利用LSB算法隐藏文字信息的MATLAB实现

LSB算法——隐藏文字信息


LSB(Least Significant Bit)最低有效位,指二进制中最低位数值。
详见WiKi:https://en.wikipedia.org/wiki/Least_significant_bit


隐写术

隐写术是一种关于信息隐藏的技术,通常利用位图的RGB三种颜色值(0~255)在微小改变的情况下难以被人眼察觉的特点。每种颜色值都可以用8位二进制来表示,而最低有效位LSB技术就是通过修改最低几位的数值来将信息嵌入到图片中去。
详见WiKi:https://en.wikipedia.org/wiki/Steganography


相关方法

LSB方面的论文有很多,在此利用以下这篇

Chan C K, Cheng L M. Hiding data in images by simple LSB substitution[J]. Pattern recognition, 2004, 37(3): 469-474.
http://www.sciencedirect.com/science/article/pii/S003132030300284X

其具体的思路如下:
1. 设图像像素为 MC×NC M C × N C C C 表示原8-bit图像

C={xij|0i<MC,0j<NC,xij{0,1,...,255}} C = { x i j | 0 ≤ i < M C , 0 ≤ j < N C , x i j ∈ { 0 , 1 , . . . , 255 } }
M M 为n-bit待隐藏的消息
M={mi|0i<n,mi{0,1}} M = { m i | 0 ≤ i < n , m i ∈ { 0 , 1 } }

2. 将 M M 重排为 M M ′
M={mi|0i<n,mi{0,1,...,2k1}} M ′ = { m i ′ | 0 ≤ i < n ′ , m i ′ ∈ { 0 , 1 , . . . , 2 k − 1 } }
其中 n<MC×NC n ′ < M C × N C ,则 m m ′ 可以表示为:
mi=Σk1j=0mi×k+j×2k1j m i ′ = Σ j = 0 k − 1 m i × k + j × 2 k − 1 − j

3. 从原图中按照既定规则挑选出 n n ′ 个像素:
{xl1,xl2,...,xln} { x l 1 , x l 2 , . . . , x l n ′ }
,嵌入过程是通过用 m m ′ 替换 xli x l i k k 位LSBs,则 xli x l i 会被替换成
xli=xli(xli mod 2k)+mi x l i ′ = x l i − ( x l i   m o d   2 k ) + m i ′

4. 提取消息的方法为:
mi=xli mod 2k m i ′ = x l i ′   m o d   2 k


MATLAB相关函数解释

  1. imread( ) 用于读取需要嵌入隐藏信息的图片,并存储为 uint8 类型的三维RGB矩阵,每个数值都位于0~255间
  2. strcat( ) 用于字符串连接
  3. unicode2native( ) 将 unicode 编码转化为相应的数字字节,相对应的 native2unicode( ) 是将数字字节转化为对应的 unicode 编码
  4. dec2bin( ) 将十进制数转化为二进制,可选参数为最少几位二进制,相应的 bin2dec( ) 是将二进制转化为十进制
  5. strjoin( ) 将元胞中的字符串数组组合成一个单字符串
  6. double( ) 将字符转化成相应的 ASCII 码,相应的 char( ) 将 ASCII 码转化成字符
  7. imwrite( ) 将矩阵存成图片

注意事项

  1. MATLAB的 imwrite() 函数存成图片时,若选用 .jpg 格式则会出现一定程度的失真,导致无法提取出正确信息,因此最好存为 .png 或其他格式
  2. 以下给出的代码以EOF作为嵌入结束的标志
  3. 以下代码适用于2-LSB
  4. 以下代码有选择地将信息嵌入红、绿、蓝中的一层

源码

CSDN的显示效果太差了,可以查看我的GitHub:
https://github.com/kemingy/Cryptography/tree/master/LSB%20steganography

嵌入函数

function [] = LSB_embed(name, message, lsb, color)
% LSBembed(name, message, lsb)  LSB in steganography (embed)
% name: the picture's path and name
% message: the data you want to hide in the picture
% lsb: lsb-rightmost LSBs
% color: 1-red, 2-green, 3-blue
%
% Author: Moming
% 2016-03-16

image = imread(name);
msg_origin = unicode2native(strcat(message, char(4)), 'UTF-8');  % UTF-8 encode, 'EOT' is the end tag
msg_bin = dec2bin(msg_origin, 8);  % convert to binary
msg = strjoin(cellstr(msg_bin)','');

len = length(msg) / lsb;
while len ~= fix(len)
    strcat(msg, char(4));
    len = length(msg) / lsb;
end
tmp = blanks(len);
for i = 1 : len
    tmp(i) = char(bin2dec(msg((i - 1) * lsb + 1 : i * lsb)) + '0');  % '0' is a kind of placeholder
end

