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Posted 26 Dec 2011

# Visual Image Cryptography Generator

, 6 Mar 2014 CPOL
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Visual cryptography is a cryptographic technique which allows visual information (pictures, text, etc.) to be encrypted in such a way that the decryption can be performed by the human visual system, without the aid of computers.

## Introduction

Merry Christmas! Before writing this article, I wrote an article about Visual cryptography. But it's in text base, that means when you enter text and it will generate a visual encrypted images for you.

Recently, I received an email for if it can do in image base, select image and return encrypted images. And answer is of cause.

## Background

Visual cryptography was introduced by Naor and Shamir at EUROCRYPT '94. They asked the following intriguing question: is it possible to devise a secret sharing scheme in which an image can be reconstructed "visually" by superimposing two shares? Each share would consist of a transparency, made up of black and white pixels. (Note that it would be more accurate to say "transparent" rather than "white".) Examination of one share should reveal no information about the image.

## Using the Code

The main difference between Image and Text is that you can control the text background in Black and White but Image contains Color, so first what we need is to convert the color into Black and White:

```private Bitmap ConvertToBackAndWhite(Bitmap source)
{
int sourceWidth = source.Width;
int sourceHeight = source.Height;

Bitmap result = new Bitmap(sourceWidth, sourceHeight);
double mid = 255d * (1d / 2d);

for (int x = 0; x < sourceWidth; x++)
{
for (int y = 0; y < sourceHeight; y++)
{
Color c = source.GetPixel(x, y);
c = (Average(c.R, c.G, c.B) > mid) ? Color.Empty : Color.Black;
result.SetPixel(x, y, c);
}
}

return result;
} ```

or you can change the color into gray scale

```public Bitmap ConvertToGrayscale(Bitmap source)
{
int sourceWidth = source.Width;
int sourceHeight = source.Height;
Bitmap result = new Bitmap(sourceWidth, sourceHeight);
for (int y = 0; y < sourceHeight; y++)
{
for (int x = 0; x < sourceWidth; x++)
{
Color c = source.GetPixel(x, y);
int luma = (int)(c.R * 0.3 + c.G * 0.59 + c.B * 0.11);
result.SetPixel(x, y, Color.FromArgb(luma, luma, luma));
}
}
return result;
} ```

and use my halftone class to convert it into black and white, read more

```List<Color> palette = new List<Color>(); //Color Palette
using (Bitmap gSource = ConvertToGrayscale(source))
{
using (Bitmap bwSource = FloydSteinbergDither.Process(gSource, palette.ToArray()))
{
m_EncryptedImages = GenerateImage(bwSource);
}
}```

After converting the image to black and white, the other is the same as text encryption. And I'm not describing the details here.

Enjoy the article and Christmas Holidays.

Cheers!

## History

• 26th December, 2011: Initial post
• 6th March 2014: Update to grayscale and halftone support

## Share

 Architect MouxIdea Limited Hong Kong
1981 Born in Hong Kong
1999 The Year Before Millennium
2000 First touch of programming - ASP(guestbook)
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2006 Microsoft MVP - C#
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2008 Microsoft MVP - C#
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2011 Start my software hut

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