[實作]Hiding data in images by simple LSB substitution

這篇是簡易版的，直接照老師上課講的方法實作，和論文實作應該有很大的出入
論文實作篇在這邊：https://iris123321.blogspot.com/2017/05/finding-optimal-least-significant-bit.html

Data hiding in image是指將secret image藏於cover image底下，就是將秘密藏於一張圖片底下，最貼近生活的例子如恐怖組織在傳遞訊息時，把機密藏在一張普通的照片底下，這樣就算被攔截也不會被發現，這時候，許多人就開始在想，那可以將一張圖片藏於一張圖片底下嗎？其實如果允許圖片失真的情況下，真的可以。

在電腦中，每一個像素(pixel)都是由RGB(red,green,blue)所組成，各占8bits，讓我們來看看如果拋棄幾個8bit中的後面幾個bit會發生什麼事

上面那張圖就可以發現，如果將R後四個bit給刪除，其實沒仔細看是看不太出來的，而刪除後面比較不重要的bit的方法稱為simple LSB (Least Significant Bit,最低有效位元)，利用人眼對於色彩的敏感度：Green>Red>Blue，來實作data hiding，做法如下圖

要將Secret image藏於cover image中，得到stego image：

• secret_R的前4個bits取代cover_R後4個bits得到stego_R,
• secret_G的前5個bits取代cover_G後5個bits得到stego_G,
• secret_R的前3個bits取代cover_B後3個bits得到stego_B,

解碼的部分就是將stego_R取後4 bits，再用0把8 bits填滿即可

來看一下demo的結果

測試圖片原圖來自google search

左上(cover image)、右上(secret image)、左下(stego image)、右下(decode image)

失真率還是蠻明顯的，但以data hiding來說已經算是成功了，接下來就是程式碼的部分，首先先準備兩張512x512 pixels的RAW檔圖：cover.raw 和 secret.raw作為測資，讀入後會輸出stego.raw

	#include <iostream>
	#include <fstream>
	#include <cstdlib>
	#include <stack>
	#define size 512
	using namespace std;
	//=============input images=============//
	unsigned char image_buf_R[size][size]; int image_R[size][size];
	unsigned char image_buf_G[size][size]; int image_G[size][size];
	unsigned char image_buf_B[size][size]; int image_B[size][size];
	//=============input images2=============//
	unsigned char image2_buf_R[size][size]; int image2_R[size][size];
	unsigned char image2_buf_G[size][size]; int image2_G[size][size];
	unsigned char image2_buf_B[size][size]; int image2_B[size][size];
	//=============output1 random result======//
	unsigned char result_buf_R[size][size]; int result_R[size][size];
	unsigned char result_buf_G[size][size]; int result_G[size][size];
	unsigned char result_buf_B[size][size]; int result_B[size][size];
	//=============Input====================//
	FILE *Fimage;
	FILE *Fimage2;
	//=============Output====================//
	FILE *Fresult;
	int main(void)
	{
	int n,i,j;
	//Load Cover Image
	cout <<"Load image.\n";
	Fimage=fopen("cover.raw","rb");
	for(i=0;i<size;i++)
	{
	for(j=0;j<size;j++)
	{
	fread(&image_buf_R[i][j],1,1,Fimage);
	fread(&image_buf_G[i][j],1,1,Fimage);
	fread(&image_buf_B[i][j],1,1,Fimage);
	image_R[i][j] = (int)image_buf_R[i][j];
	image_G[i][j] = (int)image_buf_G[i][j];
	image_B[i][j] = (int)image_buf_B[i][j];
	}
	}
	fclose(Fimage);
	//Load Secret Image
	Fimage2=fopen("secret.raw","rb");
	for(i=0;i<size;i++)
	{
	for(j=0;j<size;j++)
	{
	fread(&image2_buf_R[i][j],1,1,Fimage2);
	fread(&image2_buf_G[i][j],1,1,Fimage2);
	fread(&image2_buf_B[i][j],1,1,Fimage2);
	image2_R[i][j] = (int)image2_buf_R[i][j];
	image2_G[i][j] = (int)image2_buf_G[i][j];
	image2_B[i][j] = (int)image2_buf_B[i][j];
	}
	}
	fclose(Fimage2);
	//Do some operations
	cout <<"Do some operations.\n";
	//data hiding
	int cover_r,secret_r,cover_g,secret_g,cover_b,secret_b;
	for(i=0;i<size;i++)
	{
	for(j=0;j<size;j++)
	{
	cover_r = image_R[i][j]>>4<<4;
	secret_r = image2_R[i][j]>>4;
	cover_g = image_G[i][j]>>5<<5;
	secret_g = image2_G[i][j]>>3;
	cover_b = image_B[i][j]>>3<<3;
	secret_b = image2_B[i][j]>>5;
	result_R[i][j] = cover_r+secret_r;
	result_G[i][j] = cover_g+secret_g;
	result_B[i][j] = cover_b+secret_b;
	}
	}
	//Write Image
	cout <<"Write Image.\n";
	Fresult=fopen("stego.raw","wb");
	for (i=0;i<size;i++)
	{
	for(j=0;j<size;j++){
	result_buf_R[i][j]=(unsigned char)result_R[i][j];
	result_buf_G[i][j]=(unsigned char)result_G[i][j];
	result_buf_B[i][j]=(unsigned char)result_B[i][j];
	}
	}
	for (i=0;i<size;i++)
	{
	for(j=0;j<size;j++)
	{
	fwrite(&result_buf_R[i][j],1,1,Fresult);
	fwrite(&result_buf_G[i][j],1,1,Fresult);
	fwrite(&result_buf_B[i][j],1,1,Fresult);
	}
	}
	fclose(Fresult);
	cout <<"Finish.\n";
	system("pause");
	return 0;
	}