% use Red, Green or Blue
layer = image(:, :, color);
for i = 1 : len
    layer(i) = layer(i) - mod(layer(i), 2^lsb) + double(tmp(i) - '0');  % only to be consistent with front
end

% save the picture
image_result = image;
image_result(:, :, color) = layer;
imshow(image_result);
imwrite(image_result, 'result.png');  % jpg would lose some information

end

提取函数

function [msg_origin] = LSB_extract(name, lsb, color)
% LSB_extract(name, lsb)  LSB in steganography (extract)
% name: the picture's path and name
% lsb: lsb-rightmost LSBs
% color: 1-red, 2-green, 3-blue
%
% Author: Moming
% 2016-03-17

image = imread(name);

layer = image(:, :, color);
tmp = blanks(0);
n = prod(size(layer));

% if lsb ~= 2, then you need to change something below
for i = 1 : n * lsb / 8
    tmp((i - 1) * 4 + 1 : i * 4) = mod(layer((i - 1) * 4 + 1 : i * 4), 2^lsb);
    msg((i - 1) * 8 + 1 : i * 8) = dec2bin(tmp((i - 1) * 4 + 1 : i * 4), lsb)';
    msg_origin(i) = bin2dec(msg((i - 1) * 8 + 1 : i * 8));
    if msg_origin(i) == 4  % EOT is the end tag
        break;
    end
end

msg_origin = native2unicode(msg_origin,'UTF-8');
msg_origin = msg_origin(1:end-1);

end

改进:嵌入到RGB三层中

由于单个像素点的值表示成二进制为8位,为了方便进行嵌入,补上一位,凑成9位,分别嵌入到RGB三层中,且补上的一位以‘0’、‘1’、‘0’、‘1’的顺序出现,以达到‘0’、‘1’平衡,最后一个字符嵌入时对其补上的一位取反,作为结束标记

嵌入算法

function [] = LSB_embed(name, message)
% LSBembed(name, message, lsb)
% name: the picture's path and name
% message: the data you want to hide in the picture
% LSB in steganography (embed)
%
% Author: Moming
% 2016-03-20

lsb = 3;
image = imread(name);
msg_origin = unicode2native(message, 'UTF-8');  % UTF-8 encode
msg_bin = dec2bin(msg_origin, 8);  % convert to binary
msg = blanks(9);
for i = 1 : size(msg_bin, 1)
    msg(i, :) = strcat(msg_bin(i, :), char(mod(i, 2) + '0'));
end
msg = strjoin(cellstr(msg)','');
msg(end) = char(mod(size(msg_bin, 1) + 1, 2) + '0');  % change the last bit as the end tag

len = length(msg) / lsb;
tmp = blanks(len);
for i = 1 : len
    tmp(i) = char(bin2dec(msg((i - 1) * lsb + 1 : i * lsb)) + '0');  % convert to decimal
end

% use RGB
result = image;
rgb = 1;
[len_R, len_G, len_B] = size(result);

for R = 1 : len_R
    for G = 1 : len_G
        for B = 1 : len_B
            if rgb <= len
                % only to be consistent with front: '0'
                result(R, G, B) = result(R, G, B) - mod(result(R, G, B), 2^lsb) + double(tmp(rgb) - '0');
                rgb = rgb + 1;
            end
        end
    end
end

imshow(result);
imwrite(result, 'result.png');  % jpg would lose some information

end

提取算法

function [msg_origin] = LSB_extract(name)
% LSB_extract(name, lsb)
% name: the picture's path and name
% LSB in steganography (extract)
%
% Author: Moming
% 2016-03-20

image = imread(name);

lsb = 3;
index = 1;
rgb = zeros(0);
[len_R, len_G, len_B] = size(image);
flag = char('0');  % use to detect the end tag

for R = 1 : len_R
    for G = 1 : len_G
        tmp = blanks(0);
        for B = 1 : len_B
            tmp = strcat(tmp, mod(image(R, G, B), 2^lsb) + '0');  % '0' is useful!!! Placeholder...
        end
        tmp_bin = dec2bin(tmp - '0', 3)';
        rgb(index) = bin2dec(tmp_bin(1 : 8));
        if flag + tmp_bin(9) ~= 97  % '0'/'1' is the end tag
            msg_origin = native2unicode(rgb, 'UTF-8');
            return;
        end
        index = index + 1;
        flag = tmp_bin(9);
    end
end

end

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