˚ | −140 |

retarder films. Two encrypted images (shares 1 and 2) are inserted between two crossed polarizers. Each pixel of the shares is composed of retarder films.

Here,

that order. This configuration is expressed as

For share 2,

In this paper, we consider a method to display another image by sliding the share, as shown in

In this paper, we only use λ or λ/4 retarder films with rotation angles of 0˚ or 90˚, as shown in

After converting the original image into eight colors, we encrypt it. Concretely, suppose we have images A and B of size 2 × 2, as shown in

Step 2 is to calculate the first column of share 2. We use values obtained by subtracting the first column of image A from the first column of share 2. Using steps 1 and 2, we hence encrypt the first column of image A. Step 3 is to slide the first column of share 2 onto the second column of share 1. We use the values obtained by subtracting the first column of image B from the first column of share 2 as a second column of share 1. Using step 3, we encrypt the first column of image B. Next, we slide share 1 back and repeat step 2. We use the values obtained

Color | Components | Retardation(nm) | L*a*b |
---|---|---|---|

No film | ±0 | (0, 0, 0) | |

±280 | (99, −8, 17) | ||

±700 | (79, −57, −11) | ||

±420 | (72, 20, 79) | ||

±980 | (73, 70, −23) | ||

±1120 | (78, −90, 29) | ||

±840 | (93, −23, 78) | ||

±560 | (37, 70, −89) |

by subtracting the second column of image A from the second column of share 1 as the second column of share 2. Then, we encrypt the second column of image A. In this way, we repeat these steps until there are no more pixels of the image to encrypt.

colors.

The pseudo code for our proposed encryption method is listed in

We need to design the value of s1 so that it does not exceed the range −2240 to 2240 nm. Line 17 in

using the equation written in line 16. Equation (1) is an arithmetic sequence, and can be rewritten as

Here,

We designed and simulated two secret images using the algorithm shown in

Input: secret image A of size (n, m) and secret image B of size (n, m) |
---|

Output: share s1 of size (n, m + 1), share s2 of size (n, m) |

1. Define zero matrix S of size (n, 1) 2. For j = 1 to m do 3. For i = 1 to n do 4. a = nearest retardation to Ai,j of the eight retardations in Lab color space 5. b = nearest retardation to Bi,j of the eight retardations in Lab color space 6. While |

and

and 2 accordingly.

We also made a prototype of the dual visual cryptography using polarizers and retarder films. ^{2}. By stacking shares 1 and 2, secret image A and secret image B are decoded by sliding share 1, as shown in

In this paper, we proposed a new method of dual visual cryptography using the interference color of a birefringent material. The resolution and contrast problems in conventional visual cryptography were overcome by polarization processing. We calculated the combinations of interference colors for dual visual cryptography, and a prototype of a dual color visual cryptography device using interference color was developed. Two secret images are decoded by sliding the share. This method solves the resolution and contrast problems of visual cryptography and demonstrates the potential of interference color in visual cryptography.

Qin, H., Matsusaki, T., Momoi, Y. and Harada, K. (2017) Dual Visual Cryptography Using the Interference Color of Birefringent Material. Journal of Software Engineering and Applications, 10, 754-763. https://doi.org/10.4236/jsea.2017.108041