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， C表示原8-bit图像

C={xij|0≤i

M

为n-bit待隐藏的消息

M={mi|0≤i

2. 将

M

重排为

M′

：

M′={m′i|0≤i

其中

n′

，则

m′

可以表示为：

m′i=Σk−1j=0mi×k+j×2k−1−j

3. 从原图中按照既定规则挑选出

n′

个像素：

{xl1,xl2,...,xln′}

，嵌入过程是通过用

m′

替换

xli

的

k

位LSBs，则

xli

会被替换成

x′li=xli−(xlimod2k)+m′i

4. 提取消息的方法为：

m′i=x′limod2k

MATLAB相关函数解释

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

strcat( ) 用于字符串连接

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

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

strjoin( ) 将元胞中的字符串数组组合成一个单字符串

double( ) 将字符转化成相应的 ASCII 码，相应的 char( ) 将 ASCII 码转化成字符

imwrite( ) 将矩阵存成图片

注意事项

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

以下给出的代码以EOF作为嵌入结束的标志

以下代码适用于2-LSB

以下代码有选择地将信息嵌入红、绿、蓝中的一层

源码

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