view raw simple_LSB_substitution.cpp hosted with ❤ by GitHub

解碼程式：
input：stego.raw
output：decode2.raw

	#include <iostream>
	#include <fstream>
	#include <cstdlib>
	#include <stack>
	#define size 512
	using namespace std;
	//=============input images=============//
	unsigned char image_buf_R[size][size]; int image_R[size][size];
	unsigned char image_buf_G[size][size]; int image_G[size][size];
	unsigned char image_buf_B[size][size]; int image_B[size][size];
	//=============output1 random result======//
	unsigned char result_buf_R[size][size]; int result_R[size][size];
	unsigned char result_buf_G[size][size]; int result_G[size][size];
	unsigned char result_buf_B[size][size]; int result_B[size][size];
	//=============Input====================//
	FILE *Fimage;
	//=============Output====================//
	FILE *Fresult;
	int main(void)
	{
	int n,i,j;
	//Load Cover Image
	cout <<"Load image.\n";
	Fimage=fopen("stego.raw","rb");
	for(i=0;i<size;i++)
	{
	for(j=0;j<size;j++)
	{
	fread(&image_buf_R[i][j],1,1,Fimage);
	fread(&image_buf_G[i][j],1,1,Fimage);
	fread(&image_buf_B[i][j],1,1,Fimage);
	image_R[i][j] = (int)image_buf_R[i][j];
	image_G[i][j] = (int)image_buf_G[i][j];
	image_B[i][j] = (int)image_buf_B[i][j];
	}
	}
	fclose(Fimage);
	//Do some operations
	cout <<"Do some operations.\n";
	//data hiding
	for(i=0;i<size;i++)
	{
	for(j=0;j<size;j++)
	{
	result_R[i][j] = image_R[i][j]%16*16;
	result_B[i][j] = image_B[i][j]%8*32;
	result_G[i][j] = image_G[i][j]%32*8;
	}
	}
	//Write Image
	cout <<"Write Image.\n";
	Fresult=fopen("decode2.raw","wb");
	for (i=0;i<size;i++)
	{
	for(j=0;j<size;j++){
	result_buf_R[i][j]=(unsigned char)result_R[i][j];
	result_buf_G[i][j]=(unsigned char)result_G[i][j];
	result_buf_B[i][j]=(unsigned char)result_B[i][j];
	}
	}
	for (i=0;i<size;i++)
	{
	for(j=0;j<size;j++)
	{
	fwrite(&result_buf_R[i][j],1,1,Fresult);
	fwrite(&result_buf_G[i][j],1,1,Fresult);
	fwrite(&result_buf_B[i][j],1,1,Fresult);
	}
	}
	fclose(Fresult);
	cout <<"Finish.\n";
	system("pause");
	return 0;
	}

view raw 001.cpp hosted with ❤ by GitHub

最後，祝大家有個美好的聖誕節~ 😄😄😄